Earth, Sun and Moon
Rotation of the Earth
Each day, the Earth rotates (spins) on its axis, like a basketball rotates on the finger of someone spinning it.
Each day, the Earth rotates (spins) on its axis, like a basketball rotates on the finger of someone spinning it.
It spins so smoothly, however, that for us on the surface, it feels like we are staying still and that the rest of the universe is the one doing the spinning.
And since the stars are so far away that we don't notice any movement in them, looking at the sky actually makes us think there really is a celestial sphere out there with all the stars attached to it, which is spinning. Now, since the Earth spins from West to East, the sky's apparent spinning is from East to West (hence why all objects in the sky appear to move from East to West).
Notice in the movies below, the one on the left shows what is really happening over a day... the Earth is spinning (anti-clockwise). The one on the right shows what we 'feel' is happening... that we are staying still and that the whole sky is the one doing the spinning (clockwise).
And since the stars are so far away that we don't notice any movement in them, looking at the sky actually makes us think there really is a celestial sphere out there with all the stars attached to it, which is spinning. Now, since the Earth spins from West to East, the sky's apparent spinning is from East to West (hence why all objects in the sky appear to move from East to West).
Notice in the movies below, the one on the left shows what is really happening over a day... the Earth is spinning (anti-clockwise). The one on the right shows what we 'feel' is happening... that we are staying still and that the whole sky is the one doing the spinning (clockwise).
The motion of the stars resembles the motion of stickers on a gigantic spinning sphere.
You can learn more about how and why the stars move across the sky by clicking here and going to the bit about Stars.
What is a day on Earth?
A day on Earth in its simplest form is 24 hours... the average amount of time it takes for the sun to go from its highest point in the sky (noon), do a complete circle (some of which is under the horizon) and return to its highest point in the sky (noon the next day).
But there are different types of days as the following video shows (Note, the star at the bottom is so far away, that even when you move to the right, you'd still think it was directly below you):
You can learn more about how and why the stars move across the sky by clicking here and going to the bit about Stars.
What is a day on Earth?
A day on Earth in its simplest form is 24 hours... the average amount of time it takes for the sun to go from its highest point in the sky (noon), do a complete circle (some of which is under the horizon) and return to its highest point in the sky (noon the next day).
But there are different types of days as the following video shows (Note, the star at the bottom is so far away, that even when you move to the right, you'd still think it was directly below you):
Sidereal Day:
The time it takes for the sun to spin 360 degrees on its axis is defined as a sidereal day. In this time, a star will return to its original position in the sky. This is a very consistent 23 hours and 56 minutes
The time it takes for the sun to spin 360 degrees on its axis is defined as a sidereal day. In this time, a star will return to its original position in the sky. This is a very consistent 23 hours and 56 minutes
Solar Day:
The time it takes for the sun to return from being highest in the sky from one day to the next. Because of the Earth's movement along its orbit, this requires MORE than 360 degrees of turn, so it takes a little longer than a sidereal day. Each solar day is different to the next as the Earth moves at different speeds on its orbit at different times of the year. Some solar days are a tiny bit longer then 24 hours... some are a tiny bit shorter... but they average out to 24 hours.
The time it takes for the sun to return from being highest in the sky from one day to the next. Because of the Earth's movement along its orbit, this requires MORE than 360 degrees of turn, so it takes a little longer than a sidereal day. Each solar day is different to the next as the Earth moves at different speeds on its orbit at different times of the year. Some solar days are a tiny bit longer then 24 hours... some are a tiny bit shorter... but they average out to 24 hours.
Mean Solar Day:
The average time of a solar day throughout the year is 24 hours. This is usually what we mean by 1 day and what we use to divide our calendar up into days.
The Earth's Orbit (What is a year?)
The Earth orbits around the sun in an almost circular orbit. The orbit is slightly stretched, making the shape an ellipse rather than a circle.
