1. Newbie
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Mar 2011
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## Moon Rotation

We know that we see only one side of moon... Moon is locked in synchronous rotation with the Earth, and always keeps one side toward us... What i cant get around or i dont know is... does the moon rotate around itself or is it stationary in that manner and just rotates around earth...?

2. Originally Posted by rudra
We know that we see only one side of moon... Moon is locked in synchronous rotation with the Earth, and always keeps one side toward us... What i cant get around or i dont know is... does the moon rotate around itself or is it stationary in that manner and just rotates around earth...?
Welcome to the message board, rudra.

I'm not sure what you are asking. You seem to have answered your own question. In inertial space the Moon rotates on its axis during exactly the same period that it revolves around the Earth: 27.32 days. If you were to view the Moon from the Sun or another planet, you would see the Moon rotating on its axis. An astronaut remaining in one spot on the Moon would experience night and day, buy the entire process would take nearly a month.

The situation is illustrated beautifully in another thread on this message board: http://www.bautforum.com/showthread....Moon-movements.!!!

3. Order of Kilopi
Join Date
Dec 2004
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12,198
Originally Posted by rudra
We know that we see only one side of moon... Moon is locked
in synchronous rotation with the Earth, and always keeps one
side toward us... What i cant get around or i dont know is...
does the moon rotate around itself or is it stationary in that
manner and just rotates around earth...?
The Moon rotates. It doesn't add information to say it rotates
"around something". You can tell it rotates because an observer
sitting still on the surface would see the distant stars move
across his field of view. And that is, of course, because the
direction he is facing is constantly changing.

However, there are two locations on the Moon's surface -- the
rotational poles -- where all the stars appear to go around the
zenith without rising or setting. An observer lying on his back
at either of the poles would be facing in the same direction
all the time. A star directly overhead would stay there. But
the orientation of his body would still be constantly changing.

You could reasonably say that the Moon rotates around the
"axis" which is defined by the two poles. This is different in
principle from something which rotates around an "axle", though
there might not be any practical difference. An observer on a
wheel rotating aound a fixed axle could see the same thing as
an observer on a freely-rotating body like the Earth or Moon.

similar to how the Earth rotates and also revolves about the
Sun. The Earth and the Moon are revolving about the Sun
together, as a system.

Although the Earth and the Moon have separate rotations,
with completely different periods, they can be considered
as a system to have a rotational period equal to the period
of the Moon's revolution around the Earth. In this way,
rotation and revolution can be difficult or impossible to
distinguish from each other. The Milky Way galaxy, a
collection of billions of stars, can be said to have a period
of rotation of about 220 million years at the Sun's distance
from the center. Each individual star in the galaxy has its
own period of revolution around the galaxy's center.

A person very distant from the Milky Way would not be able
to see any individual stars, so would not be able to measure
their periods of revolution, but they could see the rotation of
the system of stars as a whole, and measure its period.

Same thing for a hurricane. Each individual air molecule
revolves around the hurricane's center, but it is only the
rotation of the whole air mass that is observed.

Please let me know if that helps or is confusing!

-- Jeff, in Minneapolis

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