ChatterBank1 min ago
Why Can't We See The Far Side Of The Moon?
What I know (in an idiots fashion.)
The moon takes 28 days to orbit the Earth and 28 days to do one Moon rotation.
The moon can always be seen somewhere from Earth. weather permitting.
I see how it works if you treat the earths orbit like a clock face, spinning 9 o'clock,3 o'clock,9 o'clock,3 o'clock (East/West and the moons orbit as spinning 12 o'clock,six o'clock,12 o'clock,six o'clock.North/South, but I've been informed we both spin East to West (9 o'clock,3 o'clock,9 o'clock,3 o'clock)
If we run an imaginery line North/South around the moon and then split the face as we see it as A and B then logic says the splits hidden will be C+D.
My tiny brain then says the moon should have completed quarter of a moon rotation and quarter of an Earth orbit after 7 days,half a moon rotation and Earth orbit after 14 days,3/4 a moon rotation and Earth orbit after 21 days and back to where it started after 28 days.
I can't get my brain round, how after 7 days, we're not seeing sectors D and A, and after 14 days we're not seeing sectors C and D, and after 21 days we're not seeing sectors B and C.
After 28 days we're back to seeing sectors A and B.
I've tried looking at simulation sites but it still bamboozles me.
Can someone explain in simple terms or recommend a good simulation website.Sarcastic comments always welcome :-)
The moon takes 28 days to orbit the Earth and 28 days to do one Moon rotation.
The moon can always be seen somewhere from Earth. weather permitting.
I see how it works if you treat the earths orbit like a clock face, spinning 9 o'clock,3 o'clock,9 o'clock,3 o'clock (East/West and the moons orbit as spinning 12 o'clock,six o'clock,12 o'clock,six o'clock.North/South, but I've been informed we both spin East to West (9 o'clock,3 o'clock,9 o'clock,3 o'clock)
If we run an imaginery line North/South around the moon and then split the face as we see it as A and B then logic says the splits hidden will be C+D.
My tiny brain then says the moon should have completed quarter of a moon rotation and quarter of an Earth orbit after 7 days,half a moon rotation and Earth orbit after 14 days,3/4 a moon rotation and Earth orbit after 21 days and back to where it started after 28 days.
I can't get my brain round, how after 7 days, we're not seeing sectors D and A, and after 14 days we're not seeing sectors C and D, and after 21 days we're not seeing sectors B and C.
After 28 days we're back to seeing sectors A and B.
I've tried looking at simulation sites but it still bamboozles me.
Can someone explain in simple terms or recommend a good simulation website.Sarcastic comments always welcome :-)
Answers
Does a simple picture help?
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The Moon rotates on its axis anticlockwise, period approx 28 days. The Moon also orbits Earth in an anticlockwise direction, period approx 28 days. Therefore an observer on Earth will always see he same face of the Moon (not the case when observing Earth from the Moon)
Due to libration, one can usually see about two thirds of the face of the Moon during a year.
Due to libration, one can usually see about two thirds of the face of the Moon during a year.
Try this completely different thought experiment...
Draw a large circle on the ground, 10 or 15 feet across. This is going to be the orbit of the 'moon'.
Get a friend and stand him in the centre. He is going to be the 'earth'.
Stand on the circle, facing anticlockwise, and start walking on the line of the circle. Your friend is allowed to turn round and round if he wants to, at any speed he likes, which is the earth's rotation (and completely irrelevant, as you will see).
After one complete circuit, you will also have turned round once, just like the moon does every 28 days. Ask your friend if he could ever see your right side and shoulder! No, all he could ever see was your left side and shoulder.
Draw a large circle on the ground, 10 or 15 feet across. This is going to be the orbit of the 'moon'.
Get a friend and stand him in the centre. He is going to be the 'earth'.
Stand on the circle, facing anticlockwise, and start walking on the line of the circle. Your friend is allowed to turn round and round if he wants to, at any speed he likes, which is the earth's rotation (and completely irrelevant, as you will see).
After one complete circuit, you will also have turned round once, just like the moon does every 28 days. Ask your friend if he could ever see your right side and shoulder! No, all he could ever see was your left side and shoulder.
The planet Mercury is also "tidally locked" to the Sun. Tidal locking eventually comes to all orbting bodies. It is caused by the gravitational forces stretching the bodies in the direction of the "pull". (This is called the "tidal effect" and can be seen quite clearly on the Earth by the rise and fall of the sea). This distorts the bodies and causes their rotation to slow down. It effects both bodies in the system and eventually the earth and moon will settle into "tidal unison". Eventually the Earth and Moon will have exactly the same rotational period, and this will also exactly equal the orbital period meaning both bodies will present the same face to each other for ever.
Hold a ball (or any object) at arms length while turning (yourself) completely around 360 degrees in place. The side of the ball facing you does not change throughout because its rate of rotation precisely matches its rate of revolution around you.
What difference (if any) do you perceive between how the ball appears as it moves around you to how the Moon appears as it orbits the Earth? If you turn your head to the right or left (to simulate Earth's rotation), does that change the side of the ball facing you?
What difference (if any) do you perceive between how the ball appears as it moves around you to how the Moon appears as it orbits the Earth? If you turn your head to the right or left (to simulate Earth's rotation), does that change the side of the ball facing you?
ingo1327
Question Author
If i hold the ball at arms length and i turn in a circle I will always see the same side of the ball...but if...as i turn in a circle, the ball in my hand also slowly turns i will eventually see the other side.That's what my brain is telling me
18:42 Thu 17th Jan 2013
But the ball does turn, (once per revolution) not your hand but your hand. If the ball did turn . . . then you would see the other side.
Question Author
If i hold the ball at arms length and i turn in a circle I will always see the same side of the ball...but if...as i turn in a circle, the ball in my hand also slowly turns i will eventually see the other side.That's what my brain is telling me
18:42 Thu 17th Jan 2013
But the ball does turn, (once per revolution) not your hand but your hand. If the ball did turn . . . then you would see the other side.
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