When the moon wobbles, how long does it take?? It happens 4 or 5 times per year?

Answer 1

I am looking at the astronomical significance of "megalithic" observatories (circles of large stones) such as Stonehenge.

I am reading that some of the alignments can only be explained if the builders already knew (thousands of years ago) about a wobble of 9 arcminutes with a period of 173 days. Yet, 9 arcminutes is a very small wobble:

The Moon's apparent diameter in the sky is a little over 30 arcminutes, so 9 arcminutes is less than one third the size of the Moon. I guess it can be seen without telescopes with careful observations over long periods. But it is not comething that jumps at you (like an eclipse, for example).

I do not know what causes the wobble. This is not libration which is a one month cycle caused by the fact that the Moon's orbit is elliptical: the Moon's speed on its orbit varies over a one month period but the Moon's rotation, on its axis, is very regular (one turn per month with no periodic change).

I have asked my bookstore to find me papers by Alexander Thom (an astronomer who was the expert in these megalithic structure).

Answer 2

Wobbles? I don't think the Moon wobbles, although I suppose it might have a long, long cycle of axis-wobbling akin to the axis precession of the Earth, which persists in 22,000-year cycles.

The Moon takes about 27 days to revolve around the Earth and takes the time to rotate once.

It's orbit about the Earth is slightly elliptical (like all celestial orbiting bodies). At perigee, it's closest point, the Moon lies 363,300 km away from the Earth. At apogee, it's furthest point, it's 405,500 km away.

Answer 3

Hi. The 'wobble', also called libration, is caused by the angle that the Moon's orbital plane has relative to the Earth. It allows us to see more than 50% of the Moon's surface. http://en.wikipedia.org/wiki/Libration . It actually happens continuously.