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Copyright © 2015..7 Joseph Mack
10 Dec 2017, released under GPL-v3.
|Nov 2017: I don't have the video yet. Maybe this year.|
A lesson (and later I'll add a video) on simple astronomical phenomena, given to 4th graders, first presentation 2-3 Dec 2015.
Material/images from this webpage may be used, as long as credit is given to the author, and the url of this webpage is included as a reference.
Table of Contents
There's four 40 min. classes in the 4th grade in this school and I give all lessons in one day. This presentation occurs over two classes, a week apart.
Light for the Sun
In 2015 I used a 500W incandescent light (2700K°) for the sun. In 2016 I used an LED light for the Sun (5600K°). The LED light is white while the incandescent light is yellow/brown. The LED light produces light with the same colour as the room's fluorescent lights. When the room's fluorescent lights are out, you can leave the video camera with the same colour temperature setting, simplifying colour control for the video. However the educational point is made equally well using either light. I just happen to have an LED light available. The incandescent light is cheap and available from large hardware stores. It's also very hot and many parts of it can't be touched. The LED light is expensive and only available from large camera stores. It's cool to the touch everywhere.
At the end of each section I give a test. This is to reinforce the student's knowledge and to give me an idea of how many kids are following the material. I generally expect to find at least one kid who absorbs the whole class and one kid who doesn't understand any of it. After giving the question, I ask kids to raise their hand when they have an answer, but not to call it out. After enough kids have raised their hands, showing that they've had enough time to think about the question, I ask one kid with their hand up. It isn't always the first kid to raise their hand. I try to let as many kids as possible get to answer a question.
On the board is a set of vocab words from the presentation. I'll tell them that they will know what these words mean by the end of the presentation.
Setup globe of the Earth on a stool/table in the middle of the darkened room. Illuminate the globe by a strong light on a stool a couple of feet away from the Earth. The light faces me (away from the class, so the class is not dazzled). A globe of the Moon is on another stool next to the Earth.
I use a hula hoop to show the position of the Moon's orbit, which is inclined to the plane of the ecliptic. The consequence of this is that you don't get solar eclipses at all of the new Moons and you don't get lunar eclipses at all full Moons.
Today 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 if you can 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.
A couple of words I'm going to be using today
Later: add vocab list
What you see before you is called an orrery. An orrery is a mechanical device to represent the mechanics of the solar system. I'm going to only talk about the Sun, the Earth and the Moon. A real orrery has motors and gears and runs by itself, with the planets moving as they do in the sky. Here I'm going to move things by hand. So it's a very simple orrery.
Here is a globe of the Earth. Over there is a light representing the sun, which I'm now going to turn on. As you can see the Sun lights up half of the globe, while the the other half of the globe is in darkness. The part of the Earth that's illuminated by the Sun represents the earth in daylight, while the half that's not illuminated represents the Earth at night. Later I will bring in a globe that represents the Moon.
The setup on the table is called an orrery. An orrery is a mechanical system that replicates the motion of celestial bodies. This is a particularly simple one. It has only globes representing the Sun, Earth and Moon. Rather than having a mechanical sytem to move the globes, I'm going to move them by hand.
ask for some countries that are in day, night. Nominate a country and ask whether it's in day or night.
We all know that the Earth rotates on it's axis.
spin the Earth slowly, so that at least some of the kids see countries being illuminated by the Sun and then going into darkness.
Notice how as the Earth rotates, that some countries come into daylight while other countries go into nighttime. The time of day thus depends on where the Sun is when you look up.
How long does the Earth take to rotate once on its axis? (24 hrs, 1 day; it's the definition of a day.)
I just said that we all know that the Earth rotates on its axis. How do we know this?
Just as an aside, many of the stars have names. Anyone know some names of stars?
- 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 (which I'll talk about later). Satellites and interplanetary space probes use Canopus for getting a frame of reference.
- Castor and Pollux - the twins in the constellation of Geminii
- Regulus - the brightest star in the constellation of Leo
- Vega, Rigel, Antares which is red and has the same colour as Mars, Aldebaran, Spica.
Most of the names of stars are Arabic. This is because the Arabs, living in the desert, had good cloudless skies to observe. It's also because during the middle or dark ages, in Europe, all academic and scientific work was repressed or banned. The 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 Europe emerged from the Dark Ages, the works of the Greeks were translated from Arabic into Latin. If it wasn't for the Arabs who kept the knowledge alive for the 1000yrs of the Dark Ages, today we wouldn't even know who Plato and Aristotle were.
What's the name of the brightest star in the sky?
The brightest star in the sky is The Sun. The Sun has a name too. Does anyone know what it is?
It's name is Sol, which is where we get the name Solar System.
Calling Sol "The Sun" is like calling someone "the kid". 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.
The Moon doesn't have its own name. It's just "the Moon". Many of the planets have moons, and these moons all have their own name but our moon doesn't have its own name. The existance of other moons wasn't discovered till the invention of the telescope. So just using the name "The Moon" was just fine. However in 1610 Galileo turned a telescope to Jupiter to find 4 moons orbiting Jupiter. Since then, we've discovered many more moons orbiting the other planets.
