4th Grade class: Day/Night, the Seasons, Lunar Phases, Eclipses

about 5 mins

I'm going to be giving two classes. In the first class I'm going to be talking about why we have

In the second class I'm going to be talking about why we have

To see what's going on in this class, you're going to have to move around. Feel free to move anytime you like; you don't have to ask - just move. You don't have to stay in your seats. When you move, move slowly so you don't trip on any power cords and you don't bump anything over.

I'm going to need the room lights turned on and off a few times. Would someone in the back table there be prepared to take on the job of turning the room lights on and off, when I ask for it? (thanks).

To the volunteer: Can we have a practice turn-off and turn-on? Thanks. If something is happening down the front of the class, that you need to see, and I ask for the lights to be turned off, you'll need to first turn off the lights before coming down here. Can you handle that?

At the end of the class, get the class to give a round of applause for the kid who did the lights.

There's a vocab list on the board. By the end of the two classes, you'll know how to use these words.

I'm going to be asking questions during the class. If you want to answer a question, please put your hand up, rather than calling out. I won't always pick the first person to put their hand up. I want as many of you as possible, to have a turn at answering a question, so I'll wait for a few hands to go up, before asking someone to answer.

I know you all want to say something, in front of the class, but please don't put your hand up, unless you have an answer. Some kids like to put their hand up, only to find that they don't have an answer. This just wastes everyone's time. So think first and then put up your hand. Just because you were the first to put up your hand, you're not neccessarily going to be asked to give the answer. There's no rush to be the first person to put your hand up, so think first and then when you have an answer, you can put up your hand.

Some of the questions are going to have answers on the board. I'm quite happy for you to get the answer off the board.

What you see before you, on the table, is called an orrery. An orrery is a mechanical device to represent the motions of the planets in the Solar System.

A real orrery is mechanical; it has motors and gears and runs by itself, with the planets moving the same way they do in the sky.

Here's a wooden orrery, that just represents the Sun, Earth and Moon. There is a light in front of the Sun, to represent the light coming from the Sun onto the Earth. You push the end of the bar and the cord makes the Earth and Moon orbit the Sun.

(class_visuals/2.2/wooden.Completed_orrery.jpg),

Here's a complicated mechanical orrery. It shows the planets known to the ancients, Mercury, Venus, the Earth and Moon, Mars, Jupiter and Saturn.

(class_visuals/2.2/orrery.large_ferguson.8AC14165A3A91FFB7554F512B3CD2F07.jpg).

One of the planets has rings. Does anyone know its name?

Saturn

Heres a photo of Saturn taken by the space probe Cassini-Huygens.

class_visuals/2.2/saturn_cassini.jpl.nasa.PIA21046-16.jpg

We've sent several space probes to Saturn. Pioneer 11 in 1979, Voyager I and II in the 1980 and 1981. These were flybys, as the final destination of these space probes, was further out in the Solar System. The space probe Cassini-Huygens, which took this photo, was in orbit around Saturn, from 2004-17.

I'm going to return to this photo towards the end of the class and by then you'll be able to tell me the season (summer, winter, spring or fall) on the part of Saturn that we can see in the photo.

Here's a painting of Saturn, by a famous artist, Chesley Bonestell. Bonestell did a lot of paintings in the '50s, before man ever went into space, to show us all what it might look like out there and to get the public interested in space exploration. It turns out he was a good predictor of what things looked like. I saw his paintings when I was about your age, and they helped inspire me to become a scientist.

class_visuals/2.2/Bonestell_Saturn_from_Titan_sasft.jpg

So back to the orrery. With my orrery, we're only going to be concerned with the Sun, Earth and the Moon. I'm going to represent the Sun, by this light (turn on). Here is the Earth and Moon. I'm going to move things by hand. So it's a very simple orrery, but it will show everything you need to know.

The stars all have names; Sirius, Canopus, Castor and Pollux, Regulus, Vega, Rigel, Antares, Aldebaran, Spica, Arcturus. Our sun is a star too. Does anyone know its name? (Sol, which is where we get the name Solar System). In every day language, we just use the name "The Sun". It's like calling the cat "The Cat", but we do is anyhow, since we only have one sun at the center of the Solar System.

The Moon has a name too (Luna), which is where we get the adjective "Lunar". Again we just use the name "The Moon" in every day conversation.

The Planets all have a name, but we just use the name "The Earth", the stuff we grow our crops on. The names of some tribes, is often "The People". So we all do it. However the Earth has its own name too (Terra), which is where we get the word terrestrial.

Today I'm only going to be talking about the Earth. Next class we'll add back the Moon.

take away the Moon.

Here is a globe of the Earth. Over there is a light representing the Sun.

turn off room lights.

As you can see, the Sun lights up half of the Earth, while the the other half of the globe is in darkness. The part of the Earth that's illuminated by the Sun, is the part of the Earth that's having daytime, while the half that's not illuminated, is the part of the Earth that's having nighttime.

The lit side of the globe is facing you. You can see the countries that are in daylight.

Name some. If you can't see from where you are, come up to look. Remember you can move around in this class without asking me, or waiting to be told you can move.

You can't see the countries that are in the night; they're facing me. Can we have a couple of people come around the back of the globe and name some countries that are in night.

Thanks to Copernicus, we now know that the Earth rotates on its axis in front of the Sun.

spin the Earth slowly,

Notice that countries that were in the back of the globe, in night time, come into day and then, as the Earth rotates some more, return to night again.

The understanding, that the Earth rotates on its axis, is relatively recent. In 1543, Copernicus (print on wall) realised that the Sun is at the centre of the Solar System and that the Earth orbits the Sun and as it does so, rotates on its axis. Until Copernicus' revelation, we thought that the Earth was stationary, at the centre of the Solar System, and that the Sun and the stars rotated around the Earth. I'll come back to this when I talk about Copernicus.

(wave hand across sky) During the day the Sun rises

in what direction? E

and then ascends into the sky, reaching its maximum elevation at noon, and then descends, setting

in what direction? W

Here's a multiple exposure photo of the passage of the Sun, through the sky, in the day. The actual path is an arc of a circle. Because of distortion by the camera, the arc is a bit flattened.

class_visuals/3.1/path_of_sun_through_sky.b85dac5eca45a98b1059014c11da9211.jpg

Let's see what the passage of the Sun through the sky looks like on the orrery. Let's pick a spot on the Earth in night; it doesn't matter where. Notice how as the Earth rotates, that the spot comes into sunlight from the dark, so it sees sunrise, with the Sun coming up through the horizon, so day has begun. Then as the Earth rotates, the Sun climbs overhead, and we have noon. Then as the Earth rotates a bit more, the Sun descends, descending through the horizon, in what we call sunset. Now our spot is back in darkness and it's night time. The Earth keeps rotating and our spot comes into day once again, and then later into night.

At night the stars move through the sky in circles, in the same way that the Sun does during the day. Remember that the Sun is just a star. It happens to be closer, so it's brighter, but it's just a star like all the other stars. Here's a time lapse photo of the stars at night. It's usually called a star trails photo. Here you can see the path of the stars is a circle.

class_visuals/3.1/southern_hemisphere_star_trail.800px-All_In_A_Spin_Star_trail.jpg

The centre of all the circles is called the pole (i.e. pole of the sky). You'll be hearing more about the poles later in this class.

The stars rise in the east, culminate overhead (culminate means reaching the highest point in the sky), and then later in the night set in the west. This is just as the Sun does during the day.

The same process of day and night happens for every spot in the Earth. This process repeats itself over and over, giving the Earth an endless succession of day and night.

After the Earth rotates on its axis, the Sun returns to the same position in the sky. To mark the passage of time, you could pick a convenient marker, for the location of the Sun, such as sunrise, sunset or noon, or the Sun going behind a building, or you could just put a stick in the ground

class_visuals/3.1/sundials-kids-800x800.jpg from https://sciencing.com/sundials-kids-4841694.html

Anyone know what this instrument is?

Sundial

It's the oldest scientific instrument. It allowed us to mark the passage of time in the day. As the Earth rotates, the shadow of the stick moves on the ground. To mark a day, you watch for the shadow to return to the same place.

How long does the Earth take to rotate on its axis?

24 hrs, 1 day; it's the definition of a day.

To summarise: The rotation of the Earth about its axis causes day and night. We see the Sun and the stars move in circles through the sky.

We'd get day and night, no matter which way the Earth rotates. In that case we have to establish, in which direction the Earth rotates about its axis. (spin globe both ways; we want to find out if it rotates this way, or that way?).

Let's find the compass directions on this globe. What are the four cardinal compass directions?

NSEW

Let's say I'm standing in NC, which direction on the globe is N,S,E,W?

In which direction do we see sunrise?

E (no matter where you are on Earth, sunrise is in the east.)

The Sun (and the stars and the Moon and everything in the sky) all rise in the east. We say that the Sun rises, but Copernicus tells us that the Sun doesn't move. What's happening is that the Earth rotates in front of the Sun and the stars.

In what direction does the Sun set?

W

The Sun (and the stars and the Moon and everything in the sky) all set in the west. After the Sun sets, we're in night again.

The fact that we see the Sun rising in the east, tells us which way the Earth rotates. (show the globe with NC in darkness just before sunrise, at sunrise and then as the Sun ascends into the sky.)

Does the Earth rotate with the eastern part of each continent leading or the western part of each continent leading? With the Sun rising in the east, which is it?

E

We know the Earth rotates eastward, because the Sun rises in the east. If the Earth rotated westward, in which direction would the Sun rise?

W

	Note
	I originally did this next bit with NC at sunrise and LA at night, but the kids can't see either spots, because they're on the side of the Earth globe away from the kids. It took me several years to notice this. (It wasn't causing me any problems.)

By looking at the globe and knowing that the direction of rotation of the Earth is eastward, we can ask about the time in LA, when it's sunset in NC.

at sunset in NC is it day or night in LA? (ask kids to assemble around the globe) (day)

How long after our sunset in NC do the people in LA get their sunset (3hrs)? How could we find that out or figure it out?

When the Sun sets in NC, you could phone a friend in LA, tell them that the Sun has just set here, ask them, to call you back, when the Sun sets there. You just have to see how long it takes for your friend to call back.

Alternately we can do it by thinking, and calculate the time difference. We know that it takes 24hrs for the Earth to rotate on its axis; we know how far it is from NC to LA (it's about 1/8 the way around the Earth). We can infer from that, that sunset in LA is 1/8 of the day, or 3hrs, later than in NC.

• If it's 6am in NC, what time is it in LA? (3am, three hours earlier in their day, the people in LA are still in night).
• Let's say the Earth rotates a bit and now it's 9am in NC, what time is it in LA? (6am, three hours earlier in their day).
• The Earth rotates a bit more and now it's 3pm in LA, what time is it in NC? (6pm. three hours later in our day).

Take Backwards Clock

In the northern hemisphere, if you face the path of the Sun through the day, you have the sun rising on your left, in the east, and setting on your right, in the west. You're facing what direction

South

Let's say we decide to tell the time by a clock with hands. Now you're all sensible people. Let's also say that you're the first person to make a clock, and you could make it whatever way you wanted. Which direction would you have the hands go?

Don't say "clockwise". Instead rotate your arms clockwise and then anticlockwise and say "this way or that way".

So people did something sensible here. That's where we get the name of the direction "clockwise" and "anti-clockwise" from.

Now we come to designing the hour hand of the clock. Would you make the hour hand go around the clock once in a day, pointing to the sun as the day progressed, starting with the hour hand pointing to downwards at midnight at sunrise the hour hand would point east, noon it would point up, at sunset it would point west, and at midnight again, the hour hand would point straight down?

Or would you have the hour hand go around the clock twice in a day so that it points up at both noon and midnight?

Well I know what I'd do. I'd have the hour hand go around the clock once a day, pointing in the direction of the Sun.

However in real life, people have chosen the hour hand to go around the clock twice in a day. I've never found an explanation for this. This is likely because there aren't sensible explanations for stupid things.

	Note
	I'm not going to confuse people with the Southern Hemisphere version of this. Even adults have a hard time thinking about it.

leave out

Let's explore sunrise a bit more. (have the kids gather around the globe.)

The kids didn't have any idea where the Sunrise line was and I had to show them. Clearly I'm not doing it right. The old text, which didn't work, is gone and I'm going to try this new section in 2018.

I've positioned the globe so that NC is having sunrise. Show me the part of the globe that is having daylight, night. Show me the parts of Earth that are on the edge between day and night. The edge between day and night is a line that goes the whole way around the Earth. What's happening there? What's happening on the eastern part of the line? (sunrise). What's happening on the western part of the line? (sunset).

Show me the where sunrise is happening?

Show me where on Earth is having sunset. i.e. where is the Sunset line?

There are two spots, near the poles, where sunrise and sunset are happening at the same time. What does it look like when you're there? Things at the poles are a little different than everywhere else on Earth. At one pole you'll be having 24 hours of darkness, and the Sun will just rise by crossing the horizon, only to start setting again. So the Sun will rise and set at the same time. At the other pole you'll be having 24 hours of daylight, and the Sun will just dip below the horizon at sunset, only to start rising again. So the Sun will set and then start rising again.