There is one spot on this elliptical path which is closer to the sun than any other. When the Earth arrives at this point, we call it perihelion and this occurs on January 3rd. There is also a point on the elliptical path which is further from the sun than any other. When the Earth arrives at this point, we call it aphelion and this occurs on July 4th.
The average time of a solar day throughout the year is 24 hours. This is usually what we mean by 1 day and what we use to divide our calendar up into days.
The Earth's Orbit (What is a year?)
The Earth orbits around the sun in an almost circular orbit. The orbit is slightly stretched, making the shape an ellipse rather than a circle.
There is one spot on this elliptical path which is closer to the sun than any other. When the Earth arrives at this point, we call it perihelion and this occurs on January 3rd. There is also a point on the elliptical path which is further from the sun than any other. When the Earth arrives at this point, we call it aphelion and this occurs on July 4th.
Over the year, a line joining the sun to the Earth would sweep around the orbit. One year is defined as the time it takes for the Earth to sweep out 360 degrees on its orbit (to complete one lap basically).
On Earth, we know a year is up because the sun's apparent position on the never-moving celestial sphere returns to where it was at the start of the year.
You can work out the length of a year on Earth (and any other planet) by doing the following.
- Identify where the sun is in the sky (relative to the stars)
- The next day the sun will have moved a little and be in a different position. Wait until the sun returns to its original position. Then you know a year is up.
- Work out how many days went by between the sun starting and finishing at the same point.
The Ecliptic Plane
The Earth orbits around the sun as if there was some invisible carpet it was rolling along.
The Earth orbits around the sun as if there was some invisible carpet it was rolling along.
In Mathematics, an infinitely big 1-D object is called a line. An infinitely big 2-D (flat) object is called a plane, and can be thought of as like an infinately big sheet of paper. The imaginary plane that earth's orbit could be drawn on is known as the ecliptic plane. It's name comes from the fact that an eclipse will only occur when an object is on this plane. Objects above it do not get in the way of the sun and Earth seeing each other and therefore don't cause eclipses.
The ecliptic plane pierces through both the centre of the Sun and the centre of the Earth. If you were to extend the ecliptic plane out to the edge of the universe, it would pierce through all the constellations of the zodiac too.
The ecliptic plane pierces through both the centre of the Sun and the centre of the Earth. If you were to extend the ecliptic plane out to the edge of the universe, it would pierce through all the constellations of the zodiac too.
Nearly all the other planets orbit on or very near to the ecliptic plane too. Pluto is an exception which orbits on an angle, massively tilted to the ecliptic plane.
From Earth, the orbits of the planets look like this:
The Earth's Tilt
The entire solar system formed from the same spinning gas cloud. So everything in the solar system spins or orbits the same way.
Every planet's orbit, spin*, and even the sun's spin is always in the same direction!!! Looking down on the North Pole, everything spins counterclockwise.
In theory, a planet's rotation on its axis should be in the same exact direction as its orbital plane, since the source of the spinning/rotating is the same... the original spinning cloud from which the entire solar system formed.
The early Earth would have spun in such a way with the equator always spinning in line with the ecliptic plane... and the Earth's axis (an imaginary line going through the north and south pole) would have been at right angles to the ecliptic plane.
However, due to collisions (the one that made the moon being a major one), the spinning Earth got a bit of a bump. This bump caused the spinning Earth to be tilted (relative to the ecliptic plane). The axis is not tilted at an angle of 23.5 degrees from where it should be (at right angles to the ecliptic plane). This means the axis now points to a star called Polaris, rather than to the same star that the sun's axis points to.
Note that this tilt means the equator stops being lined up with the ecliptic plane and is now tilted 23.5 degrees. This makes one half above the ecliptic and the other half below the ecliptic.
So the image of the Earth spinning and orbiting (2 pictures up) is wrong... what really happens today is this:
So the image of the Earth spinning and orbiting (2 pictures up) is wrong... what really happens today is this:
This causes many effects on earth which include the Seasons and Length of Day and Night varying over the year.
Ask the teacher to show you some demonstrations with a globe or with Starry Night to learn more about Seasons and Length of Day and Night if you wish.