I just said that the motion of the Sun and the stars across the sky indicates that the Earth is rotating about its axis in front of the Sun and the stars. How do we know whether the Earth is rotating in front of a stationary set of stars, or whether the Sun and the stars are rotating about the Earth?
Picking between these two possibilities, turns out to be difficult. We'd see the same thing if the Sun and the stars and the whole universe rotated about a stationary Earth or if the Earth rotated infront of a stationary set of stars. 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, revolved around them. This model is called the geocentric model of the solar system, where the Earth is the center of everything.
However to everyone's surprise, in 1543 Nicholas Copernicus (https://en.wikipedia.org/wiki/Nicolaus_Copernicus) realised that the Earth and the planets all revolved around the sun. This was called the heliocentric model of the Solar System, where the Sun is the center of the solar system.
This was a major event in the history of science.
Back then, the church was the final arbiter of truth. The church held that the Sun and planets all orbited the Earth, i.e. the solar system was geocentric. To say that the Earth orbited the Sun instead, was heresy (or going against the teachings of the church). Copernicus risked imprisonment, torture and death. He didn't dare publish his theory while he was alive and had it published posthumously.
Copernicus' fears were quite justified
- In 1600, Bruno was burned at the stake for extending the Copernical system, saying that the stars were suns just like our sun, and some of them likely would have planets, with beings like us, that had souls.
- In 1615 Galileo was put under house arrest for life, for championing Copernicus' heliocentric view of the solar system.
Copernicus' heliocentric model of the Solar System was the first step out of the Dark Ages into modern science.
Still at this stage people only thought of the universe as the Solar System. With the invention of the telescope in the 1600s showed that the stars were far outside the Solar System.
It was only last century that people realised that our sun, was a very ordinary star in a spiral galaxy consisting of about 100 billion stars. Does anyone know the name of the galaxy which is our home?Milky Way.
Here's a photo of a spiral galaxy. Our galaxy probably looks much like this. Does anyone know why I didn't show you a picture of our galaxy?we don't have one. We haven't sent a spacecraft outside our galaxy yet to get a look at it. This journey would take millions of years. However we can infer the shape of our galaxy from observing the propagation of radio waves through it.
Not only was our Sun a very ordinary star, it was located in the very outer reaches of the Milky Way. Because of light pollution now-a-days, you have to get out of the populated places to see the Milky Way. But when we get to a dark spot, and look up into the sky towards the centre of the galaxy, in the direction of Sagittarius, we see a dense mass of stars. That part of the sky looks like milk. If we look in the opposite direction, away from the center of the galaxy, towards the edge, in the direction of Gemini, the cloud of stars is quite thin.
Note Play https://www.youtube.com/watch?v=7iqqokBxYIY Jewels of the night sky: time-lapse video, Chile - Nikon D810A
We don't see this view of the sky anymore. The night sky is filled with lights. 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.
In the early 20th century, astronomers realised that the Milky Way wasn't the only galaxy. There were others. Here's a photo of Andromeda, our nearest neighbour galaxy. (2.5M light years away, 220kLight years across.) On a Moonless night, if you're in a spot with little light pollution, you can see Andromeda with the naked eye. It just looks like a fuzzy blob, but with even a small telescope, you can clearly see that it's a galaxy. It's 3° across or 6 times the size of the Moon. All the pinpoints you can see in the photo are local stars in the Milky way, which are much closer than Andromeda.
Not only that, the Milky Way is only one of billions of galaxies in the universe. If you have a really big telescope and you look between the gaps of the stars in the Milky Way, this is what you'll see. (show photo of many galaxies.) e.g. Hubble Ultra Deep Field containing 10,000 galaxies.
What we learn from science is that the universe does not revolve around us, and humans are nowhere near the center of anything. As well, if you didn't already know, it was revealed that people have an over inflated sense of their importance.
Because of Copernicus we know that the Earth rotates on its axis, while the Sun and the stars and the galaxies are stationary. The next thing in this class, for us to establish, is which direction does the Earth turn. (spin globe both ways).
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 is NSEW?
In which direction do we see sunrise? (E)
Before sunrise, you're in darkness and the Sun is below the horizon. Then at sunrise, the Sun crosses the horizon in which direction? (E) Then after sunrise the Sun ascends into the sky. The reason the Sun rises, is because the Earth rotates in front of the sun. When the Earth does a complete rotation about its axis, you get sunrise again. How long is the interval between sunrises? (24hrs).
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.)
spin the globe eastwards and then westwards
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?
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?
Let's explore sunrise a bit more.
When it's sunrise in NC, show me where else on Earth is having sunrise. ie. where is the sunrise line? (i.e. the line, on the surface of the the Earth, where night and day bump into each other. The line, which is the junction between night and day, is called the terminator. When you look up at the Moon, you can see a line where night and day join. The terminator is experiencing sunset or sunrise. Because the Sun is low in the sky, objects on the terminator cast long shadows. If you're observing the Moon with a telescope, all the interesting view are on the terminator, because you can see the shadows of the mountains and the mountains are easy to see. )
When it's sunrise in NC, show me where on Earth is having sunset. ie where is he sunset line?