The line on the surface of a planet where night and day bump into each other. is called the terminator When you look up at the Moon, you can see a line where night and day join. Depending where you are, the terminator is experiencing either sunset or sunrise. (images/moon_terminator.slideplayer.slide_4.jpg). At the terminator, the Sun is low in the sky, so objects on the terminator cast long shadows. If you're observing the Moon with a telescope, all the interesting views are on the terminator, because you can see the shadows of the mountains and the mountains are easy to see (images/moon_terminator.al_paslow.laas_org.phoca_thumb_l_moon_044054mm-x3.jpg).

(if you don't know, you can come over and look at the globe of the Earth.) When it's sunrise in NC, is it day or night in LA? in Europe (day)? Is it day/night in Australia (night)?

About 12hrs after sunrise, the Earth has rotated 180° on its axis, and NC is now at sunset. What direction do we look to see the Sun setting? (W). (show Earth rotating with the Sun sinking.)

If it's sunset in NC, show me the other places on the Earth that are having sunset. (these places are on the terminator). Name them.

Point to the part of the globe that's having sunrise. Name some countries that are having sunrise.

	Note
	(2018) I expect the kids don't understand enough spherical geometry to get this. I'm leaving it out.

Notice that the line of sunrise and sunset through NC is different. The difference is greatest in summer and winter. One line runs NE-SW and the other NW-SE.

• https://www.timeanddate.com/sun/usa/raleigh
• https://www.timeanddate.com/sun/usa/boston

• In winter sunset Boston 4:13pm, Raleigh 5:01pm, 48mins difference
• In summer sunset Boston 8:24pm, Raleigh 8:34pm, 10mins difference
• In summer sunrise Boston 5:07am, Raleigh 5:59am, 52 mins difference
• In winter sunrise Boston 7:13am, Raleigh 7:25am, 12 mins difference.

Not only does the Earth spin on its axis, the Earth orbits the Sun. When I say the Earth orbits the Sun, I mean it goes around the Sun in almost a circle, i.e. in a plane. (Walk around the Sun with the Earth, while spinning the Earth.)

Why does the Earth orbit the Sun? Why doesn't it just keep going in a straight line and disappear off into outer space?

The force of gravity, discovered by Newton.

class_visuals/4.1/GodfreyKneller-IsaacNewton-1689.jpg

This force of gravity attracts the Earth to the Sun and keeps it in orbit around the Sun. Gravity keeps the Moon in Orbit around the Earth, and holds us down on the Earth.

How long does it take for the Earth to orbit the Sun?

a year, 365.25 days; the definition of the year.

How do we know when a year has passed? (The kids might say the time for the seasons to repeat, which is correct.)

There are lots of ways of telling when a year has passed. The all involve lining up someting in the sky, just like you do for determining that a day has passed. Knowing how to tell that a year has passed is more complicated than knowing that a day has passed. Some of them are easier to do than others, and some of them are more accurate. I will get to this in a bit.

The planets of the Solar system also orbit the Sun, and in the same plane that the Earth orbits the Sun. This plane is called the ecliptic plane.

Why do the planets all orbit the Sun in the same plane?

The Solar System formed from a rotating cloud of dust. It turns out that by the laws of physics, everything in the universe winds up rotating about something. In our case of the Solar System, where all the matter was flung out from an exploding star, you start with a cloud of particles moving in random directions. There's always some particle moving in one direction more than another. Two particles that pass each other at different speeds, will gravitationally attract each other, and start rotating about their common center of mass while still moving forward in the original direction. (Show your hands moving, with one moving slightly faster than the other, and then the two objects orbiting their common center of mass.)

Here's a youtube video that explains Why is the Solar System Flat. It's produced by Embry-Riddle Aeronautical University.

seasons.video/4.1/Why_is_the_Solar_System_Flat.mp4

Note: in the last few seconds of the video, there are still some lumps in the disk. The central object in the disk becomes the Sun. Not everything in the disk falls into the Sun. These lumps in the disk become the planets.

Not in class anymore. The video handles it.

Initially assume the cloud is anything but a disk, say a spherical cloud. Some of the objects will orbit above and below the plane. Eventually they'll gravitationally attact each other, and stick together and keep moving in the same direction. The inclination of the new orbit will be the average of the inclination of the two colliding particles. Eventually, no matter what the starting shape, the particles will all come to a very thin disk. The density of particles will be higher and they'll start colliding more, forming planets, till all of the matter is cleared out of the disk and resides in the planets.

Because the planets are all in the same plane, when you look up in the sky at night, you see the planets move through the sky, (wave hand) along the same line, in the plane of the ecliptic.

Note that the Earth and the planets orbit the Sun heading in the easterly direction, the same direction that the Earth rotates on its axis. This is because they all came out of the same disk of spinning matter.

Initially a star forms in the centre of the rotating disk of material. There will be little lumps of material rotating within the disk. These little lumps become planets. As a planet forms, it will be rotating in the plane of the disk, so the planet's axis of rotation will then be perpendicular to the plane of the disk.

demonstrate perpendicularly with your arm/elbow on the table.

point out that the Earth's axis is perpendicular to the plane of the ecliptic.

This plane of the disk becomes the plane of the ecliptic. If nothing happens to the planet after it is formed, then its axis of rotation will stay perpendicular to the plane of the ecliptic.

The planets Mercury, Venus and Jupiter have their axes of rotation perpendicular to the plane of the ecliptic. (The Earth, Moon, Mars and Saturn don't.)

prints_for_whiteboard/4.2/planets_with_perpendicular_axes.odt, (is on the board)

What changes we would see throughout the year if we were on a planet like Mercury, Venus or Jupiter.

Walk around the Sun, carrying the globe of the Earth, with its axis perpendicular to the plane of the ecliptic, to show the days throughout the year.

In the day, when we look up, the sky looks pretty much the same all year; we just see the Sun moving across the sky in the same path every day.

(point in the direction away from the Sun, to where people at night would be looking).

The stars we see in the night sky, make patterns that our human mind interprets as animals and mythical creatures. We call these patterns constellations.

Here's an image http://en.es-static.us/upl/2013/07/scorpius-orion-Hyaku.jpg showing the constellations of Scorpio and Orion. (on the wall)

class_visuals/4.2/scorpius-orion-Hyaku.jpg

Can anyone name some other constellations?

Scorpio, Geminii, Sagittarius, Hercules, Orion, Andromeda, Leo, Taurus, Cassiopeia. (is on the board)

As our planet orbits the Sun during the year, at night when we look up in the sky, we see different parts of our galaxy. As a result, we see different stars and constellations though the year.

leave out

The constellations along the ecliptic are all animals or humans. We give these constellations a special name; the Zodiac (from the word zoo). There are twelve constellations of the Zodiac. Because these constellations are on the ecliptic, through the year, the Sun passes through the signs of the Zodiac. If you were born when the Sun was in a certain sign, then you can say that you were born under that sign. I was born on 1 Dec, under the sign of Sagittarius.

For a planet whose axis of rotation is perpendicular to the plane of the ecliptic, the only way you could tell that the year was progressing, would be the change of constellations at night. Every day would pretty much be the same; the temperature would be the same day after day, the length of the day and night would be equal, every day of the year. The fact that the planet orbits the Sun wouldn't be all that interesting, because all times during the year, the path of the Sun through the sky each day, and the temperature and weather, would be the same.

So you'd know that a year had passed by the return of the same stars at night. However you probably wouldn't care much, since the passage of a year didn't have much effect on your life.

It turns out that the Earth's axis of rotation is inclined to the plane of the ecliptic. This gives us the seasons, which mark the progression of the year. I'm now going to show you how the tilting of the Earth's axis, to the plane of the ecliptic, gives us the seasons.

All globes of the Earth, like the ones used in classrooms, have the Earth's axis of rotation tilted. (show the tilt) This means, as the Earth orbits the Sun (walk around the Sun), the axis points at an angle to the plane of the ecliptic. Does anyone know the angle of the tilt?

23.5°

The planets Earth (and the Moon), Mars, Saturn, Uranus and Neptune, all have their axes of rotation tilted to the plane of the ecliptic. They all have seasons.

prints_for_whiteboard/4.3/planets_with_tilted_axes.odt (is on the board)

Planets initially form with their axes of rotation perpendicular to the plane of the ecliptic. If later we see that the axis of rotation is tilted, then we infer, that in the time since the planet's formation, something has happened to the planet.

Does anyone know how the Earth's axis of rotation became tilted?

Well we don't know for sure, but here's the generally accepted explanation. When the Earth first formed, its axis of rotation was perpendicular to the plane of the ecliptic. The Earth didn't have a Moon. It's thought that very early in the history of the Solar System, a Mars sized planet crashed into the Earth. The ejecta from the collision initially would have formed a ring of debris around the Earth, which eventually coalesced into the Moon. This collision was not dead centre, so it tilted the axis of rotation of the Earth. So the tilting of the Earth's axis of rotation is thought to be a result of the how we got the Moon.

There's nothing special about the 23.5° tilt angle; it could be almost any angle at all. The actual angle is a consequence of the spot on Earth that suffered the collision, and the size and velocity of the Mars sized object.

As you can see from the list of planets with tilted axes, most of the planets have suffered some sort of collision since they were formed.

Leave out. I assume this will overwhelm the kids. Mars' axial tilt varies from 10-25° and is caused by the eccentric orbit and non-spherical distribution of mass.

Here's a list of the axial tilt Axial tilt of the planets. Mars, Saturn, Uranus and Neptune have axial tilts.

(walk the Earth around the Sun, spinning the Earth, showing the kids that the axis points in the same direction.)

Because the Earth rotates about its axis, it behaves like a gyroscope. As a result, when the Earth orbits the Sun, the axis of rotation stays pointed in the same direction in the sky. i.e. no matter where the Earth is in its orbit around the Sun. So no matter what time of year it is, the Earth's axis is always pointing in the same direction.

Show https://www.youtube.com/watch?v=xQb-N486mA4 Gyroscopes in space

seasons.video/4.3/Gyroscopes_in_space-xQb-N486mA4.webm

The fact that a gyroscope maintains its orientation in space, allows you to make what's called an intertial guidance system. In an airplane you mount a gyroscope in a gymbal mount. This allows the gyroscope to stay in its original orientation, while the airplane banks and goes up and down. An intertial guidance system can guide a plane from one side of the country to another without human intervention, by using the gyroscope to tell the orientation of the aircraft. This allows the pilot to fly and land the plane blind, i.e. in conditions when he can't see anything out the front window of the plane, like at night or in bad weather. When a computer flies a plane, as happens for much of regular passenger flights, the computer uses the inertial guidance system to keep track of where the plane is and its orientation.

If the Earth's axis always points in the same direction, where in the sky does it point?

Remember the star trails photo I showed you earlier where all the stars rotated in a circle about a point in the sky, that I called the pole. There's a pole in the northern sky and another pole in the southern sky.

If you extend the Earth's axis in the northerly direction, it goes through the North Pole of the sky. It's the point of the sky that all the stars, including the Sun, rotate around once every day as seen from the northern hemisphere. The previous star trail photo was in the southern hemisphere. Here's a star trail photo in the northern hemisphere.

class_visuals/4.3/northern_hemisphere_star_trail.ff359f5e3e15bb023018c188cf426cf0.jpg

https://www.wallpaperup.com/uploads/wallpapers/2013/03/30/67821/ff359f5e3e15bb023018c188cf426cf0.jpg

It turns out there is a star near, but not exactly on, the North Pole of the sky. This star is close enough to the pole (about 1°) to be handy for finding north at night. It's been used for navigation, by sailors all throughout history. Because the star is near the pole, it's called the pole star. This star has its own name. Does anyone know the name of the pole star?

Polaris

The fact that there is a star, Polaris, near the north pole of the sky is just a coincidence. There didn't have to be a star there at all. If instead, you extend the Earth's axis, in the opposite direction, you find that the South Pole of the sky is just a blank spot in the sky. There's no stars or anything interesting there at all.

Because of the gyroscopic effect, the Earth's axis always points to the same spot in the sky, no matter where the Earth is in its orbit around the Sun. Let's say that the Earth is in a spot in its orbit where the southern hemisphere is tilted towards the Sun, while the northern hemisphere is tilted away from the Sun. Then 6 mo later, when the Earth is on the other side of the Sun, the Earth's axis will still be pointed to the same spot in the sky. Now the situation will be reversed for the hemispheres; the southern hemisphere will be tilted away from the Sun and the northern hemisphere will be tilted towards the Sun. A quarter the way around the year, the axis of rotation will be crossways to the Sun and both hemispheres will be equally illuminated, getting the same amount of sun.

Let's see what happens, as the Earth orbits the Sun, with the Earth's axis inclined to the plane of the ecliptic, and pointing in the same direction.

(southern hemisphere summer, northern hemisphere winter) Say "the southern hemisphere is getting the Sun overhead, while the northern hemisphere is getting the Sun low in the sky".