This video gives a basic explanation of the seasons:
The Moon and its Orbit
As the Earth orbits around the sun, it is orbited itself by the moon.
As the Earth orbits around the sun, it is orbited itself by the moon.
As the moon moves around the Earth, at all times, 50% of the moon faces the sun and reflects light, being the 'bright' side of the moon. And 50% of the moon faces away from the moon, being 'dark' and not reflecting any light. Note in the diagram below, the moon is shown as multiple points on its orbit around the Earth. The white half of the moon represents the bright side and the black half represents the dark side.
Phases of the Moon
While 50% of the moon is always facing the sun and reflecting light to be seen... the amount of this reflected sunlight that reaches us on Earth changes, depending on where the Earth is relative to the bright and dark side. This causes the phases of the moon
When the moon is in between the Sun and Earth, all the reflective side faces back to the sun and away from Earth (see picture 2 above). So we on Earth only see the dark side of the moon. It rises and sets close to the sun and cannot be seen. We call a new moon, and consider it the start of an orbit.
Between being in front of the sun and coming around to the side of the Earth (relative to the sun), the little angle means a little bit of the reflected light comes towards the Earth. Thus, we get a crescent moon. As it moves towards the side, the crescent gets wider and wider (more bright moon is visible) and so we call it a waxing crescent (getting wider and wider each night).
Once the moon is side on with the Earth, exactly half of the reflected light is in view from Earth. So we see a half moon. This phase is also called the first quarter because the moon is 1/4 of the way around its orbit.
As the moon continues on, between being side on, up until it is directly behind the Earth (relative to the sun), more than 50% of the bright side of the moon is within our view on Earth. So we see a gibbous moon. As it gets closer to being behind, the percentage of moon you see gets bigger and bigger and so we say the moon is waxing (getting bigger and bigger each night).
When the moon is directly behind the Earth (but a little higher or lower so as not to fall into the Earth's shadow and not get any sun), all the moon's reflected light shines back at the Earth. So we see a full moon.
After the full moon, the moon continues on to the other side of the Earth. As it moves towards the side, less and less of the bright side is seen so we call it a waning gibbous (waning = getting smaller each night).
Once the moon is side on with Earth again (on the other side now), we again see a half moon. Since the moon is now 3/4 through its orbit, we call it the third quarter.
As the moon makes its final return to being in between the Sun and Earth, less and less of the bright side is visible from Earth. We call this phase the waning crescent (waning = getting smaller each night). Eventually, it will return to being a new moon and the cycle begins again.
Read through the information above carefully and link it to the diagram above. Notice each time the top half of the moon is the bright side and the bottom half is the dark side as the sun is at the top. Notice the green lines divide the half of the moon visible to the Earth from the half of the moon not visible to Earth. Look at the ratios of bright and dark at each phase and now it links to the phase of the moon.
Between being in front of the sun and coming around to the side of the Earth (relative to the sun), the little angle means a little bit of the reflected light comes towards the Earth. Thus, we get a crescent moon. As it moves towards the side, the crescent gets wider and wider (more bright moon is visible) and so we call it a waxing crescent (getting wider and wider each night).
Once the moon is side on with the Earth, exactly half of the reflected light is in view from Earth. So we see a half moon. This phase is also called the first quarter because the moon is 1/4 of the way around its orbit.
As the moon continues on, between being side on, up until it is directly behind the Earth (relative to the sun), more than 50% of the bright side of the moon is within our view on Earth. So we see a gibbous moon. As it gets closer to being behind, the percentage of moon you see gets bigger and bigger and so we say the moon is waxing (getting bigger and bigger each night).
When the moon is directly behind the Earth (but a little higher or lower so as not to fall into the Earth's shadow and not get any sun), all the moon's reflected light shines back at the Earth. So we see a full moon.
After the full moon, the moon continues on to the other side of the Earth. As it moves towards the side, less and less of the bright side is seen so we call it a waning gibbous (waning = getting smaller each night).