When it's sunrise in NC, is it day or night in LA? in Europe? What time is it in eastern Australia when it's sunrise in NC?
We can see by looking at the globe and knowing the direction of rotation of the Earth, that it's nighttime in LA when it's sunrise in NC. How long after sunrise in NC do the people in LA get sunrise (3hrs)? How do we know that?When the Sun rises in NC, you could phone a friend in LA, tell them that the Sun has just risen here, ask them to call back when it's sunrise there. and see how long it takes for your friend to call back.
But better than that we can calculate it. We know that it takes 24hrs for the Earth to rotate once on its axis; we know how far it is from NC to LA (it's about 1/8 the way around the Earth) and we can infer from that the sunrise is 1/8 of the day or 3hrs later in LA.
- If it's 6am in NC, what time is it in LA? (3am three hours earlier).
- 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).
- The Earth rotates a bit more and now it's 3pm in LA, what time is it in NC? (6pm three hours later).
About 12hrs after sunrise, the Earth has rotated 180° on its axis, and we're 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.
On the globe show me where it's sunrise, sunset?
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.
Q: who were the people that kept knowledge alive in the period known in Europe as the Middle Ages or the Dark Ages?
A: the Arabs.
Q: the names of most stars come from which language?
Q: what's the brightest star in the night sky?
A: Sirius. It's in the constellation of Canis major or the big dog.
Q: what's the brightest star in the sky?
A: the sun.
Q: What's the name of the person who proposed the heliocentric model of the Solar System, i.e. that the Sun was at the center of the Solar System, rather than the Earth being at the center?
Q: name two people who were punished by the church for supporting the Copernican system.
A: Bruno, Galileo.
Q:What's the name of the galaxy that our Solar System is in?
A: The Milky Way
Q: Name the nearest galaxy to us.
Q: Why do we have day/night?
A: the Earth rotates on its axis in front of the sun.
Q: how long does it take for the Earth to rotate once on its axis?
A: 24hrs, 1 day (the definition of a day)
Q: which direction does the Earth rotate; eastwards or westwards?
A: eastwards. (we know this because the Sun rises in the east.)
Q: If it's sunrise in NC, is it day or night in Europe?
(When some of the kids have their hand up, and the pace of kids putting their hand up has slowed, give the rest of the kids a hint. Position the globe of the Earth in front of the sun, with NC at sunrise and your finger on NC, telling the kids what you're doing. When they look at the globe, they will see Europe in the daytime.)
Q: If it's sunrise in NC, is it day or night in LA?
A: night. It's actually early morning before sunrise.
Q: Let's say there's an Olympic skiing event in the middle of the day in Norway, that's being broadcast live, world-wide on TV. What time of day (approximately) will you be seeing it in NC?
A: about sunrise in NC.
Not only does the Earth spin on its axis, it orbits the sun. When I say the Earth orbits the sun, I mean it goes around the Sun in almost a circle, ie in a plane. (Walk around the Sun with the Earth, while spinning the earth.) The plane, in which the Earth and all the planets orbit the sun, is called the ecliptic plane.
Why are the planets in the same plane?
The solar system formed from a rotating cloud of dust. It turns out that everything in the universe rotates about something. This is because not everything is moving at the same speed. Two particles moving 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 video on youtube that explains Why is the Solar System Flat. It's produced by Embry-Riddle Aeronautical University.
Initially assume the cloud is anything but a disk, say a spherical cloud. Some of the objects will orbit above and below the plane. Since they're all rotating in the same direction, 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 ecliptic plane, when you look up in the sky at night, you see the planets move through the sky, in the plane of the ecliptic, so you'll see them moving along the same line.
How long does the Earth take to orbit once around the sun? (1yr, it's the definition of a year).
In the day, when we look up, the sky looks pretty much the same all year; we just see the very bright sun. The Sun is bright because it's so close to us.
But at night things are different. When we look up, we only see the stars. These are suns, just like our sun, only a lot further away. Consequently they are a lot dimmer than the sun. Our Sun and all the stars we see at night are all members of the our galaxy, The Milky Way.
The stars we see in the 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.
Can anyone name some other constellations?
scorpio, geminii, sagittarius, hercules, orion, andromeda, leo, taurus, cassiopeia.
(Walk around the Sun with the globe of the Earth, pointing in the direction away from the sun, to where people at night would be looking.) As our planet orbits the Earth during the year, at night when we look up in the sky we see different parts of our galaxy; so we see different stars and constellations though the year.
Along the ecliptic, the ancients constructed constellations of animals and humans. These constellations have a special name; the Zodiac (from the word zoo). Because these constellations have the ecliptic running through them, 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.
With what I've told you, 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 amount of sunlight and nightime would be 12hrs each, every day. The fact that the Earth orbits the Sun wouldn't be all that interesting, because all times during the year would be the same.
There's one small fact I've left out; the Earth's axis of rotation is inclined to the plane of the ecliptic. This gives us the seasons. I'm going to show you how we get the seasons. Having seasons makes it easy for us to tell what part of the year it is.
All globes of the Earth, like the ones used in classrooms, have the Earth's axis of rotation tilted. (show the tilt) Does anyone know the angle of the tilt? (23.5°).