	Note
	Northern hemisphere winter and southern hemisphere summer occur at the same time. The seasons are opposite in the two hemispheres.

As you go closer to the poles, the Sun gets lower in winter.

Here's a multiple-exposure photo of the Sun in mid-winter from St George, ME. ME is further north than NC, so they have a shorter time of daylight in winter than we do. Notice how low the Sun is in the sky compared to the previous multi-exposure shot of the Sun, (copied from class_visuals/3.1)

class_visuals/4.3/time_lapse.winter_solstice.St_George_Maine.AP_02121905856.jpg

Here's a time lapse photo from further north, taken at mid-winter, from just outside the Artic Circle. The Sun here is even lower than it was in ME.

class_visuals/4.3/time_lapse.winter_solstice.just_outside_artic_circle.31008.jpg

Next orbit the Earth to the other side of the Sun (how long does this take? 6mo), pointing out that you're keeping the direction of the Earth's axis fixed, till we have the northern hemisphere with the Sun high in the sky (northern hemisphere summer). Ask which hemisphere is getting the overhead Sun now. Say "the northern hemisphere is getting the Sun overhead".

Do the reverse of above and show that the northern hemisphere summer is southern hemisphere winter.

What month has the longest days for Northern hemisphere summer and the shortest days for Southern hemisphere winter?

Jun

leave out - too complicated for kids. class_visuals/5.3/equinox_solstice-north.jpg

Show spring and fall and that they are opposite in each hemisphere. Show that in spring and fall that the amount of day and night is the same and it's the same in both the northern and southern hemispheres.

At spring and fall, the amount of daylight and night are equal at about 12 hrs, all over the globe. The date when they exactly equal, about 22 Mar and 22 Sep, is called the equinox, from noct meaning night. From noct we get the word noctural, meaning being awake at night and asleep during the day.

What season is it now i.e. in real life (not in the models on the table)

(The class is normally given in Dec, so it's southern hemisphere summer and northern hemisphere winter.)

The seasons are opposite in the two hemispheres; when it's summer in one, it's winter in the other; when it's spring in one, it's the fall or autumn in the other.

I showed this view of Saturn a bit earlier. It was taken by the space probe Cassini-Huygens. (point)

class_visuals/4.3/saturn_cassini.jpl.nasa.PIA21046-16.jpg

Saturn's axis is inclined at 27° to the ecliptic plane. (This is about the same tilt as Earth's axis.) So we know that Saturn has seasons. Let's look at the hemisphere of Saturn that's above the plane of the rings. What season is that part of Saturn having?

In case they need clues (wait for a few kids to put their hands up)

we can see the pole is fully illuminated. and the illumination extends a fair way past the pole. Saturn's axis is inclined at 27° to its orbital plane. We see that the pole is tilted about 30°, so the Saturn's axis (and that of the rings), is maximally tipped over.

We see a shadow of the planet on the rings. This confirms that the planet is illuminated from the top. Note that the shadow doesn't extend as far as the outer edge of the rings. We also see (bottom right) a shadow of the rings on the planet. This means that the sun is higher in the sky in this hemisphere. It will be having longer daylight than the hemisphere below the plane of the rings.

The top of the planet is tipped towards the Sun. The hemisphere we're seeing is near the peak of summer.

Just so you know, the temperature on Saturn is about -170°C. Summer isn't all that warm.

Here's a view of Saturn taken about 25 years earlier by Voyager. Notice how the shadow of the planet extends way out past the end of the rings. Compared to the previous photo, is the angle of the Sun closer to the equator or to the poles.

sun is closer to the equator. Notice the shadow of the rings on the planet is in about the middle of the planet.

In this photo, Saturn is near the equinox, the length of day and night is going to be close to equal. One half of the planet is in spring and the other is in fall. We can't tell which is which in this photo.

Saturn is a long way out in the Solar System, and so takes about 28years to orbit the Sun. Because of the gyroscopic effect, the direction of Saturn's axis does not change, and neither does the plane of the rings, as Saturn orbits the Sun. (Show the plane of the rings not changing as Saturn orbits the Sun). (Talk about the angle of the rings changing as observed on Earth. Talk about seeing Saturn with no rings from Reedy Creek.)

class_visuals/4.3/saturn_seasons.universe_today.jpg

So the tilt of the Earth's axis produces the seasons and makes these differences from summer to winter, during the year

Unlike a planet whose axis is not tilted to the ecliptic, if you live on a planet with a tilted axis of rotation, the passage of the seasons tells you when a year passes.

Adjusting our lives to the seasons is important in staying alive. We grow our crops in summer, so we must plant our crops in spring, to be ready for summer. Animals and plants must time reproduction, according to the seasons, so the babies are born in spring, to be able to eat the food available in summer. Otherwise the offspring will not have grown enough to survive the following winter or the dry season. Some animals migrate to avoid winter or the dry seasons.

Name some animals that migrate to avoid winter

list is on wall, prints of images are too small for kids to see. Are now presented from projector

birds, deer, butterflies (monarch), bats, salmon, the mass migration of many animals in Africa,

whales https://www.learner.org/jnorth/images/graphics/gwhale/maps/map_annual.gif

class_visuals/4.3/map_annual_whale_migration.gif

Whales go to the poles in summer, to feed. They then go to to the tropics in winter to give birth. In the tropics, the water is warmer and the babies have a better chance of surviving.

The arctic tern https://en.wikipedia.org/wiki/Arctic_tern

class_visuals/4.3/Arctic_tern-wikipedia-2009_07_02_-_Arctic_tern_on_Farne_Islands-NE_England-_The_blue_rope_demarcates_the_visitors_path.jpg

flies every year from the Arctic to the Antarctic and back again, a round trip of 56,000 miles.

class_visuals/4.3/Arctic_tern-Sterna_paradisaea_distr_mig.png

This distance is just over double the distance it would do if it did the trip in a straight line.

Ancient civilisations used the stars to tell the seasons. For the Egyptians, the river Nile provided their livelihood. Each year it would flood, bringing not only water for their crops, but fresh silt to grow them on. (https://en.wikipedia.org/wiki/Flooding_of_the_Nile) The flooding of the Nile is the result of the yearly monsoon, between May and August, causing enormous precipitations on the Ethiopian Highlands, whose summits reach heights of up to 4550 m (14,928 ft). The flooding of the Nile is celebrated in Egypt by a holiday. Having enough food to eat is a big deal and just about every society celebrates having enough food to make it through winter.

The US has a celebration of having food for winter. What's it called? (Thanksgiving.)

While the ancient Eqyptians didn't know why the Nile flooded, they were able to predict when it would flood. They noticed that the river flooded on the heliacal rising of Sirius, the Dog Star, the brightest star in the night sky.

class_visuals/4.3/Kepler_Observatory_Linz_under_the_Stars.jpg from https://en.wikipedia.org/wiki/Heliacal_rising

So what's a "heliacal rising"? "Heliacal" is an adjective meaning "of the Sun" The phrase "heliacal rising of Sirius" means that you can see Sirius at sunrise. To be able to see a star, it has to be far enough away from the Sun, in an angular sense, that the Sun is below the horizon and the star is in black sky. It turns out that in July, the Sun has moved far enough away from Sirius that you now can see Sirius at sunrise. The photo shows the helical rising of Sirius from the Kepler Observatory in Linz, Austria. The photo shows the Sun below the horizon, but brightening the sky, with Sirius, Orion, Taurus and the Pleiaded visible.

	Note
	If run out of time, just drop this.

Because of the tilt of the Earth's axis, as you go through the year, the height of the Sun above the horizon at noon changes. In the middle of summer, the Sun is highest. In the middle of winter the Sun is lowest.

As you watch the Sun go across the sky in summer, it's high, and in winter it's low.

(sweep hand across the sky to show the two arcs, summer and winter, of the Sun.)

As you go from winter to summer and then to winter again, the Sun will get higher in the sky at noon, then stop and then get lower again. The day when the Sun is highest in the sky, the Sun is said to stand still. The word for this is "Solstice" (sun stationary). In winter, the Sun gets lower every day until the middle of winter, when the Sun stops descending and starts climbing again. Again when the Sun is at it's lowest at noon, it is said to stand still and we use the word "Solstice" again.

So we have a summer solstice and a winter solstice.

prints_for_whiteboard/4.5/solstice_equinox.odt

summer solstice

winter solstice

In the spring and the fall, the Earth's axis is pointing crossways to the Sun (show in model). The result of this is that for one day, the whole of the Earth from pole-to-pole is illuminated, and every spot on Earth gets 12hrs of day and 12 hours of night. Equinox means equal night. (nox comes from the same word where we get noctural, meaning being awake during the night and sleeping during the day.)

Here are the approximate dates of the equinoxes: 22 Mar, 22 Sep. These occur in the spring and the fall.

Half way between the two poles is a line around the middle of the Earth that we call the equator. At the equinox, the Earth's axis is titled across from the Sun, and the Sun is over the equator.

There's another pair of lines called the Tropics at 23.5° from the equator.

At midsummer in the northern hemisphere (22 Jun), The Sun is over the northern tropic, called the Tropic of Cancer. Because the Earth's axis is tilted by 23.5° the latitude of the tropic is 23.5°N.

At midsummer in the southern hemisphere (22 Dec), the Sun is over the southern tropic, call the Tropic of Capricorn. Its latitude is 23.5°S.

As the year progresses, the latitude at which the Sun is overhead moves between the two Tropics.

It turns out that the Sun is only ever overhead between the two Tropics i.e. between 23.5°N and 23.5°S. The area between the two Tropics is called "the tropics". It's always warm there and people like to go there for holidays in winter. Hawaii, Cancun and the Carribean are in the tropics.

The Sun is always overhead somewhere in the tropics.

Outside the tropics the Sun is never directly overhead at any time of year.

point to the lines that are the Arctic and Antarctic circles.

There are two circles on the Earth, the Artic, and Antarctic circles, 23.5° from the poles. The Arctic Circle is in the Arctic, i.e. in the northern hemisphere. The Antarctic Circle is in the Antarctic, i.e. in the southern hemisphere. At the solstices, if you're inside one of these circles, there is 24 hours of light at the summer solstice and 24 hours of dark at the winter solstice.

pick a point inside the Arctic Circle, and rotate the Earth to show that it's illumintated throughout the day.

Let's see how we get 24hrs of daylight. In northern hemisphere summer, because of the tilt of the Earth's axis, all of the area inside the Arctic Circle, is illuminated. This includes the area that's on the side of the Earth away from the Sun, that everywhere else on Earth is in darkness and is having night. This means that in the middle of their summer, people inside the Arctic and Antarctic Circles have 24 hrs of sunlight. So even when the clock says that it's midnight, the Sun is above the horizon and we see daylight. These two parts of the Earth, inside the Arctic and Antarctic circles, are poetically called the land of the midnight sun.

Similarly, at the winter solstice, inside each of these circles, you have 24hrs of darkness.

leave out

If you're near the poles, the pole of the sky, is almost overhead.

(point to a spot on the globe in the Arctic Circle and show that the Earth's pole points to a spot almost overhead.)

The pole in the sky is the point to which the Earth's axis points and about which the Sun and the stars and the sky rotate through the day. If you're near the poles, because the pole of the sky is nearly overhead, the stars and the Sun move through the sky in circles with a low inclination.

(show with hand, the path at low latitude and at high latitude. In summer, the Sun at its lowest point will be above the horizon.)

Here's a timelapse photo, from inside the Arctic Circle, taken in midsummer, every 15mins. It's from north of Fairbanks, AK. Because the spot is inside the Artic Circle, and it's near the summer solstice, they're having 24hrs of daylight. The camera is fixed and the Sun moves through the sky.

class_visuals/4.7/time_lapse.summer_solstice.north_of_Fairbanks.AK.UAMI201_BLOG.jpg

The timelapse only covers a small part of the day. Can anyone guess what time of day it was when the Sun was lowest in the sky?

It's the time when the clock says midnight.

Here's a video from inside the Antarctic Circle, taken in midsummer for the southern hemisphere, when the Antarctic is having 24hrs of daylight. It's at Scott Base, Antarctica, lat 77.51S. In this video, the camera follows the Sun. Since we're having 24hrs of sunlight, the Sun never sets.

(start at 0:42), from Scott Base, Antarctica, lat 77.51S

solstice.video/4.7/Flat_Earth_Debunked-A_time_lapse_of_the_24_hour_sun_in_Antarctica-Zc-WlTaG7WY.mkv

(show the path of the Sun in the class room, in winter, if we're near the pole.) Here's a time lapse from Fairbanks taken 2 Dec (about a week ago). It's winter, so we expect the Sun to be low in the sky and the amount of daylight to be short.

class_visuals/4.7/time_lapse.winter_2_Dec.Fairbanks_AK.lat_65N.fpd6c_2.jpg

leave out

You know that the Earth's axis is tilted at 23.5°. In the middle of summer, when the Sun is highest in the sky, how many degrees of the Earth's curvature, on the side of the Earth facing away from the Sun, is illuminated? In other words, how big is the Arctic circle?

23.5° from the north pole to the Arctic circle.