Once the moon is side on with Earth again (on the other side now), we again see a half moon. Since the moon is now 3/4 through its orbit, we call it the third quarter.
As the moon makes its final return to being in between the Sun and Earth, less and less of the bright side is visible from Earth. We call this phase the waning crescent (waning = getting smaller each night). Eventually, it will return to being a new moon and the cycle begins again.
Read through the information above carefully and link it to the diagram above. Notice each time the top half of the moon is the bright side and the bottom half is the dark side as the sun is at the top. Notice the green lines divide the half of the moon visible to the Earth from the half of the moon not visible to Earth. Look at the ratios of bright and dark at each phase and now it links to the phase of the moon.
Period of the Moon
The time it takes for the moon to sweep around 360 degrees is 27.3 days. In this time, the moon will return to the same spot on the celestial sphere, as seen from Earth. This is known as a sidereal month.
More common however, is a synodic month, the time it takes for the Moon to go from one full moon to the next full moon. In the image above, note the green bit of angle represents the extra the moon has to travel to return to a full moon. This is due to the Earth moving along its orbit and thus causing the moon to have to rotate more than 360 degrees. The extra time it takes for the moon to get in front of the sun again is around 2 days, making a synodic month is 29.53 days. The very concept of a month is related to the moon's orbit (month = moon-th)
More common however, is a synodic month, the time it takes for the Moon to go from one full moon to the next full moon. In the image above, note the green bit of angle represents the extra the moon has to travel to return to a full moon. This is due to the Earth moving along its orbit and thus causing the moon to have to rotate more than 360 degrees. The extra time it takes for the moon to get in front of the sun again is around 2 days, making a synodic month is 29.53 days. The very concept of a month is related to the moon's orbit (month = moon-th)
The Moon Rotates on it's axis
Not only is the moon orbiting around the Earth, it is also spinning on its axis (like the Earth does). This allows each place on the moon to have night and day (just like on Earth). Unlike the Earth, which rotates at an angular speed of 360 degrees in 24 hours, the moon rotates much more slowly. The moon's rotational speed has been slowed down by the Earth itself. When a moon orbits close to its parent planet, the gravity of that parent planet constantly swinging it around on its orbit will slow the spin down until the time it takes to orbit is the SAME as the time it takes to rotate 360 degrees. So a day on the moon is 27.3 days, the same as the time it takes to complete an orbit around the Earth.
A consequent of this is that while each side of the moon gets the same amount of day and night (time facing the sun that is), one side is locked onto facing the Earth and the other side is always facing away from the Earth. So from Earth, we only ever see one side of the moon.
Not only is the moon orbiting around the Earth, it is also spinning on its axis (like the Earth does). This allows each place on the moon to have night and day (just like on Earth). Unlike the Earth, which rotates at an angular speed of 360 degrees in 24 hours, the moon rotates much more slowly. The moon's rotational speed has been slowed down by the Earth itself. When a moon orbits close to its parent planet, the gravity of that parent planet constantly swinging it around on its orbit will slow the spin down until the time it takes to orbit is the SAME as the time it takes to rotate 360 degrees. So a day on the moon is 27.3 days, the same as the time it takes to complete an orbit around the Earth.
A consequent of this is that while each side of the moon gets the same amount of day and night (time facing the sun that is), one side is locked onto facing the Earth and the other side is always facing away from the Earth. So from Earth, we only ever see one side of the moon.
The other side is sometimes inappropriately called 'the dark side of the moon' which is not right, it isn't dark if you lived on it, it is only 'dark' in terms of not being visible from Earth. It is better to call this side we don't see, the far side of the moon. Russian spacecrafts were the first to fly around the moon and photograph this side. The far side looks very different and is much more heavily cratered. Some people think this is due to it being on the 'outside' of the Earth/Moon system and thus getting more asteroids. This isn't true, asteroids can hit at any point. The real reason for the far side of the moon being more cratered is that there has been less volcanic activity on that side. On the near side of the moon which we see, the crust is thin and this has allowed lava to flow and solidify, covering up craters in its wake. The 'smooth' dark areas on the near side of the moon are called 'seas' because they were once thought to be oceans on the moon. What these 'seas' actually are are solidified lava flows that smoothed the surface. The far side of the moon, with a thicker crust, makes it harder for lava to come up to the surface and smooth out the heavily cratered ancient surface.