When a planet is first formed, it's assumed that the axis of rotation is perpendicular to the ecliptic. This is a consequence of the way the planets formed from a disk of rotating material. If the axis of rotation is different, then something has happened to the planet, like it had a major collision with another body.
Does anyone know why the Earth's axis of rotation is tilted?
Well we don't know for sure, but here's a likely explanation. When the Earth first formed, it didn't have a Moon. Its axis of rotation was perpendicular to the plane of the ecliptic, by virtue of being formed out of a spinning disk of dust and rock. It's thought that early in the history of the solar system, a Mars sized planet crashed into the Earth, forming the Moon. This collision tilted the axis of rotation of the Earth.
Here's a list of the axial tilt Axial tilt of the planets. All the planets except Mercury, Venus and Jupiter have significant axial tilts. The rest of the planets have had major collisions as part of their formation.
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. So no matter what time of year it is, the Earth's axis is always pointing in the same direction.
(walk the Earth around the sun, spinning the earth, showing the kids that the axis points in the same direction.)
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 get a plane from one side of the country to another without human intervention, by using the orientation of the gyroscope to tell the orientation of the aircraft. It also 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 you extend the Earth's axis in the northerly direction, you find it points to a spot in the sky near, but not exactly on, a star. Does anyone know the name of this star? (Polaris, the pole star.) The stars at night move through the sky, just like the Sun moves through the sky during the day. If you look up in the sky at night, you'll see the stars all rotate around a point in the sky. If you do a long exposure photograph of the night sky, you'll see star trails.
show star trails photo
The fact that there is a star, Polaris, near the north pole of the sky is just a coincidence. If you extend the Earth's axis in a southerly direction, it doesn't point to anything interesting at all, just a blank spot in the sky.
Let's see what having the Earth's axis inclined to the plane of the ecliptic does for us.
Show Earth with southern hemisphere with the Sun overhead (southern hemisphere summer), that the sky moves through the sky with the Sun at high elevation. Show that at the same time, the Sun moves through the sky at a low elevation in the northern hemisphere (norther hemisphere winter).
We know that in summer the daylight is longer than the night time. Show that in the summer, the Sun illuminates more of that hemisphere, and so more of the 24 hour day is spent in sunlight.
Say "the southern hemisphere is getting the Sun overhead". 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.
Show that northern hemisphere summer is southern hemisphere winter.
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.
What season is it now in the northern hemisphere? southern hemisphere. (The class is normally given in Dec, so it's southern hemisphere summer and northern hemisphere winter.) Let's say it's 1 Jan today; what's the season in the northern hemisphere, the southern hemisphere?
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.
So the tilt of the Earth's axis produces the seasons and makes the weather/temperature/amount of sunlight/rain different in each part of the year. Adjusting our lives to the seasons is important in staying alive. We grow our crops in the summer. Animals and plants must time reproduction according to the seasons or their offspring will not survive winter. Some animals migrate to avoid winter.
Name some animals that migrate to avoid winter
birds, deer, butterflies (monarch), bats, salmon, the mass migration of many animals in africa,
The arctic tern https://en.wikipedia.org/wiki/Arctic_tern flies every year from the artic to the antarctic and back again, a round trip of 56,000 miles. This distance is just over double the distance it would do if it did the trip in a straight line.
Because the Earth's axis is tilted, the amount of daylight varies throughout the year. Let's see how this happens.
(point to area in the Arctic circle.) In northern hemisphere summer, land in the Arctic circle, that's on the side of the Earth away from the sun, is illuminated too. This means that in the middle of summer, people inside the Artic circle have 24 hrs of sunlight. (rotate the Earth.)
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, the area inside the Antartic Circle is receiving no light at all.
How big (in degrees) is the Antarctic Circle? (23°).
(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 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.
The consequence of this is that 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 and 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.
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 and living at night.)
Here are the approximate dates of the solstices: 22 Dec, 22 Jun, the peak of summer and winter.
Here are the approximate dates of the equinoxes: 22 Mar, 22 Sep. These occur in the spring and the fall.
We've found two circles on the earth, the Artic, and Antarctic circles, which are 23.5° from the poles. Inside these lines, at the solstices, there is 24 hours of light in summer and 24 hours of dark in winter.
There's another pair of lines called the Tropics. At midsummer in the northern hemisphere (22 Dec), at what latitude is the sun directly overhead? (23.5°N)
As the year progresses, (walk around sun with the Earth) the sun moves south till on 22 Jun, at what latitude is the sun directly overhead? (23.5°S).
Half way between these two lines and also halfway between the poles is the equator, the line that cuts the sphere of the Earth into two equal halves.
It turns out that the sun is ever only directly overhead between these two lines of latitude, the Tropics at 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 northern hemisphere tropic is called the Tropic of Cancer, because at the northern hemisphere summer solstice, the Sun is in the Sign of the Zodiac called Cancer. The southern hemisphere tropic is called the Tropic of Capricorn, because at the southern hemisphere tropic, the Sun is in the Sign of the Zodiac called Capricorn
At latitudes outside the tropics the sun is never directly overhead at any time of year.
The amount of daylight we get each day varies throughout the year. Why is this?
