moon

[merejeans]

I know it takes approx 28 days for the moon to orbit the earth, and also spin on its own axis, but how long does it take to come back into exactly the same position, say, where I saw it last night at 11 pm.
I am sure I heard years ago that it takes a very long time, is this so.

[fo3nix]

What do you mean by "exactly"?

If you want to see it accurately to some very small length scale (say, the diameter of a golf ball), then you may never see it again in exactly the same position.

[mibn2cweus]

"The Moon's orbit is inclined 5.145396° with regard to the ecliptic (the plane in which the Earth's orbit around the Sun lies or, more precisely, the plane in which the centre of gravity of the Earth-Moon system [its barycentre] orbits the Sun), so as seen from the centre of the Earth the Moon drifts up and down slightly more than five degrees in the course of each orbit. The dark grey wedge shows the limits of the Moon's excursion above and below the plane of the ecliptic.

"The Moon's orbital inclination, combined with the inclination of the Earth's axis of rotation, causes the Moon's declination, as observed from the Earth, to vary between ±28.5° when the Moon's inclination adds to that of the Earth, and ±18° when the two inclinations oppose one another; the maxima and minima of declination repeat every 18.6 years, the period in which the ascending node of the Moon's orbit precesses through a full circle."

Copied from here: http://www.fourmilab.ch/earthview/moon_ap_per.html

Drawing here: http://en.wikipedia.org/wiki/File:Lunar_perturbation.jpg

Full Moon Declination Simulator: http://astro.unl.edu/classaction/animations/lunarcycles/fullmoondec.html

Note that this only applies to the range where a full moon might appear.

Lunar standstill: http://en.wikipedia.org/wiki/Lunar_standstill

Moon Data Applet: 15:34 Thu 01st Oct 2009

[mibn2cweus]

Moon Data Applet: http://www.jgiesen.de/moondata/index.html

Thanks to AB, you might need to manually Copy/Paste any broken links into your address bar.