How craters are formed
Craters are formed when an asteroid, comet, or part of a comet, crashes into the surface of a planet or moon.
The size of the crater is normally bigger than the asteroid or comet itself.
When the impact occurs, there can be so much energy that the rocks around the impact site melt.
On the moon, there is a thin layer of soil that lies above the harder rocks below. When an impact occurs, that soil is blasted out in all directions, making the ray patterns that you can see and making craters kind of look like spiders.
You can make your own craters!
Craters are formed when an asteroid, comet, or part of a comet, crashes into the surface of a planet or moon.
The size of the crater is normally bigger than the asteroid or comet itself.
When the impact occurs, there can be so much energy that the rocks around the impact site melt.
On the moon, there is a thin layer of soil that lies above the harder rocks below. When an impact occurs, that soil is blasted out in all directions, making the ray patterns that you can see and making craters kind of look like spiders.
You can make your own craters!
Activities
Craters
Using a tub, flour, cocoa, and rocks, you can make craters! Please request to the teacher the lesson before so we can get it ready.
Make a Slide Show showing the phases of the moon using an overhead projector and a styrofoam ball
Imagine trying to teach a Grade 6 or a little sister or brother why we have phases of the moon. Using an overhead projector, a styrofoam ball, maybe another ball for Earth, and a camera (use your phone???), demonstrate the phases of the moon and the position of the Moon and Earth relative to the sun in each case.
Moon Documentary (40 minutes)
From the History Channel's The Universe Series, here is a documentary about the moon (in 10 minute parts):
https://youtu.be/aUT5Lsq9sP0?list=PLpO6WaxxmXtN2JjN58O_MLRCj2ApkFn9K
https://www.youtube.com/watch?v=KVZg99WB20E
https://www.youtube.com/watch?v=1GvGXwuRmnk
https://www.youtube.com/watch?v=n6zzyVFVHWw
A Day on Mars
The length of a day on your planet can be worked out by sky-watching from wherever you are.
Use Starry Night to travel to the surface of Mars, and then work out both a solar day and a sidereal day on Mars. Once you've done Mars, you can do it for all the other planets too!!!
A Year on Mars
The length of a year on your planet can be worked out by sky watching from wherever you are.
Use Starry Night to travel to the surface of Mars, and then work out how long a year on Mars is.
HINTS: Turn daylight off. Add a new time Time Step called Mars Solar Day which goes for 88775.2440 seconds and use this (that will ensure each step, the sun is back close to the meridian... it's the equivalent of only looking at the sun at noon each day and thus not having to worry about the rotation. Also, turn on Constellations:Zodiac and Constellations:Labels in View. Also, turn on Grid in Celestial Guides to make a grid on the celestial sphere that moves with the stars. This will help you to identify the exact location of the sun in the sky relative to the stars, rather than just getting roughly where it is based on the constellation it is in. Be as accurate as you can with location of sun and timing of how long it takes to return to that exact same location one Mars year later.
Once done, you can TRY working out a year of the other planets from their surfaces, but they can be trickier!!! Venus and Mercury have years shorter than their days, so that makes life very tricky. You'll have to think of a clever way to get around this!!! Also, Jupiter doesn't have an exact solar day because different cloud bands spin at different rates!!! For whatever location on Jupiter you are at, you'll probably need to calculate your own Solar Day using the same technique from the activity above, and then use this as your time step. Same with Saturn.
Find the Earth's Tilt and the Tilt of Other Planets
Whatever planet you are on, you can work out the tilt of your own home planet based on the sun's varying altitude on the meridian in the sky throughout the year. This PDF takes you through what to do: http://www.avilafm.com/tilt.pdf
Work Out How Much Later the Moon should rise the next day
Using some basic maths, you can work out WHY the moon rises at different times each day and even learn how you can predict when the moon will rise the next day. Check out http://www.avilafm.com/moonrise.pdf for more.