The Earth's axis is tilted to the ecliptic. (by 23.5°)
How many degrees of latitude is the Artic and Antarctic circle from the poles?
How many degrees of latitude are the Tropics from the equator?
No I'm not going to do this for 4th graders, fun though it would be.
Q: The Earth orbits the Sun in a plane called the ecliptic. All the other planets also orbit the Sun in this plane too. Why do the planets all orbit the Sun in the same plane?
A: They were formed from dust and rock, which was orbiting the center of mass of the rotating matter. Through collisions, the matter formed a very thin disk. Any matter orbiting out of the plane of the disk, would eventually collide with matter in the disk. Once the disk formed, further collisions formed the planets, all in one plane.
Q: How long does it take for the Earth to orbit the Sun once?
A: 1 year (the definition of a year).
Q: When you look up at the sky in the night, you see patterns of stars, that the human mind makes into people, animals and objects. These patterns are called constellations. Name a few constellations.
A: scorpio, geminii, sagittarius, hercules, orion, andromeda, leo, taurus, cassiopeia.
Q: Why do we have seasons?
A: the Earth's axis is inclined to the plane of the ecliptic. The inclination is 23.5°.
Name some effects of having seasons
the hours of daylight change through the year, the elevation of the sun changes. Both of these affect the amount of heat and light we receive, making it warmer in summer and cooler in winter. Depending on where you live, the changing seasons can bring rain or dry weather, or ice and snow.
Q: The planets Mercury, Venus and Jupiter have their axis of rotation perpendicular to the ecliptic. Do these planets to have seasons?
|I added this into these notes in Dec 2016, but I haven't given it in class. I barely have time for the other material and the kids would be overloaded if I gave this. Perhaps I could give another class on sundials. I could move the section on the sunrise/sunset line into here. Maybe I could talk about analemmas|
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 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 once 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 Earth has a natural satellite, the Moon. (Orbital plane is tilted 5° to the ecliptic, the Moon's axis is tilted 1.5° to the ecliptic; wikipedia. This information is just for me. I don't tell the kids.)
The Earth is unique in the solar system in that it has a Moon which is of comparable size to the primary body, in this case the planet Earth. There are many moons throughout the solar system, but all of them are small compared to the planet. The Earth-Moon system is more like a double planet, than a planet with a Moon. The diameter of the Earth is about what? (8000 miles). The diameter of the Moon is about what? (2000 miles). The Moon is about 1/4 the diameter of the Earth.
The Moon orbits the Earth, in the plane of the ecliptic, i.e. in the same plane that the planets orbit the sun. Can anyone guess 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 on its axis 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. Because nearly all the bodies in the Solar System rotate the same way, we can conclude that in the time since the formation of the Solar System, there haven't been too many really massive collisions.
How long does it take for the Moon to orbit once around the Earth? (a month, actually about 28 days - the original definition of a month).
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). You'll see why a week was chosen as a useful time interval in a few minutes.
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. I'm going to mark the crater Tycho (point to image on white board). Tycho is a relatively young (108Mya) crater in the southern hemisphere of the Moon, that faces the Earth. It's a bright rayed crater that is easily visible to the naked eye from Earth. Let's watch the Moon go around the Earth. I'm going to move the Moon in a way that the 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 with the same side facing the Earth.
lights off please
Show the Moon lit by the sun, with its own day/night. Start with the full Moon and Tycho lit by the sun. Ask if Tycho can see the Sun and hence if Tycho is in daylight or night? Point to Tycho and have the Moon orbit the Earth (lunar month). Show when it's sunrise, mid-day, sunset, midnight on Tycho.
surprisingly, at this stage, most of the kids have not realised that the Moon, while orbiting the Earth (taking a month) has just gone through one day/night cycle, i.e. its own version of a day.
Does anyone know how long the day is on the Moon? If you don't know, let's do it again and see.
Let's see how long it takes for Tycho to go through a day and night cycle. Remember Tycho is on the side of the Moon that always faces the Earth. OK so how long is a day on the Moon?
show lunar day == month. a month is the time it takes to orbit the Earth.
How long does it take for the Moon to orbit the Earth?
1 month, 28 days (the original definition of a week)
How long is a day on the Moon?
Why is the Moon's day the same length as the time it takes for the Moon to orbit the Earth?
Because as the Moon orbits the Earth, it keeps the same side facing the Earth.
If we're out in space, in the direction of the sun and we look back at the Earth-Moon system, we will always see a fully illuminated Moon no matter where it is in its orbit around the Earth.
If we're out in space, in the other direction, away from the sun, and we look back at the Earth-Moon system, we will see the dark side of the Moon all the time, no matter where it is in its orbit around the Earth.
If we're sitting on the Earth and looking at the Moon as it orbits the Earth, sometimes we'll see the side that's facing away from the sun and hence is dark. Sometimes we'll see the side of the Moon that's facing the sun and hence is light. In between these times, we'll see a partially illuminated Moon.
(draw diagram) Because the angle Earth-Moon-Sun changes during the Moon's orbit of the Earth, our view of the illuminated part of the Moon changes. Another way of putting it is that the Moon has phases because as the Moon orbits the Earth, we get a different view of the illuminated part of the Moon.
