Tuesday, December 17, 2013

Smallest Full Moon of 2013 is Tonight

Tonight, the Moon is going to be as small as it will get at Full stage the entire year. When the Moon reaches Full phase today, it will be roughly two and a half days from apogee, the farthest point from Earth in the Moon's orbit. So, what else of this 'Mini Moon?'

As was first discovered in the early 1600s by
 Johannes Kepler, all orbiting bodies move in elliptical (slightly elongated) orbits, the Moon is no exception. The fact the lunar orbit is elliptical is the root of the whole 'mini Moon' event that will happen today. Because of the elliptical orbit, the Moon is not always the same distance from Earth, but a varying distance that can change by as much as about 27,000 miles. Tonight, the Moon will come to a point in its orbit that will bring it very close the farthest point in its orbit, a position called apogee.

In practical terms, the fact that the Moon is farther than usual will not amount to much. In a telescope, however, things will be different to an experienced observer. As seen in a telescope, the Moon will be slightly smaller than it is when it is full at perigee (the point in its orbit closest from Earth), especially when put side by side in a composite photograph.

Oh, yes, next month's Full Moon will be a true apogee Moon, with Full taking place less than two hours from apogee, meaning that the January Full Moon will be about as small as a Moon can get.

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