Is our Moon unique?

[BGOLDIE]

Hi all,
I've been thinking lately about our moon and thought it strange how it's orbit time exatly matches it's rotation (so we only ever see the one side) and that it's axis must be exatly 90 degrees to it's orbit (or we would see some of the dark side at different times) and that it has been like this for eons.
Is our moon the only known moon that does this?
Also, does our moon appear to be the same size as the sun because the sun is 400 times bigger than the moon and the sun is 400 times further away from earth than the moon is?
Thanks
Goldie

[supernova1965]

Good question I know we can see small parts of the dark side at times but I don't know the answer to that and look foward to the answer. And that it is rare to only have one moon even Mars has two and we are at the special time when it is in the perfect place to have total eclipse's in the future and in the past the moon wont be in the right place to see this great event:D O yeah and welcome to IIS:welcome:

[[1ponders]]

From memory I think Io is the same. Ie is in synchronous orbit (tidally locked)

[mental4astro]

Hi Goldie,

:welcome: to IIS, mate.

The moon showing only one face to us is a phenomenon called 'tidal locking'. It is somewhat unique that its period matches its day so well. I would say it is a feature of the formation of the Moon with the current popular theory of the formation of the Moon being the result its formation being due to a catystrophic collision between the Earth and another body similar in size to Mars.

The Moon shows about 59% of its total surface to us due to a rocking motion called 'libration'. The link below is stunning in how this rocking manifests over one Lunar day:

http://commons.wikimedia.org/wiki/File:Lunar_libration_with_phase_Oct _2007_450px.gif

I vaguely remember that the are a couple of other Moons, either of Jupiter or Saturn, or both, that show this tidal locking. I'm not entirely sure today. Good question.

There is one, almost creepie, Lunar feature called Mare Orientalis, the Eastern Sea, first identified with the first Soviet lunar orbiter that showed us the far side of the Moon. Due to libration, Mare Orientalis is just visible to us, but not really identifieable as seen in orbiter photos.

That the Moon appears as big as the sun is also a freakish coincidence. But this too fluctuates as the Moon's orbit around the Earth isn't cirular. When it is at its furthest, the Moon appears to sit within the Sun's disk, making the Sun's appear bigger than the moon. This is called an 'annular eclipse':

http://www.hampsteadscience.ac.uk/astro/galleries/eclipses/eclipse_solar_annular_05-10-03_pearce.jpg

Mental.

Mental.

[mental4astro]

There you go! Ta Paul!!! I thought so.

[Jazza]

It's because of Tidal Locking.
Both bodies (moon and earth) gravitationally attract each other. This causes a tidal 'bulge' on the side of the body facing the other. Since the Earth is more massive than the Moon, the effect is greater on the Moon. Essentially the moon looses angular momentum due to the bulge, and slows down until the rate of roatation matches the rate of movement of the bulge around the moon (which is the same rate the Moon orbits the Earth).

The same thing but taking more time is occurring with the Earth, so eventually it will be tidally locked to the moon too, and the same side of the Earth will always face the same side of the moon.

Hope that makes sense!

EDIT: haha beaten to it :)

Jay

[OzRob]

That is only true for the current time. The Moon is very slowly moving from us. In the past it appeared larger and in the future it will be smaller. At some point (I can't remember how many years it is) there will be no more total eclipses as the Moon will be too small to cover the Sun's disk. Only annular eclipses will occur.

Nice libration video..:thumbsup:

[BGOLDIE]

Thanks people for your replies, and thanks to Mental for the links. :thanx:

[Robh]

A full list of moons that are tidally locked can be found here ...
http://en.wikipedia.org/wiki/Tidal_locking
You are correct about the size and distance of the Sun being about 400 times that of the Moon. Hence, its similar apparent size.

Regards, Rob