As a result, the Moon goes through what are called phases.
point to poster on wall showing phases of the Moon.
We see phases of the Moon, because different parts of the Moon are light and different parts are dark. The Moon can be a crescent when less than half of the Moon is lit. The Moon can be gibbous when the Moon is half to fully lit. When the Moon is fully lit, it's said to be full. When the Moon is unlit from the Earth, it is said to be new. (hopefully have a diagram of the phases).
Let's see where the phases come from. (lights off)
If the Moon and the Sun are on the same side of the Earth, and you're standing on the day side of the Earth and you look to the Moon, you will see the dark side of the Moon. In practice, the Moon will be in the glare of the Sun and you won't be able to see that the Moon is there at all. This Moon is called the new Moon. If you're standing on the night side of the Earth, can you see the new Moon? (no, it's below the horizon.)
Can you see the new Moon in the day? (yes, in principle) at night? (no).
(demonstrate with orrery)
Let's say it's noon and the Moon is new, then the Moon is directly overhead (remember, it's in the same place in the sky as the sun).
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.
Let's also say it's sunset. So the Earth has rotated 90° about its axis. Where are you on the Earth? (show line.) Where is the Moon; overhead or on the horizon? (overhead). (The Earth has rotated 90° on its axis and the Moon has moved 90° in its orbit around the Earth. It will be in the same position, relative to you, i.e. still overhead.) If you look up to the Moon now, 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.
If you're around the back of the Earth, at night, a bit before midnight (point to spot that can just see the Moon), can you see the first quarter Moon? (just). Is the Moon on the horizon or overhead? (horizon). A Moon on the horizon is either just having risen or just about to set. If it's midnight at your location, the 1st quarter Moon is on the horizon. Is it rising or setting (setting)?
If you're in daylight at about noon, can you see the first quarter Moon? (just). Is it on the horizon or overhead? (horizon). Is it rising or setting? (rising).
If you're at sunrise, where are you? Can you see the first quarter Moon? (no).
Have the kids standing on the Sun side of the Earth looking towards the Moon which will be on the other side of the Earth. Tell them to pretend that they're standing on the night side of the Earth facing the Moon.
If the Moon is opposite the Sun and you're standing on the night side of the Earth, you'll see a fully illuminated Moon. What phase of Moon is this (full Moon)? Let's says it's midnight, where are you on the Earth? Where is the Moon? On the horizon, overhead? (directly overhead). This is called a full Moon.
If you're standing on the day side of the Earth, which direction are you facing? (towards the sun.) Can you see the full Moon?
no, the full Moon is below the horizon, it has not risen yet, or it has set. You can see the full Moon if you're in darkness, i.e. anytime from sunset to sunrise.
If the Moon is full, you can't see it during the day. You can only see it at night.
Now let's look at the 3rd or last quarter Moon. Let's say we're in NC at sunset (have the kids orient the earth), can we see the 3rd quarter Moon? (no) We can only see the 3rd quarter Moon when (at sunrise).
It turns out you can't see all the phases of the Moon from just anywhere. You have to be at the right place and right time.
Something that I think is particularly beautiful is the crescent Moon. You get a crescent Moon a couple of days after the new Moon and a couple of days before the new Moon. Let's look at the Moon a couple of days after it's new. (move the Moon about 30°). You'll all have to stand on one side. This is called the waxing crescent Moon (waxing == getting bigger) or the new crescent Moon.
I'm now going to move the Moon to the end of the lunar month when we again get a crescent Moon, called the waning or old crescent Moon. Can you see the waning crescent Moon at sunset (no, the Earth is in the way). What time of day do you see the waning crescent Moon (sunrise)? If you're up early, you can see the old crescent Moon in the early morning. So in winter when you get up in the dark, go out and look for the waning crescent Moon.
You will notice that most of the time, the terminator, i.e. the boundary between light and dark, is curved. However at 1st and 3rd (or last) quarter, the terminator is straight. It's easy to tell when this happens. As well it's easy to tell a full Moon just by looking at it. The new Moon, the quarters and the full Moon are all a quarter of a month, or a week apart. Because it's easy to mark the passage of a week, it became a useful length of time.
(Position NC at sunset with the Moon just a day or two after new). Let's say it's sunset. Where are you on the Earth? (show line.) If you look up to the Moon, what do you see? (This phase is called a crescent Moon or a new Moon). (have everyone stand behind the Earth.) Is the Moon near the horizon or overhead?
If you're at sunrise, where are you? Can you see the new (crescent) Moon? (no). Can you see the old (crescent) Moon? (yes).
The pattern of illumination of the Moon as seen from the Earth changes through the month. Why is this?
It's because as the Moon orbits the Earth, we get a different view of the illuminated part of the Moon.
Can we see the full moon at night/day?
Why can't we see a new moon?
we only see the dark side of the Moon and it's in the glare of the Sun.
How long does it take for the Moon to go from 1st quarter to full?
Point to globe of the Earth, but don't put the Moon in till you give the answer
I'm at sunset and I see the Moon directly overhead. What phase is the Moon?
