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jimmy
2003-Dec-24, 12:19 AM
Seems unnatural to me that the moon isn't spinning, please enlighten!

Littlemews
2003-Dec-24, 12:54 AM
Our moon is spining just look carefully....

Matthew
2003-Dec-24, 12:54 AM
The moon does rotate. Its just that the orbital time of the moon, and the rotation time are the same. The same side of the moon does not always face Earth. Both the near side and the dark side face Earth, but the dark side is almost always in darkness, when looked upon from Earth. At maximum we can oly see 7% of the dark side.

zephyr46
2003-Dec-24, 12:57 AM
Thats a great question, I can't wait to hear from somebody who knows.
It does spin, but it's day is as long as it's year, if that makes any sense ?

A nice picture of the moon, from APOD (http://antwrp.gsfc.nasa.gov/apod/ap020316.html)

http://antwrp.gsfc.nasa.gov/apod/image/0203/moon_gal_big.jpg

jimmy
2003-Dec-24, 01:18 AM
Thanks matthew, but I'd like to have an explanation why?

DippyHippy
2003-Dec-24, 01:58 AM
Matthew's right, the Moon takes exactly the same amount of time to orbit the Earth as it does to spin once on it's axis - in this way, the same side is always kept towards the Earth and it only appears not to spin. In fact, this is a very common phenomenon in the solar system - for example, only 2 of Saturn's 30 satellites spin at a different rate from their orbits.

Basically, the Moon used to be a lot closer to the Earth but over the course of billions of years, it has slowly drifted further away from us. As it does so, it's actually gradually slowing down both it's own spin and the spin of the Earth until eventually the Earth and Moon will become tidally locked. When that happens, the Moon's orbital and spin period will exactly match the spin period of the Earth - which by that time will probably be about a month (I think :blink:) The Moon will then appear to hang over one area of the Earth - like a geostationary satellite - and one half of the Earth will never see the Moon at all.

It's difficult to explain how the illusion of the Moon's phases works without the aid of a diagram but if you want to know more, you might be interested in reading this. (http://www.sciencedaily.com/encyclopedia/Moon_phase)

Matthew
2003-Dec-24, 04:48 AM
Thats a great question, I can't wait to hear from somebody who knows.
It does spin, but it's day is as long as it's year, if that makes any sense ?

A nice picture of the moon, from APOD

Great pic, but isn't it a bit multi colored?

kashi
2003-Dec-24, 10:00 AM
I never knew that DippyHippy. You learn something new everyday I guess!

Tinaa
2003-Dec-24, 05:40 PM
Here is an exercise that may help you, it did help me. The moon does spin very slowly. Take a ball and hold out in front of you. You are the Earth and the moon is the ball. As you rotate in a circle, the ball rotates too, only it just makes one rotation, which is why we only see one side of the moon. I had a hard time visualizing it until I did it myself. If you want to do further, have a bright light source shining as you turn and you can see the phases of the moon. my kids thought it was cool!

TheThorn
2003-Dec-24, 08:39 PM
There have been some very good explanations of the effect here, for instance:

[/I]"Matthew's right, the Moon takes exactly the same amount of time to orbit the Earth as it does to spin once on it's axis - in this way, the same side is always kept towards the Earth and it only appears not to spin."[I]

But, as always, if yo go a bit deeper, it's more complicated than that.

The moon's orbit is not circular. At apogee it is about 405,000 km away from the Earth, and at perigee it is only about 363,000 km away. That means that it varies noticeably in apparent size through the month 10% or so). But it also means that it doesn't exactly keep the same side facing us all the time.

Anything in orbit moves more slowly when it is near apogee than when it's near perigee (Kepler's 2nd law). The result of this is that while the moon is near perigee it moves around it's orbit faster than it rotates on its axis, and when it's near apogee it moves around it's orbit slower than it rotates. They call this libation, and it means we get to see parts of the "back side" of the moon every month.

One of the most impressive gif's I've ever come across on the 'net illustrates this effect with time-lapse photography of the moon covering a whole month:

APOD: 2003 August 10 - Lunation (http://antwrp.gsfc.nasa.gov/apod/ap030810.html)

It's slow to load, but worth every second.


As to "why" - tides. Just like the moon raises tides in the earth's oceans, the earth raises tides in the moon's rocks. Much bigger tides because the earth is much more massive. These tides are locked in place right now, because the moon rotates once per month, but back when the moon rotated more than once per month, these tides moved through the lunar crust causing friction which created heat in the crust. The energy for that heat has to come from somewhere, and it came from the rotation of the moon. So its rotation gradually slowed down until it matched it's orbital period, and the tides no longer moved through the crust.

DippyHippy
2003-Dec-26, 12:36 AM
That's a very impressive image Thorn - thanks for posting that :)

TheThorn
2003-Dec-26, 02:48 AM
You're welcome. I was impressed by it myself, and thought it was worth sharing.

damienpaul
2003-Dec-26, 02:55 AM
it certainly was Thorn, very impressive,