In the southern hemisphere, which is having winter at the same time as the Northern hemisphere is having summer, the area inside the Antarctic Circle is receiving no sunlight at all.

How big (in degrees) is the Antarctic Circle? (23.5°).

(put finger on spot and rotate globe from sunrise to sunset for that spot.) If you look at other parts of the northern hemisphere in summer, you'll see that more than half of the globe is illuminated. This means that in summer, daylight is longer than 12hrs. As you go towards the equator, the amount of sunlight in summer drops to 12hrs. sunrise equation The summer hemisphere is getting 12 hrs of daylight at the equator with the amount of daylight increasing as you go towards the pole, when you get 24 hrs of daylight in the Arctic circle.

The reverse is happening in the southern hemisphere, which is having winter. The amount of daylight drops from 12hr at the equator to 0hr at the Antarctic circle.

You'll notice that although the axis of the Earth is always tilted at 23.5° to the plane of the ecliptic, that the angle it makes in the direction of the Sun changes through the year. In winter the northern hemisphere pole is pointed away from the Sun, in summer it's towards the Sun and in spring and fall it's straight up.

No I'm not going to do this for 4th graders, fun though it would be.

The original definition of noon was the time that the Sun is highest in the sky. When people were limited to walking or riding a horse for their transport, your world wasn't very big and everyone set noon for their location. The time you used is called local time.

No-one really cared about the time very much anyhow. No-one had clocks; they were expensive and needed a lot of attention. The best you could do was have a sundial in the middle of town. Usually people just looked up in the sky and said it was early or late morning, or early or late afternoon and that was it.

It turns out that places east and west of you had different times for noon and so clocks in different towns had different times. Let's see how this works. The Earth rotates in what direction (eastwards)? Raleigh is east of Durham. When does Raleigh have noon, compared to Durham? Do they have noon before, the same time or after Durham (before).

Raleigh is 0.3° east of Durham. The Earth rotates 360°/day or 1° every 4mins. This means that Raleigh has noon 1.2mins or 1min 12secs before Durham. This means that the local time for Raleigh and Durham are different. If you're in Durham at noon, and you phone a friend in Raleigh and ask the time, they'll say it's 1 minute and 12 seconds past noon.

The time difference between the noons for Durham and Raleigh was not a big deal when it took you a day to ride between the two towns. But if you could travel faster over long distances, the time differences become a problem. What was the first form of transportation to arrive that was faster than a horse? (not cars. they didn't arrive till the early 1900s). The train. American's don't think of trains very much. This is because of the way the train system was set up here. The US government, priding itself in being a weak goverment, gave the rights to setup the trains to cooporations, giving the railroads much public land to build the trainlines. The corporations became fabulously rich and having no obligation to serve the public and their customers, fleeced them and became hated by all americans. This lead to the term "robber barons".

The rest of the world, having strong governments, had the government setup the railroads, to serve the citizens. The trains became communication, travel and shipping channels for the citizens, and provided good jobs. In the rest of the world, the trains are loved.

Not only could you move people and goods around quickly, you could send information quickly too. The electric telegraph was developed in the early 1800s and by 1830 was capable of delivering accurate time signals all over Britain making it possible to synchronize clocks all over the country. How fast do signals travel over the telegraph wires? (speed of light in wires, which is about 2/3 the speed of light in a vacuum).

Although many systems of electric telegraph were developed, everyone eventually adopted one system. Does anyone know what it was (The Morse system)? It was the cheapest and simplest - it used only one wire, with the Earth (ground) as a return; the information was encoded in dots and dashes called the Morse code. Ham radio operators (of which I am one) still use morse code. (play cq.mp3)(also mp4 of a ham calling CQ).

What sort of telegraph was used before the electric telegraph? (optical).

On ships and land, semaphore was used. A person would hold up a flag in each hand, each pair of flag positions representing a letter. (show positions of hands). The person receiving the message would watch with a telescope. On land, the stations would have to be on hills or towers.

Ships also use flags. Show the Blue Peter, which means "going to sea". It's the logo for the organisation Outward Bound, which runs adventure camps for young adults. I did Outward Bound when I was in college. At the time, I thought it was the best month I'd spent in my life.

At the battle of Trafalgar in 1805, in which England defeated the joint navies of Spain and France, giving England the supremacy of the seas, Admiral Nelson raised this signal (show flags) England Expects. The significance of the victory and Nelson's death during the battle led to the phrase becoming embedded in the English psyche, and it has been regularly quoted, paraphrased and referenced up to the modern day

Note that there is no flag for "duty" and the word is spelled out in letters.

In Nov 1840, the British Railways instituted Railway time. The railways set their clocks to Greenwich Mean Time (GMT). Greenwich is a place just outside of London which has a famous observatory. What do you know about Greenwich and the observatory?

You can indicate your coordinates on the surface of the Earth with your latitude and your longitude. If you look down on the Earth from the north pole, you can see that the circumference of the Earth is a circle. You can divide the circle into 360°. The problem is where to set the 0° point. In a conference a while ago, everyone agreed that the Greenwich Observatory would be the reference point. Your longitude then is so many degrees east or west of Greenwich. Durham is at a longitude of -79° or 79° west if Greenwhich.

Greenwich is the place where the Longitude is 0°. All maps in the world already used Greenwich as their reference point. Greenwich was an obvious place to use for your time reference.

I have been to the Greenwich Observatory and stood next to the brass plug in a block of stone with a line ruled on it, that marks the 0° longitude line. Doing this is really cool, if you're a science geek.

The key purpose behind introducing railway time was twofold: to overcome the confusion caused by having non-uniform local times in each town and station stops along the expanding railway network and to reduce the incidence of accidents and near misses, which were becoming more frequent as the number of train journeys increased.

This system of time, when everyone in a time zone, is on the same time, is called "Mean Time".

This scheme worked well for England, which doesn't extend very far east or west. However if you're a long way from Greenwich and you're using GMT as your time reference, you might find that it's dark and the middle of the night, when your clock says it's noon. Where would you have to be for it to be night time at noon in Greenwich? (put finger on Greenwich and show kids the opposite side of the globe) SE Asia, Australia, NZ, Alaska.

What people decided to do was to divide the world up into 24 time zones. The Earth rotates on its axis every 24hrs.

show time zone map.

If you look down on the Earth from the north pole and you divide the Earth into 24 sectors, it will take 1hr for the Sun to pass overhead through that sector. With the time zone system, everyone in a time zone agrees to set their clock to the same time. When you move into the next sector, you move your clock ahead or back an hour. There are 4 time zones in the the continental US; E,C,M and W. each an hour apart. LA in the western time zone is 3 hours behind Durham in the E time zone. Because administrative (state and country) borders are not always at 15° intervals, the actual lines of timezone have been moved for convenience. (see timezone_map.jpg)

In 1883, the US and Canadian railroads adopted timezones. (Railroads create the first time zones.

The timezone system still only works if you don't move very far. If you interact with people in different timezones, then everyone adopts GMT as their time. Pilots all set their watches by GMT. Unix computers, which are designed to connect to computers anywhere in the world, all are set to GMT. (Windows computers, which are designed to only work in an office, use the local timezone.) Greenland covers 4 timezones, but sets it clocks all to the same timezone. Russia which covers 11 timezones, has all its clocks set to Moscow time and quite sensibly does not use the timezone system. I spend a lot of time on a computer interacting with people in different timezones. I set all my computers and cameras, anything with a clock, to GMT. Anything local, like my alarm clock and my microwave oven, I set to local time.

The Moon orbits the Earth, in the plane of the ecliptic, (walk the Moon around the Earth) i.e. in the same plane that the planets orbit the Sun. Can anyone tell me why this is so?

The Moon formed in the same disk and in the same plane as all the planets.

Not only that, the Moon orbits the Earth in the same direction that the Earth orbits the Sun AND the Moon rotates eastwards on its axis, i.e. in the same direction as the Earth rotates on its axis. (spin the Earth, and walk the Moon around the Earth, while rotating the Moon.) Can anyone guess why that would be so?

The Solar System formed from a spinning disk of matter. All the objects in the Solar System, when they first formed, all would have been spinning in the same direction and would stay that way, unless there was a massive collision.

How long does it take for the Moon to orbit the Earth? (a month, actually about 28 days - the original definition of a month).

	Note
	Orbital Period 27.3 days, Synodic Period 29.5 days. I'm not going to go there.

leave out

We now use a solar calendar and have to make the month a bit longer, to fill out a whole solar year of 365.25 days.

If the Moon takes one month to orbit the Earth, how long does it take for the Moon to go a quarter the way around the Earth? (a week, the original definition of a week). In a few minutes, I'll show you why a week was chosen as a useful time interval.

When the Moon orbits the Earth, it always keeps the same side facing the Earth. So from the Earth, we only ever see one side, or one half, of the Moon. We don't ever see the back side of the Moon from the Earth. I'm going to mark the crater Tycho (point to image on white board) (2019 now on screen).

class_visuals/7.1/Tycho_LunarDay15.jpg

Tycho is a relatively young crater, formed by the impact of an asteroid 108Mya. It's in the Moon's southern hemisphere, facing the Earth. Because it's young, the ejecta, the rocks thrown out from the crater, are still light coloured, rather than the dull weathered grey of the rest of the Moon. The ejecta form rays emanating from the crater. Tycho is a bright, rayed crater. Although you can't see it with the naked eye, in a pair of binoculars, it is one of the obvious features of the Moon.

leave out. It's interesting to me, but probably more than the kids can fit in.

I said Tycho, at 108Mya, is a relatively young crater. But young relative to what?

prints_for_whiteboard/7.1/Tycho_age.odt

Important dates in history:

• how old is the Earth and moon? (About 4.6Gya. The Earth/Moon system formed right at the beginning of the Solar System.)

  So Tycho is young relative to the age of the Moon.

• How old are the craters on the Moon? (4.1-3.8Gya)

  The Moon formed as a molten ball of rock and would have cooled to a relatively smooth sphere. So the Moon formed without any craters.

  Craters were created shortly after the formation of the planets and the Earth-Moon system, but before the planets all settled into their current orbits.

  What happened was that before the planets all settled into their current orbits, yet another Mars sized planet is thought to have come too close to Jupiter. When it did, it was broken up into fragments by the tidal effect of Jupiter's strong gravity field. The fragments of the broken up planet were flung out into the Solar System. Then, over a period of 300Myr, from 4.1-3.8Gya, the chunks of the broken up planet crashed into all the planets, not just the Moon, till there were no more pieces left in orbit around the Sun. This period of crater formation, bombarded all the planets and is called the late heavy bombardment. Today we see craters on the Moon and on Mercury. The craters that were formed on the other rocky planets, Venus, the Earth and Mars, are assumed to have since been obliterated by erosion and techtonic subduction of the crust.

The formation of the Moon involved two Mars sized planets. The first one crashed into the Earth, tilting the Earth's axis and giving us the Moon. The second one broke up after coming too close to Jupiter. The resulting fragments rained down on the Moon (and the other planets), giving us the craters on the Moon and throughout the rest of the Solar System.

So when I say that Tycho is a young crater, it's young relative to the age of the other craters on the Moon. It was formed by a recent impact of an asteroid and not by the much earlier Late Heavy Bombardment.

The impact that created Tycho would have made a gigantic bright flash, easily observable on Earth. Back 108Mya, were there there any life forms on Earth that could have seen the impact?

Let's recall some more important dates in history. Was there life on Earth 108Mya?

Yes. Life started on Earth in the arduous conditions of the Late Heavy Bombardment 4.1-3.8Gya. It seems that life started on Earth shortly after its formation. It must be very easy to get life started on Earth, although no-one has figured out how.

So there was life on Earth 108Mya. What sort of life forms were on Earth 108Mya? Would the dinosaurs have seen the impact? Would humans have seen it?

108Mya was the middle of the Cretaceous period, the last of the three periods of the Mesozoic (or middle animal) Era. The Mesozoic was the era of the dinosaurs and extended from 252-66Mya. The dinosaurs went extinct 66Mya.

(As I've found out, the kids don't know any of this. Presumably they've been too busy being creative, learning New Math and compulsorily learning languages that none of the adults in their world use in their daily life.)

So yes, dinosaurs would have seen the impact that created Tycho. Modern humans didn't come along till much later, about 1Mya.

During the Cretaceous

• flowering plants were taking over the plant world
• Gondwana, which consisted of South America, Africa, Antarctica, Australia, and the Indian Subcontinent, was breaking up

  class_visuals/7.1/Gondwana_420_Ma.png from wikipedia

OK, we've talked about Tycho. Let's watch the Moon go around the Earth. Remember that the same face of The Moon is turned towards the Earth. and that Tycho faces the Earth. In the orrery, I'm going to move the Moon so that as the Moon orbits the Earth, that Tycho always faces the Earth.

The Moon has its own day and night too. Let's see what happens to Tycho, as the Moon orbits the Earth, keeping the same side facing the Earth,

Now ask the kids how long the lunar day is (one month). All (well enough) of them get it.

