The other side of the Moon

  • #1
Summary:
The other side of the moon.
I have learned that the moon has rotation as well the Earth, so why we can see only one side of the moon? With the moon's rotation, at some point, we must see all sides. Can anyone explaine why it does not happens?
 
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  • #3
phinds
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Summary:: The other side of the moon.

I have learned that the moon has rotation as well the Earth, so why we can see only one side of the moon? With the moon's rotation, at some point, we must see all sides. Can anyone explaine why it does not happens?
For some reason, that misunderstanding seems to be quite common. It is, of course, exactly as orodruin pointed out but it seems to be pretty common that people can't easily picture it.

The easiest way to see it, I think is this: hold a ball out at arm's length and with your arm held stiffly out in front of you turn yourself around 360 degrees. Your head is the Earth and the ball is the moon. You only ever see one side of the ball. BUT ... someone standing off to the side sees all sides of the ball because it IS rotating. They also see that your nose is always pointed at the same point on the ball. That's tidal locking **.

** it should be noted, however, that the Earth doesn't have a nose.
 
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  • #4
Janus
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Also, there is libration.
While the Moon rotates at a constant angular velocity, it doesn't orbit so. Because its orbit is a bit elliptical, its orbital speed varies over the orbit. Thus, its rotation and its orbital speed don't stay exactly matched. So, as seen from the Earth, the Moon "rocks" from side to side a bit as it orbits, allowing us to see more that just 50% of its face.
Also, the Moon's rotational axis is tilted by about 6 degrees to its orbit. Thus when it is on one side of the Earth we see more of its North pole, and when it is on the other side, more of its South pole.

This shows How this motion looks over the course of a month ( it also shows how the angular size of the Moon changes as it distance from the Earth varies over it orbit.)
https://en.wikipedia.org/wiki/Libra...ion_with_phase_Oct_2007_(continuous_loop).gif
 
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  • #5
sophiecentaur
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Summary:: The other side of the moon.

I have learned that the moon has rotation as well the Earth, so why we can see only one side of the moon?
The facts about the two rotations producing the Moon always presenting the same face to us all the time are written above. The mechanism that locks the Moon's motion that way is because the Moon and Earth are not perfectly solid spheres. They both distort a bit as they go round each other. That asymmetry is what causes the right forces to lock the smaller body to face 'inwards' all the time. In the long run, we expect the Earth to end up facing the Moon all the time and for the Moon to orbit further away. (I'm talking Billions of years here so don't throw your Tide Tables away just yet.)
 
  • #6
Hey, guys! Seems like soon we will find out more about dark side of the Moon! I recently read about this: China's lunar program, also known as the Chang'e mission, in addition to relatively routine tasks like taking samples of lunar soil, also has a completely unprecedented goal: to land on the other side of the Earth's satellite. If China succeeds, it will become the world's first country to do that. The landing of the Chang'e-IV descent vehicle on the lunar surface will take place in two stages. First, a satellite station will be thrown behind the Moon, which will provide communication with the Earth, and only then the rest of the mission - with the descent vehicle and the rover - will go to the dark side of the Earth satellite, which will land on its surface.
 
  • #7
phyzguy
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Hey, guys! Seems like soon we will find out more about dark side of the Moon! I recently read about this: China's lunar program, also known as the Chang'e mission, in addition to relatively routine tasks like taking samples of lunar soil, also has a completely unprecedented goal: to land on the other side of the Earth's satellite. If China succeeds, it will become the world's first country to do that. The landing of the Chang'e-IV descent vehicle on the lunar surface will take place in two stages. First, a satellite station will be thrown behind the Moon, which will provide communication with the Earth, and only then the rest of the mission - with the descent vehicle and the rover - will go to the dark side of the Earth satellite, which will land on its surface.
Please don't call it the "dark side". It is light 50% of the time and dark 50% of the time, just like the side that faces the Earth.
 
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  • #8
Ok, I won't, my bad :rolleyes: I am not good at physics and I came here to learn new and I am glad to hear an answer from Science Advisor:smile:
 
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  • #9
Ibix
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As phyzguy notes, both sides of the Moon are light half the time. I think "dark side" of the moon is supposed to refer to its mysterious nature because it's not visible from Earth. But since we've now got detailed photographic surveys and satellites parked where they can see it the term has fallen out of favour. That won't stop some sources using it, but it's probably more confusing than not.

We'll see what the Chinese mission tells us. I think the comms satellite is probably the most interesting thing long term, assuming it's built to last.
 
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  • #10
Orodruin
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As phyzguy notes, both sides of the Moon are light half the time.
Well ... Technically the side facing us is dark slightly more than the far side (which is a more appropriate name) due to Lunar eclipses only occurring when that side would otherwise be lit. :wink:
 
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  • #11
sophiecentaur
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  • #13
sophiecentaur
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But Gary Larson is better...
Horses for courses - GL is Far Side. Floyd are Dark Side. Both excellent in their way
 
  • #14
jim mcnamara
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Okay. We got a bit out hand.... thread locked.
 
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