How long is a month?
Using Starry Night, calculate the time it takes for the moon to complete 1 sidereal month (so moon returns to same point on the celestial sphere - just like finding a sidereal year but doing it with the Moon instead of the Sun). Also find the time it takes for the moon to go from being IN LINE with the Sun (so directly above it or below it) and then falling behind and being caught by the sun around 1 month later). Be as accurate as possible. Compare with accepted values.
Craters
Using a tub, flour, cocoa, and rocks, you can make craters! Please request to the teacher the lesson before so we can get it ready.
Make a Slide Show showing the phases of the moon using an overhead projector and a styrofoam ball
Imagine trying to teach a Grade 6 or a little sister or brother why we have phases of the moon. Using an overhead projector, a styrofoam ball, maybe another ball for Earth, and a camera (use your phone???), demonstrate the phases of the moon and the position of the Moon and Earth relative to the sun in each case.
Moon Documentary (40 minutes)
From the History Channel's The Universe Series, here is a documentary about the moon (in 10 minute parts):
https://youtu.be/aUT5Lsq9sP0?list=PLpO6WaxxmXtN2JjN58O_MLRCj2ApkFn9K
https://www.youtube.com/watch?v=KVZg99WB20E
https://www.youtube.com/watch?v=1GvGXwuRmnk
https://www.youtube.com/watch?v=n6zzyVFVHWw
A Day on Mars
The length of a day on your planet can be worked out by sky-watching from wherever you are.
Use Starry Night to travel to the surface of Mars, and then work out both a solar day and a sidereal day on Mars. Once you've done Mars, you can do it for all the other planets too!!!
A Year on Mars
The length of a year on your planet can be worked out by sky watching from wherever you are.
Use Starry Night to travel to the surface of Mars, and then work out how long a year on Mars is.
HINTS: Turn daylight off. Add a new time Time Step called Mars Solar Day which goes for 88775.2440 seconds and use this (that will ensure each step, the sun is back close to the meridian... it's the equivalent of only looking at the sun at noon each day and thus not having to worry about the rotation. Also, turn on Constellations:Zodiac and Constellations:Labels in View. Also, turn on Grid in Celestial Guides to make a grid on the celestial sphere that moves with the stars. This will help you to identify the exact location of the sun in the sky relative to the stars, rather than just getting roughly where it is based on the constellation it is in. Be as accurate as you can with location of sun and timing of how long it takes to return to that exact same location one Mars year later.
Once done, you can TRY working out a year of the other planets from their surfaces, but they can be trickier!!! Venus and Mercury have years shorter than their days, so that makes life very tricky. You'll have to think of a clever way to get around this!!! Also, Jupiter doesn't have an exact solar day because different cloud bands spin at different rates!!! For whatever location on Jupiter you are at, you'll probably need to calculate your own Solar Day using the same technique from the activity above, and then use this as your time step. Same with Saturn.
Find the Earth's Tilt and the Tilt of Other Planets
Whatever planet you are on, you can work out the tilt of your own home planet based on the sun's varying altitude on the meridian in the sky throughout the year. This PDF takes you through what to do: http://www.avilafm.com/tilt.pdf
Work Out How Much Later the Moon should rise the next day
Using some basic maths, you can work out WHY the moon rises at different times each day and even learn how you can predict when the moon will rise the next day. Check out http://www.avilafm.com/moonrise.pdf for more.
How long is a month?
Using Starry Night, calculate the time it takes for the moon to complete 1 sidereal month (so moon returns to same point on the celestial sphere - just like finding a sidereal year but doing it with the Moon instead of the Sun). Also find the time it takes for the moon to go from being IN LINE with the Sun (so directly above it or below it) and then falling behind and being caught by the sun around 1 month later). Be as accurate as possible. Compare with accepted values.