Note

The first two years I taught this class, the kids didn't get that the lunar day was a month. I assumed that it was some Piaget type of conceptual problem. Then in 2017, it occured to me I wasn't teaching it right. The thing that worked was asking the kids to explicitely track Tycho and call out the time of day for Tycho on the Moon at each quarter. It turns out that when kids don't get something, it can usually be fixed by me figuring out how my teaching is wrong.

Remember the cardinal directions, NSEW? Let's say we're on Tycho, what directions are NSEW (get some kid to come up)?

As the Moon orbited the Earth, what direction was the Moon rotating to keep the same face to the Earth? (E) (Orbit the Moon again, if the kids don't get it.)

eastwards

Why does the same side of the Moon face the Earth? From the Moon you can see the Earth rotating on its axis in front of you, showing different continents as the Earth's day progresses. Why doesn't the Moon spin in front of the Earth in the same way?

In the very early days of the Solar System, the Moon did rotate on its axis. Then through a mechanism I don't understand and can't explain to you, the spinning slowed down. This happened very early in the Earth/Moon system. Also it turns out that the Moon isn't homogeneous; it's lumpy inside. This means that mass is not distributed evenly throughout the Moon. Due to a process called tidal locking, the lumpiness keeps one side facing the Earth.

So the short answer is that I don't know. However tidal locking occurs elsewhere; e.g. the planet Mercury keeps the same side facing the Sun.

For this next phase (ho! ho!) of the talk, I need to wear my fashionable and highly covetted Lunar Phases hat.

	Note
	Tell kids to come out and position themselves so that they can see what I'm showing them.

lights off

The words in the next couple of paragraphs are tortuously difficult to understand. The point is to show what's happening with the models. Words aren't great for communicating some things. Some things are better described in diagrams.

If we're out in space (in the classroom, this is on the kid's side of the Sun) with the Sun between us and the Earth-Moon system, we will always see a fully illuminated Moon, no matter where it is in its orbit around the Earth. We will also see a fully illuminated Earth.

orbit the Moon around the Earth to show that the Moon stays fully illuminated.

Instead if we're out in space, with the Earth-moon system between us and the Sun (in the classroom, this is where I stand, between the blackboard and the table with the orrery), we will always see the dark side of the Moon, no matter where it is in its orbit around the Earth. We will also see a dark Earth.

orbit the Moon around the Earth to show that view of the Moon stays dark.

Instead if we're out in space, with the Earth-moon system and the Sun to the side of us, we will see half of the Moon illuminated, no matter where it is in its orbit around the Earth. We will also see a half illuminted Earth.

orbit the Moon around the Earth.

If we change our viewpoint again and now we're sitting on the Earth and looking at the Moon as it orbits the Earth, we see that some of the Moon is illuminated and some of it is dark, depending on where the Moon is in its orbit around the Earth.

Our view of the illuminated part of the Moon changes, because the Sun-Earth-Moon angle changes as the Moon orbits the Earth.

As a result of viewing the Moon from the Earth, the Moon goes through what are called phases.

point to poster on wall showing phases of the Moon. (hopefully we have a diagram of the phases).

Let's see where the phases come from.

lights off. demonstrate with orrery.

7.3.2. First Quarter

Have the kids stand on the Sunrise side of the Earth and look across the Earth towards the 1st quarter moon on the opposite (sunset) side of the Earth.

Now let's take the Moon 1/4 the way around its orbit of the Earth. How long does it take for the Moon to travel 1/4 the way around the Earth? (1 week) (first quarter). So the Moon has gone 90° in its orbit around the Earth.

(to the class) Point to the area of the Earth where you can see the first quarter moon.

the half of the Earth facing the Moon.

If you look at the Moon from Earth, what phase of Moon do you see? (first quarter Moon). The Sunward half of the Moon is illuminated, while the rear facing half of the Moon is dark.

At first and last quarter, the terminator (the line between light and dark) is straight. This straight terminator makes it easy to know when a week has elapsed.

	Note
	You don't get a very straight terminator in this setup. If the orrery was scaled properly, the Moon would be 30' away, and you'd see a straighter line.

Let's find where in the sky the first quarter Moon will be, if you're at noon, then sunset and then midnight.

(show with orrery). Point to a place with noon, sunset and midnight and ask the students to say whether it's noon, sunset, midnight or sunrise at that spot.

• noon: the Moon is in what direction - overhead/horizon (horizon)? Is the Moon in the east/west (east)? Is the Moon rising/setting (rising). (show Earth rotating eastwards, with the Moon getting higher in the sky.) Everything rises in the east.

  At noon, the sky will be what colour - blue/black (blue, you're in daytime)? At noon, you'll see the first quarter moon rising silhouetted against blue sky.

• sunset: moon is where - horizon/overhead (overhead)? If you look at the Moon shortly before sunset (rotate Earth, so just before sunset), the sky will be what colour - black/blue (blue, because it's still day time) and you'll see the first quarter moon overhead against a blue sky. If you look at the Moon shortly after sunset, about an hour later (rotate Earth), the sky will be what colour black/blue (black, because now it's night time, and the stars will be out). Shortly after sunset, you'll see the first quarter moon overhead against a black sky.

  If you're around at sunset with the moon in the sky, you'll see that over a period of about an hour, the Moon will change from being against blue sky, where the edges are fuzzy and where the Moon is not that much brighter than the sky, to standing out brightly against black sky and with sharply defined edges. It's quite dramatic. People like me, like watching this sort of thing. It's easy to see this on a long car trip at sunset or sunrise. Just look around to see if you can find the Moon.

• midnight: the Moon is in what direction - overhead/horizon (horizon)? Which horizon - east/west (west)? The moon is rising/setting (setting)? (show the Earth rotating eastwards. the Moon goes below the horizon.) Everything in the west is setting. Because it's midnight, the sky will be what colour - blue/black? (black), At midnight, you'll see the first quarter moon setting against a black sky.

If it's sunrise, where are you? (have them point to model.) If it's first quarter moon and you're at sunrise, can you see the Moon? (no, the Moon is below the horizon).

So as you go from noon to noon, you see

• noon: moon rising in the east agains blue sky
• sunset: moon overhead going from blue sky to black sky as the sun sets.
• midnight: moon setting in the west against black sky
• sunrise: moon below the horizon, not visible

So like the Sun, the Moon rises in the east, goes overhead and then sets in the west.

Here's a video of the phases of the Moon through the lunar month video of phases of the Moon.

class_visuals/7.4/Moon_Phases_in_HD_over_a_Flat_Earth--ncuV0mzpb4.mkv

At the terminator (the Sunrise or sunset line), it's sunrise or sunset. The mountains cast long shadows making it easier to see them. If you're observing the Moon through binoculars or a telescope, you usually look at features along the terminator, because the shadows give you a better idea of the features you're looking at.

leave out

If we knew the orientation of the telescope to the horizon, we could tell whether the Moon was rising, going parallel to the horizon (overhead) or setting. If we assume the horizontal is the horizon, the Moon is rising except at 1:00 when the Moon is overhead, and the last two, then the Moon is descending. (the old moon sets against a blue sky, so the scope must be tilted to the horizon.)

	Note
	Leave this out for 2018. I need to cut the presentation down.

The first calendars were lunar. It was easy to see the changing phases of the Moon. It was obvious that the position of the Moon affected the tides, and this affected your fishing, or collecting clams and oysters by the shore. As well at night, the Moon provides some light. This light allows some movement at night, an otherwise dangerous time to move, as you could be eaten by a nocturnal predator. So if you're out hunting and gathering, and you know the Moon is going to be in the sky after dark, you might be able to stay out a little later, if you know you can get back home in moonlight. There are prehistoric cave paintings with sets of 28 tick marks, indicating that someone's job was to track the Moon.

With a lunar calendar, you counted time in moons. If something happened a long time ago, you might say "it was many moons ago".

The advantage of the lunar calendar was that it was simple. The month only lasted 28 days, and it was easy to see the passing of the month with the phases of the Moon.

However the seasons were determined by the Sun. Hunting depended on the seasonal migrations of deer and buffalo. You had to store up food for winter when animals were scarce and you risked dying of cold if you went out hunting. Our ancestors survived for 100,000s of years in the tough conditions of the ice age Europe, living much like the Innuit of northern Canada do now. Your survival was more determined by the Sun than the Moon.

It turns out that once you stop hunting and gathering and society becomes agricultural, it's more useful to have a solar based calendar. The solar based calendar keeps in synch with the seasons. All agricultural activities depend on the seasons; planting crops in the spring, the arrival of the new babies from the farm animals in spring, and harvest in the fall.

With a solar calendar, you counted time in years. If something happened a long time ago, you might say "it was many years ago".

However if you want to have a solar calendar with 12 months filling out the year, you have to lengthen the month from 28 to 30 or 31 days. These longer months no longer stay in sync with the 28 day period of the lunar month. So you can't have a calendar which stays in sync with the Moon and in sync with the Sun at the same time. You have to pick one.

A solar calendar is more complicated. It requires some understanding of astronomy and someone to keep track of 365 days in a year.

Modern European man is on a solar calendar, because we've stopped hunting and gathering, and we became an agriculture based society.

Cultures which predate modern astronomy, such as the Chinese and the Judao-Christian cultures, have calendars which are still partially lunar based.

For agricultural societies, the beginning of the year was in spring, when after being trapped by the cold of winter, suddenly in spring you became very busy. You planted your crops and the baby farm animals were all born. So just about all societies and cultures have their new year celebrations in spring; well all cultures except one, and that's ours. We have new year in the middle of winter; that's because the Romans shifted the new year for political and religious reasons. Do you remember from the icosahedron class that October isn't the 8th month? That's because the politicians moved the new year from March to Jan. The politicians don't care about the concerns of the farmers.

	Note
	I added this in 2017, but didn't deliver it (I haven't finished writing it either).

Here's a movie of the Moon crossing the face of the Earth, taken by a satellite which is monitoring global warming, the Deep Space Climate Observatory, from a distance of 1,000,000 miles above Earth, or about 4 times the distance of the Moon from the Earth.

https://www.nasa.gov/feature/goddard/from-a-million-miles-away-nasa-camera-shows-Moon-crossing-face-of-earth, dscovrepicMoontransitfull.gif, EPIC_View_of_Moon_Transiting_the_Earth-DMdhQsHbWTs.mkv, EPICEarthMoonVideo.mov

Since the same side of the Moon always faces the Earth, this movie shows side of the Moon that faces away from the Earth, called the far side of the Moon. Note the uniform appearance of the far side of the Earth, compared to the familiar near side of the Moon that we see from Earth. (show photo of full Moon as seen from Earth). (Have Lyn stop the animation in the middle and point to the far side of the Moon as seen in the movie and to the image of the full Moon on the white board.) Compare the uniform appearance of the far side of the Moon with the side that faces the Earth that is covered in dark smooth basalt lava flows called Mare (Maria pl. which means seas, this is where we get the word "marine"). People originally thought that the maria were oceans like we have on Earth. But when the telescope was invented and turned on the Moon, it was obvious that the maria were solid rock.

Both sides of the Moon are covered in craters. There just happen to be a lot more craters on the far side of the Moon than there are on the near side of the Moon. These craters are from asteroids slamming into the Moon. The cratering mostly happened in the early days of the Solar System, in a period called the Late Heavy Bombardment. https://en.wikipedia.org/wiki/Late_Heavy_Bombardment (4.1-3.8Gya), when yet another Mars size object broke up, presumably after having come too close to Jupiter, the biggest planet in the Solar System. The breakup scattered debris throughout the Solar System, which came raining down on all the planets and the Earth and the Moon.

The reason the far side of the Moon is so uniform in the video, is that it's densely covered in craters, more densely covered in craters than the near side of the Moon. Any maria that had formed on the far side of the Moon, would have been obliterated by the Late Heavy Bombardment.

So why is the far side of the Moon densely packed with craters, while the near side of the Moon is relatively sparsely covered in craters? It turns out that the near side of the Moon is protected from asteroid bombardment by the Earth. Most of the asteroids heading to the near side of the Moon will hit the Earth first, after being attracted by the Earth's greater gravity. If the asteroid is heading from outer space towards the far side of the Moon, there will be no Earth to protect it.

When the Moon formed it would likely be rotating much faster than it is now. But some time after its formation, it became tidally locked to the Earth, with the same side of the Moon always facing the Earth. The same thing is true of the other Moons in the Solar System; they all rotate with the same face to the primary body.

Let's see if we can determine the order in which these 3 events happened: synchronous rotation, late heavy bombardment and the formation of the Maria.

We know this about the Moon:

From this we can order the bombardment event and the slowing down of the Moon's rotation to become synchronous with the Earth.

• bombardment then synchronous rotation
• synchronous rotation then bombardment

A: synchronous rotation, then bombardment. (If the reverse, then the Moon would be evenly cratered.)

So synchronous rotation was the first to happen.

Next we have to determine when the Maria flows occured.

We know this about the Moon

From this we can order the bombardment and the maria flows.

• bombardment then maria flows
• maria flows then bombardment

bombardment then maria flows. If it was the reverse, then the maria would have the same density of craters as the rest of the near side of the Moon.