(same again, don't put Moon in till the answer) I'm at midnight and I look up and see the Moon overhead. What phase is the Moon?
I look on the calendar and find that the Moon is 3rd quarter. What times (or part) of the day can I see it?
Any time between midnight, through sunrise to noon.
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.
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. The politicians don't care about the concerns of the farmers.
Are we using a solar or a lunar calendar?
Why do we use a solar calendar?
it stays in sync with the seasons
why do hunter/gather societies use a lunar calendar?
it's simple to keep time.
In what season do most agriculture based societies have their new year?
|I added this in 2017, but didn't deliver it.|
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. 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.
- bombardment then maria flows
- maria flows then bombardment
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.
An eclipse occurs when a celestial body (i.e. planet or moon) casts a shadow on or blocks the view of another celestial body.
A lunar eclipse occurs when the Earth casts a shadow on the Moon (hold Moon up behind the Earth, show shadow). This occurs when the Moon is at what phase? (full).
Where do you have to be to see a lunar eclipse? (anywhere it's night time, i.e. half the world sees a lunar eclipse.) The Earth is 4 times the diameter of the Moon, and it takes a couple of hours for the Moon to move through the Earth's shadow.
Two weeks later, the Moon moves around in front of the sun. When the Sun is blocked by the Moon, we have a solar eclipse. The phase of the Moon is what? (new).
The names Solar and Lunar eclipse are a little confusing. You'd hope that the name for the eclipse would indicate the object that has the shadow on it, or maybe the object that disappears from view. Whatever it is, you'd hope that the same convention would be used for all eclipses. However a lunar eclipse occurs with the Moon in the earth's shadow. while a solar eclipse occurs with the Earth in the Moon's shadow. so maybe a solar eclipse should be called an earth eclipse. If you were standing on the Moon during a Lunar eclipse, you would probably call it a Solar eclipse. If you were standing on the Sun's surface, you might call it a Lunar eclipse. Instead the name used came from what humans, standing on the earth, saw. A solar eclipse occurs when you can't see the sun, i.e. when the shadow is on the earth. A lunar eclipse occurs when you can't see the moon, i.e. when the shadow is on the moon. Human language is like this; it isn't precise. The listener has to pay attention and disambiguate the speaker's words on the fly.
Where do you have to be to see a solar eclipse? (right under the shadow, you can't see it from everywhere on the daylight side.) The Moon is small compare to the Earth, so the shadow of the Moon on the Earth is small and only a small part of the earth sees a solar eclipse. The shadow of the Moon only a couple of miles wide under it. The only way to see a total solar 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 solar eclipse, where part of the Sun is blocked by the Moon. A partial solar eclipse can be seen for several 100 miles on either side of the area that's totally eclipsed.
If the sun, Moon and Earth all orbited in the same plane, as I've told you, how often would we have eclipses? (twice a month). We'd have a lunar eclipse, every full Moon and a solar eclipse every new Moon.
In fact eclipses are relatively rare. You only get them about twice a year. Let's see why.
To get eclipses every two weeks, the Moon would have to orbit the Earth in the same plane that the Earth orbits the sun. (show Earth orbiting the Sun in the horizontal plane, and then the Moon orbiting the Earth, in that same plane.)
In fact, the Moon orbits the Earth in a plane that's tilted 5° to the plane of the ecliptic.
show exaggerated tilt with hoop.
Why doesn't the Moon orbit the Earth in the same plane as the ecliptic, the plane where all the other planets do their orbiting?
It's left over from the collision that formed the Earth-Moon system. The collision wasn't dead-centre, so the Earth's axis of rotation was tilted. Another effect of the collision not being dead centre was that the the collisional debris was sprayed out in a plane tilted to the ecliptic. This collisional debris later coallesced to form the Moon, which now orbited the Earth in a plane tilted 5° to the plane of the ecliptic.
Now how often do we get eclipses?
As the Earth orbits the sun, the plane of the Moon's orbit stays (essentially) fixed, just as the direction of the axis of the Earth's rotation stays fixed.
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 that the planets travelled on too.
We've shown that eclipses are only possible twice a year. Let's say we get two solar eclipses and two lunar eclipses a year. So if you can travel anywhere you can see two solar eclipses and two lunar eclipses a year.
You can see a lunar eclipse from anywhere on the night side of the Earth. Can you see a lunar eclipse from the day side of the Earth (no)?
Can you see a solar eclipse from the night side of the Earth? (no) Can you see a solar eclipse from the day side of the Earth? Sometimes. It depends where you are. The shadow of the Moon on the Earth's surface is small. You have to be exactly in the right spot.
Back in the '80's I travelled from MD to GA to see a total solar eclipse. We had great weather and we saw it perfectly. This year I went to MO to see the eclipse: I've been waiting for this eclipse for 20yrs. It rained.
If you stay in one place, how often are you going to see a total solar eclipse? Never unless you travel. The partial eclipse has a wider path, so you can expect to see a partial solar eclipse a couple of times in your life.
how often do you see a lunar eclipse?
What time of day can you see a lunar eclipse? (night time - the sun has to be on the other side of the earth to see the shadow of the earth on the moon.)
