The Moon's rotation: not-so-useful info

[Guest guest]

Hi,

 

If you go out on several different nights and look at the Moon, you will always

see the same features, at about the same position. It looks as if the Moon

doesn't rotate! But it does; in fact, if it didn't, you'd have seen all of it!

Grab two cricket balls (or whatever roughly spherical objects you have handy).

Mark one with an "X"; this represents a feature on the Moon. Now put the other

one down on a table; this is the Earth. Place the Moon model on the table about

one foot away with the X facing the Earth model. Now move the Moon model as if

it were orbiting the Earth, taking care to make sure that the X faces the Earth

model at all times. Surprise! You'll see that to keep the X facing the Earth

model, you have to rotate the Moon model as it goes around the Earth model.

Furthermore, you can see you have to spin it exactly once every orbit to keep

the X facing the Earth model. If you don't rotate it, the Moon model will show

all of its "sides" to the Earth model as it goes around.

 

Now, I have been a bit tricky here. We are talking about two different frames of

reference; one on the surface of the Earth looking out at the Moon, and one

outside the Earth-Moon system looking in. You performed the experiment from the

latter frame, and saw the Moon rotating. From the former, however, you can see

for yourself the Moon doesn't rotate. What is being argued here is that in one

frame the Moon rotates, in another it does not.

 

There are three things to be learnt here:

 

1) the Moon rotates as it orbits the Earth (as seen by an outside observer); 2)

it rotates one time for every orbit (to that observer); and 3) like I said

earlier, if it didn't rotate, we would eventually see all of the Moon as it

orbited the Earth.

 

We actually can see a bit more than just the one face of the Moon. Because the

Moon's orbit is not a perfect circle but an ellipse, its spin and rotation

don't exactly match up. This means that sometimes the spin lags behind the

orbital speed, and sometimes it moves ahead. This in turn means that sometimes

we can "peek" around a bit onto the far side of the Moon. This is called

"libration". You can see it yourself! If you happen to observe the Moon a week

after perigee (closest point to the Earth) and then two weeks later, a week

after apogee (farthest point from the Earth) you can see that the face looks

like it has rotated a bit. This is easiest to see with binoculars or a small

telescope. It's very hard to see with the naked eye, but remember, our ancient

astronomers knew about this effect long before the invention of the telescope!

 

Ramapriya

hubli (AT) vsnl (DOT) com