From this we have two pairs: synchronous rotation then bombardment; bombardment then maria flows.

If you're a scientist, you try to check your answer. If you've just made a new medicine or a rocket ship, you want to be confident that it's going to work. The ordering of the two pairs implies that synchronous rotation occured before the maria flows. How are we going to test this?

We know this about the Moon;

From this we can order the slowing of the Moon's rotation to become synchronous with the Earth and the maria flows

• synchronous rotation then maria flows
• maria flows then synchronous rotation

Let's test synchronous rotation then Maria flows. If it was the reverse, then the Maria would be on both sides of the Moon. However the next thing that happened was the bombardment. This would obliterate the Maria on the far side of the Moon. So we can't be sure of this one.

Let's test Maria flows first then synchronous rotation next. If it was the reverse, then we would possibly see Maria flows on one side or other of the Moon, but we don't know enough to know which side it would be on.

The evidence is consistent with synchronous rotation first and maria flows 2nd, but we can't exclude the reverse. Science is like this. You don't always get the complete answer. You have to find another way to get the missing information.

The most likely ordering of events then is synchronous rotation, then bombardment then maria flows.

It turns out, because objects in the Solar System, are orbiting in the same plane, that now and then, a planet, moon, or asteroid, will line up in a way that blocks the view of another object.

class_visuals/8.1/2001_planets.jpg

Depending on what happens, it's called an eclipse, occultation or transit. All of these events are called Syzygys, a Greek word, that means things lining up. No-one ever uses the word Syzygy however. Since you already know whether the event of interest is an eclipse, an occultation or a transit, you talk about the eclipse, occultation or transit. You don't talk about the Syzygy.

However there's a lot of empty space in the Solar System, so Syzygys don't happen all that often. Because they're rare, when one does happen, people get excited about it. In the pre-scientific era, people thought that gods used nature to send them messages. So people would watch the behaviour of the plants, animals and weather to see if they could figure out what the gods were trying to tell them.

As an example, until quite recently, some cultures (the US) regard a black cat crossing your path as portenting bad luck. Other cultures (Japan) regard the same event as portending good luck.

The astronomers, priests and shamans, wanting to keep their power over the peasants, declared that they alone understood these messages from the gods, and that the peasants didn't. Because of this, eclipses were frightening and were used to subjugate the masses.

This cut two ways, of course. The King, being very important, wanted to know what the eclipse portended for him personally, just as the peasants did. If the royal astronomer said that the crops would do well or that the King would be victorious in battle and would smite his enemies, and that didn't happen, then the astronomer would likely have his head chopped off.

Eventually the ancient Greeks and the Persians figured out that they were looking at a completely mechanical system, and could predict eclipses. Now-a-days, eclipses are of visual interest to the public, and of scientific interest to astronomers.

I will talk about two types of Syzygys today

On the Orrery, show Moon being eclipsed.

These next two are the same thing.

If you're on the day side of the Earth, do you see the Lunar Eclipse?

No, the Moon is below the horizon.

A Lunar Eclipse occurs when the Moon is at what phase? (full).

The Earth is 4 times the diameter of the Moon, so the Earth's shadow is large compared to the Moon. It takes a couple of hours for the Moon to move through the Earth's shadow.

Where do you have to be (on Earth) to see a lunar eclipse?

anywhere it's night time, i.e. half the world sees a lunar eclipse.

	Note
	The Moon is not occulted; there is nothing blocking our view of the Moon.

The Moon keeps orbiting the Earth and a little bit later, the Moon arrives in front of the Sun.

How long does this take?

2 weeks.

The phase of the Moon now is what? (new). The Moon casts a shadow on the Earth. The thing that's eclipsed is what?

the Earth.

Can anyone guess what we call this Eclipse?

A terrestrial eclipse.

The Moon is small compared to the Earth, (it's a quarter the size of the Earth), so the shadow of the Moon on the Earth is small and only a small part of the Earth sees a terrestrial eclipse. The shadow of the Moon is only a couple of miles wide at the most. While you can see a Lunar eclipse from anywhere on the night side of the Earth, the only way to see a terrestrial eclipse is to travel to the exact location. If you stand on Earth, on either side of the area that's totally eclipsed, you will see a partial eclipse, where only a part of the Sun is blocked by the Moon. A partial terrestrial eclipse can be seen for several 100 miles on either side of the area that's totally eclipsed.

Where on the Earth do you have to be to see a terrestrial eclipse?

On the day side of the Earth.

right under the shadow. you can't see it from just anywhere on the daylight side.

If you're standing on the Earth and looking at the Sun during a terrestrial eclipse, the Sun is occulted (or hidden) by the Moon. The Earth observers see a Lunar Occultation of the Sun. If you were standing on the night side of the Moon, you'd see the shadow of the Moon on the Earth, so you would be seeing an eclipse. Since the Earth is eclipsed, you're seeing a Terrestrial Eclipse. The Terrestrial Eclipse and the Lunar Occultation of the Sun are the same event. It's just that you see different things depending on your location. If you're on one celestial body seeing an eclipse, then an observer on the eclipsed body, looking back, is seeing an occultation.

Let's go back to the Lunar Eclipse. If you're standing on the night side of the Earth, you see a Lunar Eclipse. If instead, you were standing on the day side of the Moon, you would see the Earth blocking the view of the Sun. Are the people on the Moon, facing the Earth seeing an eclipse or an occultation?

hint: if we're seeing an object in shadow, we're seeing an eclipse. If an object disappears from view, i.e. it's hidden, then it is being occulted.

We're seeing an occultation.

The people on the Moon are seeing a Terrestrial Occultation of the Sun.

If you were standing on the night side of the Moon during a Lunar Eclipse, what would you see?

Nothing. The Earth and the Sun would be below the horizon.

We have a small nomenclature problem here. You've probably never heard of a terrestrial eclipse. Most people call a terrestrial eclipse, a Solar Eclipse, even though there is no shadow being cast on the Sun by the Moon and the Earth is not emitting light.

What I've just told you, about the name Solar Eclipse, illustrates another example of unclear thinking that is accepted in the world.

Remember; just because everyone does it, doesn't make it right. Remember Alfred Wegener. What did he say that no-one believed and everyone said he was wrong?

That at some time in the long distance past, all the continents were together in a supercontinent that we now call Pangea.

You get to decide whether you want to be an Alfred Wegener, be one of the few, and be right, or join the masses and have lots of company, and be wrong.

Why astronomers still use an obviously wrong name is a matter of conjecture. Astronomy is the oldest science, and has kept all of its obscure medieval terminology, whose purpose originally was likely to keep astronomical knowledge within a small circle of adepts.

While geology and biology have a rapidly changing nomenclature that keeps pace with modern knowledge (and which also makes it hard to keep up), astronomy had been using the same terms for 1000s of years, even though the universe is undertood in quite different terms now. As a result the name "Solar Eclipse" that we use for a terrestrial eclipse, comes from the pre-scientific era. In both the common Syzygys, I've discussed, something disappeared from view, so the ancients called them both eclipses.

When you walk out of this classroom, you won't find many people who know what a terrestrial eclipse is.

When I was young, I found the names for the eclipses very confusing. I found that if I remembered to think with the education of a caveman, I could talk about eclipses to other people.

I've told you that the Moon orbits the Earth in the plane of the ecliptic (the plane that the Earth and the planets orbit the Sun). With the Sun, Moon and Earth all moving in the same plane, how often would we have eclipses? (twice a month). We'd have a lunar eclipse every full Moon and a terrestrial eclipse every new Moon.

It turns out that eclipses only occur twice a year. Let's see why.

In fact, the Moon doesn't orbit the Earth in exactly the plane of the ecliptic. The Moon orbits the Earth in a plane that's tilted 5° to the ecliptic plane. (Moon's orbital plane is tilted 5° to the ecliptic, the Moon's axis is tilted 1.5° to the ecliptic; wikipedia.)

show exaggerated tilt with hoop.

Can anyone guess why the Moon doesn't orbit the Earth in exactly the ecliptic plane?

It's left over from the collision that formed the Earth-Moon system. The collision wasn't dead-centre. Not only did the collision tilt the Earth's axis of rotation, but the collisional debris was sprayed out in a plane tilted to the ecliptic. This collisional debris later coallesced to form the Moon, which now orbits the Earth in a plane tilted 5° to the plane of the ecliptic.

Now with the Moon's orbit tilted to the ecliptic plane, let's see how often we get eclipses. I'm going to exaggerate the inclination of the Moon's orbit around the Earth.

As the Earth orbits the Sun, the plane of the Moon's orbit stays fixed, just as the direction of the axis of the Earth's rotation stays fixed. It's the same gyroscopic effect.

Have one kid hold and spin the Earth, and another kid hold the hula hoop. I'll hold the Moon.

Do the Lunar Ballet. First walk around the room, showing the Moon orbiting the Earth though a year, but not showing the eclipses. Spin the Earth (day/night), and have the Moon orbiting out of the plane of the ecliptic. You don't have enough hands to hold the hula hoop (unless we get Lyn Streck to do it).

Let the line normal to the plane of the Moon's orbit tilt backwards a little bit. Have the Earth's axis tilt somewhere else. use a few kids to walk the Earth Moon system around the room. Have the kid with the Earth spin it slowly.

Show that eclipses do not occur for most months. Show that eclipses can only occur twice a year.

We've seen that eclipses only occur as the Moon crosses the ecliptic plane. This is where the word ecliptic comes from. The ancients noticed that eclipses only occured when the Moon was in a certain position in the sky. They joined up all the places where eclipses occured, and found this was the line in the sky that the planets travelled on too.

We've shown that eclipses are only possible when the Moon crosses the ecliptic plane AND if that place is on the line that goes through the Earth and the Sun.

We get approximately two terrestrial eclipses and two lunar eclipses a year. If you can travel anywhere, anytime, you can see two terrestrial eclipses and two lunar eclipses a year.

We last saw a Lunar Eclipse here in NC in Jan (2019). The next Lunar Eclipse here is in May 2021.

not in the 2 week class.

The Earth has a natural satellite, the Moon, that orbits nearly in the plane of the ecliptic. If the Moon orbited exactly in the plane of the ecliptic, we would get lunar eclipses every orbit of the Moon i.e. every month. However the orbit of the Moon is sufficiently tilted that we only get eclipses at two points of the Earth's orbit, i.e. twice a year.

The actual time of year depends on the actual plane of the Moon's orbit. The plane of the Moon's orbit itself rotates, with a cycle of 18.6 years (Saros cycle),; so the time of year for the eclipses returns every 18.6 years.

As well as the natural satellite, the Moon, the Earth has many artificial satellites, in fact thousands of them. One set of them is in orbit around the Earth in the plane of the equator at a distance of 36,000km miles. These are called geostationary satellites, because their orbit takes the same time as the earth's rotation. A geostationary satellite then is above the same point on the Earth's surface all the time. These satellites are good for monitoring the weather, vegetation and for communications, such as providing TV services to people on the ground. They orbit in the plane of the equator, so they're directly above the equator. What is the angle between the plane of the equator and the plane of the ecliptic? (23.5°). Remember the Earth's axis is titled to the ecliptic.

How often are they eclipsed? Twice a year. They are also eclipsed at the equinoxes. (They aren't orbiting in the plane of the ecliptic.) How long are they eclipsed? There's two answers to this eclipse seasons

A geosynchronous satellite gets its power from solar cells. When it's eclipsed, it's in the dark and must get its power from batteries. A geosynchronous satellite must have enough battery capacity to run for 72 min without any solar power. Batteries are very heavy and so it's often simpler to not provide full services during the eclipse.

The size of the Earth's shadow is large enough that a lunar eclipse takes about 4hrs.

(images from https://en.wikipedia.org/wiki/Saros_%28astronomy%29) The shadow of the Earth is 4 times the size of the Moon, so the Moon can take longer or shorter paths across the Earth's shadow;

When the Moon is eclipsed, it's in a shadow that's not completely dark. The Moon sees sunset through the Earth's atmosphere, the whole way around the Earth's circumference. So an eclipsed moon is red rather than black. The reddened eclipsed moon is much darker than the Moon lit by the Sun, but still bright enough that it's easily visible in the sky. So the Moon doesn't go completely dark for most lunar eclipses.

artist's idea of a lunar eclipse, as seen from the Moon. While the people on Earth are seeing a Lunar eclipse, the viewer is seeing an occultation of the sun by the Earth.

class_visuals/8.6/www.vox.com_science-and-health.2019.1.16.18167039.lunar-eclipse-2019-blood-moon-supermoon.why_the_moon_is_red.png

	Note
	with sunrises on the right, the side of the Earth that we see is rotating to the right. This means that the N pole of the Earth is uppermost. Since from where we're standing on the Moon, we see the ground in front of us and the Earth just above the horizon, we must be at the N pole. (show the Moon in eclipse and have the kids walk around and figure out where on the Moon they'd be.)

It's only when the Moon goes across the diameter of the Earth's shadow, that it gets black enough that it's hard to find in the sky. I've seen about half a dozen lunar eclipses and only one of them was so black that I couldn't see the eclipsed moon anymore. In a chordal eclipse, one side of the Moon will be almost black, while the opposite side will be bright red.