How many lunar eclipses are there every year? (2). If you stay in the one spot, and are prepared to stay up all night, on average, how many lunar eclipses will you see each year? (1) (half of the eclipses will occur when you're in daytime.) If it's cloudy, you'll see fewer eclipses.
The next lunar eclipse visible here in NC, will be at midnight 20-21 Jan 2019 and will last 5:12(hrs:mins). That's a sunday night through early monday morning. If the sky is clear, I'll be watching it.
(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 cross the earth's shadow along the diameter of the shadow, along a chord of the shadow, or along a tangent to the shadow.
The size of the earth's shadow is large enough that a lunar eclipse takes about 4hrs.
Now when the moon is behind the earth, it's in a shadow that's not completely dark. The earth has an atmosphere and for the same reason that the sky becomes red when the sun is setting, the moon sees a ring around the earth of atmosphere, that's red. As viewed from the moon, during an eclipse, the sun is just below the earth's horizon. So an eclipsed moon is red rather than black. The reddened eclipsed moon is much darker than the moon lit by the sun, but the eclipsed red moon is easily visible in the sky. It's only when the moon goes across the diameter of the earth's shadow, that it gets black enough that you can find it anymore, unless you know where it is. I've seen about a dozen lunar eclipses and only one of them was so black that I couldn't see the eclipsed moon anymore.
If the moon goes across the diameter of the shadow, the place on the moon's disk where the light appears at the end of the eclipse is opposite the place where the light disappears. If the moon goes across the shadow of the earth, chordally, then one side of the moon will stay brighter than the other, and the new bright area when the moon emerges will be close to the bright spot that disappeared when the moon went into the earth's shadow.
We're about to see a lunar eclipse, where the moon goes across a chord of the earth's shadow. The moon will stay red the whole time. The bright area lit by the sun on emergence from the earth's shadow will be near the location of the bright spot when the moon entered the earth's shadow. The part of the moon that's illuminated by the sun is very much brighter than the red moon when it's eclipsed. You don't get to see the red of the moon until the moon is totally in shadow. The camera has to adjust its exposure to handle the difference, so you will see some artifacts like the moon getting brighter and dimmer on a short time scale. As well the moon is moving through the sky in the same way that the sun moves in 4 hours, so 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.
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.
A solar 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.
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. 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
We've known how to predict eclipses since the days of the Chaldeans (Saros cycle) in 600-500 BC. These were people who lived in what became Persia.
Acropolis, dragon's head, dragon's tail. nodes
Humans have a large interest in figuring out how things work. It helps you get food, makes you comfortable, and stops you from doing things that will kill you.
After society became agricultural, and produced more food than was needed to feed the people who did the farming, you could have people who didn't need to work in the fields all day. You wound up with a king and his court, an army, people who could plan earth works, irrigation and build buildings (engineers), troubadors, and tax collectors and accountants, to make sure the people fed the king and all the other people who weren't farmers anymore. Because of the necessity to record taxes, writing developed.
A class of people developed to explain all the things that no-one understood. These are the priests, mathematicians, astronomers and philosophers. For some of these people, is was convenient to attribute the unknown to the action of gods. Since the King told the peasants that he was descended from the gods, it was most important that the King be able to maintain this fiction. For this he had to be forewarned of communications from the gods.
All these people, I'll call the leisure class.
An eclipse, where the moon disappeared out of the sky, clearly was a communication from the gods. If you were the King's astronomer, you could expect to loose your head if you didn't predict an eclipse. The King had to tell the populace that he, the mighty one, by his great strength, would in the coming weeks, be doing battle with the forces of evil, who would attempt to swallow the moon. When, after great struggle, the moon reappeared, the King would be declared victorious and the populace could be appropriately grateful.
It wasn't too long after the arrival of the leisure class, that they learned to predict eclipses. They did so without our modern understanding of celestial mechanics i.e. how the planets move. We don't know how they did it, but here's how they could have done it.
Let's go back to our model. The plane of the moon's orbit is inclined to the plane of the ecliptic, not by much, only 5° but it's enough that you only get eclipses at the two points, where the moon's orbit crosses the plane of the ecliptic. These points are called nodes. There is one point, where the moon is going from the southern hemisphere of the sky, to the northern hemisphere. This point is called the ascending node. The other point is 180° away and is where the moon is moving from the northern hemisphere of the sky to the southern. This other point is called the descending node.
So let's say you're a priest and it's your job to follow the moon. You know that all the planets follow the ecliptic. You also notice that during the month, the moon swings above the ecliptic and then below the ecliptic, for half the month each. The phase of the moon, as it crosses the ecliptic, varies throughout the year. Priests have been recording eclipses for ages and you've all noticed that eclipses of the moon only occur when the moon is full and it crosses the ecliptic. Clearly the ecliptic is important, even if you don't know why.
Here's a photo, xistock-157180942.jpg.pagespeed.ic.atZTcttunj.jpg, of the Acropolis from Visiting Greece: Climbing to the top of the Acropolis by Judy Freeman. The rectangular building with all the columns is called the Parthenon. We're looking north. Approximately what time of day is it? (hint, what's the elevation of the sun?) (sunset).
These words are on the white board. I tell the students that I will talk about these things during the presentation.
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