We're about to see a video of a lunar eclipse, where the Moon goes across a chord of the Earth's shadow. Most of the Moon will stay red the whole time. Only one side will look near black.

The part of the Moon that's illuminated by the Sun is white. The part of the Moon which is eclipsed is red. The white part is very much brighter than the red part. Because of the difference in light intensity, you won't see the red of the Moon until the moon is almost totally in shadow.

The Moon is moving through the sky in the same way that the Sun moves over a long period of time. To keep track of the Moon over the 4 hours of an eclipse, the camera has to be on a tracking mount, usually a telescope.

2015 Super Moon Lunar Eclipse in HD through a telescope This is an edited version of https://www.youtube.com/watch?v=DHJ1YgeRPTI, by scannerguy1968. It's a time lapse. The actual eclipse took about 4hrs.

Note

We're going to see the eclipse twice. The first time I'm going to stop the video and point out a few things. The second time, I'm going to let it run. 0:11: the full moon is rising through the trees. The trees appear tilted because the camera is tilted. 0:36: a plane flies across the bottom of the Moon. Because the Sun and the Moon are so large, you often have planes photobombing the Sun or the Moon. People have movies of the ISS crossing the face of the Sun. 1:21: he brightens the exposure in an attempt to show the reddened eclipsed part of the Moon. This has the side effect of washing out the features on the illuminated part of the Moon. 1:41: the Moon is fully eclipsed and he turns up the exposure to see the reddened moon. 1:43/1:58: after turning up the exposure, two stars become visible (top left quadrant). The two stars appear to move away from the Moon. In fact the stars are stationary in the sky; the Moon, not the stars, is the one that's moving through the sky. 2:37: he turns down the exposure, allowing the features on the illuminated part of the Moon to be seen. Tycho is visible in the bottom right of the Moon.

In a terrestrial eclipse, the Sun is occulted by the Moon. A terrestrial eclipse is a little different to a lunar eclipse

The video Total Solar Eclipse, 21 Aug 2017 Oregon Star Party. https://www.youtube.com/watch?v=4J9uWUW_BrU by Bill Basham.

eclipse.video/8.6/Total_Solar_Eclipse-4K_Time_Lapse-August_21_2017-Oregon_Star_Party-4J9uWUW_BrU.mkv

In the '80's I travelled from MD to GA to see an annular terrestrial eclipse. We had great weather and we saw it perfectly.

In 2017 I went to MO to see a total terrestrial eclipse: I've been waiting for this eclipse for 20yrs. I chose that spot as historically it had the lowest chance of cloud east of the Rockies at that time of year. It rained. Every other spot east of the Rockies had a perfect view. You learn to be philosophical when observing nature. You don't always see what you hoped to see.

Here is the terrestrial eclipse of 2017 as seen from a satellite. Notice the shadow of the Sun as is goes across the USA. What are all the lights flashing that we see (lightning)?

eclipse.video/8.6/20170821_Solar_Eclipse_GLM.mp4

from https://www.youtube.com/watch?v=cytOTtG1y0Q. Hubble sees three shadows on Jupiter. The moon producing the first shadow seen, is not in the video.

class_visuals/8.8/Hubble_Spots_Rare_Triple_Eclipse_on_Jupiter-2004.03.28-cytOTtG1y0Q.mp4

Archeoastronomy

Because light travels in straight lines, for an eclipse to occur, the objects had to be in a line, and you know exactly where the objects involved are. This allows you to get a better idea of where things are in the Solar System.

As an example, in Mesopotamia, archeologists when excavating an ancient city, that existed a couple of 1000 yrs ago, found records of a solar eclipse. The date and time of the eclipse was not recorded, but they knew the name of the King, and the period in which he ruled, thus giving them some idea of the date of the eclipse. By back calculation, astronomers found an eclipse that passed near the city. The date was compatible with the era of that King, but the calculated location for the eclipse was about 100 miles away. Astronomers realised that the length of the day, which changes very gradually with time, had changed at a different rate than they thought. They fixed up the rate of change of the length of the day, to move the eclipse, so it would be visible from that city. So this record of the ancient solar eclipse allowed a more accurate determination of how the length of the day has changed with time.

accurate orbital parameters for asteroids

Recently NASA sent a space probe to the asteroid Bennu. They knew where Bennu was, to within a 100 miles or so. If you're sending a space probe a billion miles through space, on a trip that takes several years, 100 miles may not seem a big deal, but if when you get there, you find that the asteroid is 100 miles away, rather than sitting next to you, then you're not going to get good photos of it. NASA needed a better idea of where Bennu was. They found that Bennu was going to pass in front of a star, and cast a shadow, which crossed the southern tip of Argentina.

What continent is Argentina on? (South America)

They sent a team of of about 30 people with telescopes to Argentina, to sit on the coast and see which ones saw Bennu pass in front of the star. From that they got a much better fix on the orbit of Bennu, enabling the rendezvous of the space probe with Bennu.

I couldn't find the details for the Bennu occultation, but here's a image showing an occultation, by asteroid Nemausa, of some star (that I don't know). (from https://www.universetoday.com/130328/tales-two-asteroid-occultations-io-nemausa-cast-shadows-north-america/)

class_visuals/8.1/nemausa_occultation.51-nam.jpg

locating stars and the Moon

Here's a video the Lunar Graze of Aldebaran from El Paso https://www.universetoday.com/130328/tales-two-asteroid-occultations-io-nemausa-cast-shadows-north-america/ El_Paso_Grazing_Occultation_of_Aldebaran_by_the_Moon-July_29_2016-s-IYWt9sZlQ.mp4

Aldebaran blinks out 4:52-4:57, 5:01-5:07, 5;39-5:50, 5:50-5:51, 6:20-6.22.

determining the size of the Solar System

Mercury Transit video from https://www.youtube.com/watch?v=dzKHbeRDrWI

./class_visuals/8.1/Transit_of_Mercury_on_November_11_2019-dzKHbeRDrWI.webm

not in lecture.

The Chaldeans, who lived, back in 600-500BC, in what is now known as Persia, were great astronomers. They figured out all there was to know about the motion of the Moon, and they knew how to predict eclipses perfectly. Thanks at least partially to the Dark Ages, it took another 2,000 years before there were any further great advances in astronomy. The Chaldeans also knew that eclipses were due to the Sun, Earth and moon lining up. Thus the circular terminator on the Moon during a lunar eclipse was the shadow of the Earth. From this they infered that the Earth was round, just like the Sun and the Moon. From the curvature of the shadow of the Earth on the Moon's surface, they could tell the relative size of the Earth and the Moon. However they didn't know the size of the Earth.

The size of the Earth was figured out by a guy called Eratosthenes in 240BC. He was the head of the library at Alexandria and he read that at the town of Syene, in southern Egypt, on midsummer's day, that a stick cast no shadow and that on looking down a deep well, you could see a reflection of the Sun.

Syene https://en.wikipedia.org/wiki/Aswan, https://en.wikipedia.org/wiki/Syenite, https://en.wikipedia.org/wiki/Aswan_Dam,

The video on Eratosthenes How Eratosthenes calculated the Earth's circumference from https://www.youtube.com/watch?v=Mw30CgaXiQw

Here's a clip from Carl Sagan's Cosmos TV show, about the well at Syene. Eratosthenes from https://www.youtube.com/watch?v=G8cbIWMv0rI

Once we knew the diameter of the Earth, we knew the diameter of the Moon. The distance to the Moon was determined by Hipparchus about the same time, using Lunar Parallax.

21 mins, being read at full clip.

I'm going to talk about the Solar System, the stars, the galaxy we live in and the universe.

All the bright stars have names. Can anyone name some stars?

prints_for_whiteboard/8.1/stars.odt

• Sirius - the brightest star in the night sky. It's in the constellation of Canis major, or the big dog.
• Canopus - the brightest star in the southern hemisphere. It's at the south pole of the ecliptic plane. Satellites and interplanetary space probes use Canopus for setting a frame of reference.
• Castor and Pollux - the twins in the constellation of Geminii
• Regulus - the brightest star in the constellation of Leo
• Antares - the brightest star in the constellation of Scorpio. It's red and has the same colour as Mars.
• Vega, Rigel, Aldebaran, Spica.

There are star chart apps for cell phones. You hold up your cell phone to the sky and it shows the stars and planets in that part of the sky.

The names of most stars are Arabic. This is because the Arabs, living in the desert, had good cloudless skies to observe the night sky.

It's also because during the middle or dark ages, which lasted about 1000yrs, from about 500AD to 1500AD, in Europe, all academic and scientific work was repressed or banned by the church. Europeans lost all of the knowledge of the ancients. It was the Arabs who kept knowledge alive by translating the Greek works into Arabic. When, 1000 years later, Europe emerged from the Dark Ages, the works of the Greeks were translated back from Arabic into Latin. If it wasn't for the Arabs, who kept knowledge alive for the 1000yrs of the Dark Ages, today we wouldn't even know who the famous Greek philosophers, Plato and Aristotle were.

Anyone remember Plato ... and the Platonic Solids? Can anyone name a Platonic Solid? (tetrahedron, hexahedron/cube, octahedron, dodecahedron and icosahedron) You took home a globe of the Earth that was in the shape of a Platonic Solid. What was it's name? (Icosahedron). What does "icos" mean in Greek? (20). The icosahedron has 20 faces or heads.

What's the name of the brightest star in the sky?

This might seem to be a trick question. The brightest star in the night sky is Sirius. The Sun is a star too, so the brightest star in the sky is actually The Sun. By convention we think of stars as the pinpoints of light we see in the sky at night, so most people don't think of The Sun as a star. The pedantic, or most correct answer, to that question, is the Sun. People with only a casual acquaintance with astronomy, and who aren't careful with their words, will give Sirius as the answer. However a person who thinks and communicates clearly, will always say exactly what they mean. Such a person will always describe Sirius as the brightest star in the night sky.

We've just heard the names of a few stars. The Sun is a star and it has a name too. Does anyone know The Sun's name?

It's name is Sol, which is where we get the name Solar System.

Calling Sol "The Sun" is like calling someone by a generic name, like "the kid" or "the cat". But we only have one star in the center of our Solar System, so calling it the Sun works out OK enough. The Sun doesn't seem to mind and today we'll call it "The Sun" too.

leave out

On Earth we use the term "day" to describe the period from midnight to midnight. The time for other planets to rotate on their axes is different to Earth's. The period for Mar's rotation is a bit longer than Earth's day; it's about 24hrs and 40mins.

There are robots running around on the surface of Mars, taking photos, digging holes and testing soil chemistry. These robots are on wheels, so they're called rovers. Here's an artist's idea of what one of them, Opportunity, looks like, on Mars.

class_visuals/9.1/NASA_Mars_Rover.Opportunity.jpg

Scientists planning activities for these robots, have to go to work according to whether it's day or night on Mars, at the robot's location, not whether it's day or night on Earth, at the scientist's location. To avoid confusion as to whether they're talking about an Earth day, or a Martian day, they use the word "sol" for the Martian day. They talk .about "yestersol", "tosol", "nextersol" "morrowsol" "solmorrow", and for the period when you're not doing anything "soliday". (https://en.wikipedia.org/wiki/Timekeeping_on_Mars) You may have to get ready for one of the robots doing something later tosol, that is, later during the robot's day on Mars, even though the activity will be happening tomorrow, for the scientists on Earth.

Our moon, which we call "The Moon" has a name too

Luna

This is where we get the adjective "lunar". We only have one moon, so in English we just say "the Moon".

on the board from Lyn

The existance of other moons in the Solar System wasn't discovered till the invention of the telescope. In 1610 Galileo turned a telescope to Jupiter to find 4 moons orbiting Jupiter.

You can't see Jupiter's moons with the naked eye, but this is the view you'll get with a small telescope.

class_visuals/9.1/galilean_moons.jpg from SciencePhotoLibrary.

Since then, we've discovered many more moons orbiting the other planets. Most of the planets have multiple moons, and these moons all have their own names too.

Our planet, which we call the Earth, meaning the stuff we grow our crops on, has a name too.

Terra

from where we get the adjective terrestrial, meaning "of the Earth". So we have terrestrial plants, meaning plants that grow on ground, rather than plants that grow in the ocean, We also have the word extra-terrestrial meaning from outside the Earth. Extra-terrestrials are fictional beings from other planets, and feature in Sci-Fi stories. There is also the name Gaia/Gaea, but that more from mythology.

I said that by looking at the motion of the Sun and the stars across the sky, (arc hand across the sky) we can tell that the Earth is rotating about its axis. But we actually can't tell that just by looking. It could be that the Sun and the stars are rotating about a fixed Earth.

Picking between these two possibilities, turns out to be difficult. From our position, sitting on the surface of the Earth, we'd see the same thing in both cases.

Back in the old days, people, being people, thought that they were the centre of the universe and that the Sun, the planets, the stars and everything else that existed, were all created for their benefit, and all revolved around them. Many people still think that the Universe was created for their benefit. This model of the Solar System, where the Earth is at the centre, is called the geocentric (earth centric) model.

class_visuals/3.3/480px-Tychonian_system.svg.png from https://space.fm/astronomy/images/content/480px-Tychonian_system.svg.png

In this system all the planets and the Sun orbit the Earth. But if you look carefully, actually everything except the Earth is orbiting the Sun and the Sun with all the planets is orbiting the Earth. This was the way everyone accepted how it worked for at least 2000 yrs.

Then, to everyone's surprise, in 1543 Nicholas Copernicus (https://en.wikipedia.org/wiki/Nicolaus_Copernicus) realised that the Solar system would be much simpler, if the Earth and the planets all revolved around the Sun. This model of the Solar system, with the Sun at the centre, was called the heliocentric (Sun centric) model.

class_visuals/9.2/201412291419879087230803_fd73b3749f426f88d23d61c07f484d70-201412291419879573213637.png https://courses.lumenlearning.com/sanjac-earthscience/chapter/introduction-to-the-solar-system/

Copernicus' heliocentric model of the Solar System was much simpler than the geocentric model. His discovery was a major event in the history of science. This was the first step out of the Dark Ages.

Copernicus said that the Earth is just another planet going around the Sun and the Sun is the centre of the Solar System.

Copernicus' model of the Solar System may not seem a big deal to you, and may even seem obvious. But back in Copernicus' day, you weren't free to have your own ideas. Back then the church was the arbiter of truth. The church held that the Earth was the centre of the Solar System, and that Sun and planets all orbited the Earth, i.e. the Solar System was geocentric. If you said otherwise, that Sun was the centre of the Solar System, and that the Earth orbited the Sun, as Copernicus said, you were committing heresy (i.e. saying something contrary to the teachings of the church). If Copernicus revealed that the Solar system was heliocentric, he risked imprisonment, torture and death. Because of the treatment he expected from the church, Copernicus didn't dare publish his theory while he was alive and had it published posthumously.

(does anyone know what "posthumously" means?)

Copernicus' fears were justified

Copernicus lived before the scientific era. In the pre-scientific era, the truth was established by people in power. In the case of the Solar System, the church decided how the Solar System worked and didn't need to ask the Solar System.

The scientific method is different. Instead of truth being dictated by people in power, science requires evidence. That means you ask Nature, (i.e. the physical world) and you do that by conducting experiments. Nature does not lie, nor does it dissemble, nor does it make mistakes. Nature patiently gives consistent answers without favouring anyone. No matter who you are, whether you're black/white, male/female, adult/child, no matter what your creed or belief system, Nature always gives the same answer. If you drop a ball a 100 times or 1000 times, the ball always behaves the same way. If you disagree with Nature, it won't punish, intimidate or mock you. You'll just find out you're wrong. Nature will still continue to answer you, even if you're wrong. Because of the way Nature responds to questions, we have a lot of respect for Nature and we trust that its answers are true.

Now lets say we have two explanations (these are called theories) to explain something. In particular, how do we choose between the heliocentric model of Copernicus and the geocentric model of the church, when both explain what we see.

In science, if you have two theories that explain the evidence, and one is simpler than the other, you accept the simpler theory and you discard the complicated one. This principle is called Occam's Razor. Copernicus' heliocentric system was simpler than the geocentric system, so in science, Occam's Razor says to pick the simpler system, in this case Copernicus'.

leave out

Let's find out who would have benefitted from Copernicus' heliocentric model of the Solar System.

Copernicus made his discovery in 1543. When Colombus sailed the ocean blue, and discovered the New World, did he know about Copernicus' heliocentric model of the Solar System?

Did the people who started the colony on Roanoke Island in eastern NC know about the Copernical heliocentric model of the Solar System? Yes, that colony was started in 1585. They were probably too busy trying to survive to care about what Copernicus had to say.

How about the people in the Mayflower (1620)?

Even after accepting that the Sun was the center of the Solar system, people didn't know of anything beyond the Solar sytem. As far as they knew, the Solar System was the whole universe. Just outside the orbit of the outermost planet that they knew, Saturn, were the stars, fixed on a rigid transparent sphere, At this stage of human knowledge, the Solar System, surrounded by the fixed stars, was the universe.

Copernicus lived in the era before telescopes. With the arrival of telescopes, the view of the Solar System, as being all there was in the universe, was quickly demolished. In the 1600s, the telescope revealed that the stars were at immeasurably large distances outside the Solar System. The universe suddenly was revealed as being a lot bigger than anyone thought and consequently the Solar System by comparison was very small. The stars were all found to be part of the Milky Way. The bright stars we see with our eye are just the close by stars within the Milky Way. With a telescope, there were many more stars, all at a much greater distance.

Let's talk about the Milky Way. The ancients, when they looked up in the sky, saw a band of what looked like milk painted across the sky. They called it the Milky Way. The ancients could easily see the Milky Way, just by looking up. Even 100 years ago, before artificial lighting polluted the night sky, you could see the stars and Milky Way from anywhere, including cities. Where I grew up, in Sydney, after about 10pm, when everyone was home for the night, they turned off the street lights and you could see the sky clearly. But for us today to see the Milky Way, with street lighting all night, we have to go to a place with no artificial lighting, i.e. a place with no light pollution. To do this, you have to get away from populated areas.

Here's the Milky Way from a dark site.

seasons.video/9.3/Jewels_of_the_night_sky-time-lapse_video_Chile-Nikon_D810A-7iqqokBxYIY.mkv

https://www.youtube.com/watch?v=7iqqokBxYIY Jewels of the night sky: time-lapse video, Chile - Nikon D810A

This is what the sky really looks like at night. To me, not having seen this, would be like not having heard music, or seen a flower, or trees or animals, or the ocean, or the Grand Canyon. We don't see this view of the sky anymore in populated areas. The night sky is washed out by the pollution of street lights.

Here's an image of eastern USA at night from space. You can see that in populated areas it's very bright at night. (Lyn has this)

class_visuals/9.3/usa_night_custom-bf181f2a1fd8866176c80f091085b80af94cb934-s800-c85.jpg

Can anyone locate Durham? (see Pamlico and Albermarle Sound. Go West to Raleigh and then NW to Durham/Chapel Hill. The twin cities of Greensboro and Winston-Salem are west of Durham.)

Would anyone like to volunteer to identify a city on this map.

Note the bright fuzzy lights in (I think) N Dakota. There are no cities there. This is natural gas being burned at oil rigs, because it's cheaper to burn it, filling the atmosphere with CO2 which causes global warming, than to capture it and burn it somewhere else to do useful work. It's the perverse economics of fossil fuels.

There are 9,096 stars (down to magnitude 6.5) in the sky capable of being seen by the human eye. (https://www.skyandtelescope.com/astronomy-resources/how-many-stars-night-sky-09172014/) Since you can only see half the sky at any one time (the Earth blocks the other half), there are 4500 stars visible on a clear night (no clouds). If you're in a city (limiting magnitude 2), then there are only 70 stars you can see in the whole sky, or 35 at any one time.

Here's some photos of what the sky would look like without city lights. They're from a book called Dark Cities. I met the author on a visit to Dismal Swamp. It's a park in eastern VA. He took photos of cities and then photos of the sky from a dark spot at the same latitude, and joined them together, to show people what the sky would look like without artificial lighting, or in a blackout.

class_visuals/9.3/thierry_cohen.new_york.dark_cities.02.jpg

class_visuals/9.3/thierry_cohen.san_francisco.dark_cities.jpg

Notice all the stars in the sky. You can't see that anywhere around here.

When there are blackouts in big cities, people call up the police, wanting to know what all that stuff up in the sky is. They think they're being invaded by aliens or the sky is on fire. What they're seeing is stars and the Milky Way.

As the telescopes got bigger, astronomers realised that the stars we see are all members of the Milky Way. The Milky Way is a galaxy. The word "galaxy" means milk. It's where we get the words "lactose, galactose and lactation".

In fact the Milky Way is a spiral galaxy. Here's a photo of a spiral galaxy.

presumably Lyn has a photo of a spiral galaxy. There's a nice high res photo of M101 in Wikipedia.

class_visuals/9.3/M101_hires_STScI-PRC2006-10a.1280x1001.jpg

Our galaxy looks much like this.

It turns out that our sun is just one of billions of stars in the Milky Way. When you look at the other stars in our galaxy, you find that our Sun is a very ordinary star. It's not even located in a special part of the galaxy. Our sun is located in the very outer reaches of the Milky Way, in one of the spiral arms.

class_visuals/9.3/andromeda.you-are-here_2.png

Our galaxy looks like this photo of galaxy M101. Our Solar System is on one of the spiral arms. At night, when look towards the center of the galaxy, we see a dense mass of stars. This part of the sky looks like milk. This is in the direction of the constellation Sagittarius.

If we look in the opposite direction, i.e. away from the center of the galaxy, we're looking towards the edge of the galaxy, and the cloud of stars is much thinner, but still an obvious milky band. There we're looking towards the constellation of Geminii.

If we look up or down in the plane of the galaxy, we see don't see many stars at all.

Not in class any more.

Here's a view of the Earth at night. It's been assembled from multiple shots taken when there's no cloud cover. It's called the black marble, this being the night time view of the blue marble, another name for the Earth, that's come about by looking at pictures of the Earth from space. Black Marble

Here's a view eastern USA at night. Anywhere near lights, you're only going to see the brightest of stars. You won't see the Milky Way except in an area with no lights. The link contains two videos; The night side of Earth (celebrating how cool the Earth looks at night) (2mins https://www.youtube.com/watch?v=Q3YYwIsMHzw ) and Earth from the ISS at night (5 mins https://www.youtube.com/watch?v=Ip2ZGND1I9Q). The last one is really nice, but not directly on the subject. The first one doesn't make the point any better than the image of eastern USA.

Just like earlier astronomers thought that the Solar system was the whole universe, now they realised that the Milky Way was the whole universe. Our Sun wasn't at the centre of it.

Astronomers initially didn't realise the Milky Way was just another galaxy. When astronomers first built telescopes big enough to see galaxies, they thought that the galaxies were close by and part of the Milky way. The telescopes back then weren't good enough to see that galaxies contained stars; Back then, the pictures astronomers had of galaxies just showed fuzzy blobs.

Then early in the 20th century, astronomers built telescopes big enough to see that the galaxies contained billions of stars. They immediately recognised that the galaxies were outside of the Milky Way. The galaxies had to be a very long, long way away. For us to see the galaxies so far away, they had to very, very big. The astronomers realised that the galaxies were as big as the Milky Way. So the Milky Way wasn't the whole universe at all; it was just another galaxy. Astronomers realised that the size of the universe was thousands of times bigger than they thought. There was nothing special about the Milky Way, except that we lived in it.

All the stars we see at night are suns within our galaxy, The Milky Way. There are stars in all the other galaxies too, but they are too far away to see as points with our eyes, or even with ordinary telescopes.

The nearest galaxy to us is Andromeda. In a dark spot, i.e. with no light pollution, you can see Andromeda with the naked eye as a faint fuzzy blob. With binoculars or a small telescope, you can clearly see that it's a galaxy.

naked eye class_visuals/9.3/andromeda.VLT.chile.wikipedia.jpg

class_visuals/9.3/Andromeda_Galaxy-wikipedia-with_h-alpha.jpg

Andromeda is quite large, 3° across or 6 times the diameter of the Moon. In this photo, you can't see the individual stars in Andromeda; they're just a blurr. All the pinpoints you can see in the photo are stars local to the Milky way, which are much closer than Andromeda.

leave out

Andromeda is 2.5M light years away, and is 220kLight years across. This means that Andromeda is far enough away that the light we see now from Andromeda, left 2.5Myrs ago.

Remembering important dates in history, what was happening on Earth 2.5M years ago?

• Were there any humans around 2.5Mya? No. Modern humans evolved only about 1Mya. Our ancestors who were alive 2.5Mya, were still in Africa Smithsonian Family Tree and are called Australopithecines. They walked upright, but still could climb trees easily.

• Had the Ice Ages started 2.5Mya? Yes Wikipedia Ice Ages. The Ice Ages started 2.6Mya, covering Greenland, the Artic, the Antarctic, North America, Northern Europe and Siberia, with sheets of ice, thousands of feet thick.

  Here's a map of the ice sheet Humanities7: ice age world maps (images/earth-last-ice-age.jpg). Notice the ice came down to Memphis TN.

If you have a really big telescope, like Hubble, and you look between the gaps of the stars in the Milky Way, this is what you'll see; galaxies everywhere. (show photo of many galaxies.) e.g. Hubble Ultra Deep Field containing 10,000 galaxies.

class_visuals/9.3/hubble_ultra_deep_field_heic0406a.jpg

So the Milky Way is only one of billions of galaxies in the universe.

What we learn from science is that the universe was not created for us, that it does not revolve around us. Humans are nowhere near the center of anything. We're on a planet, one of many revolving around a very ordinary star, on the outer edges of a very ordinary galaxy, that is one of billions of galaxies in the universe.

If you didn't already know, science has revealed that people have an over inflated sense of their importance.