Lunar Perigee and Apogee. Although the Moon may look the same each time it is full, there is actually quite a difference in its size throughout the year. The Moon's orbit around the Earth is an elipse, so at some times it is full when it's close to us, and other times it's full when far away in its orbit. The closest part of its orbit is called perigee, and the distant part of the orbit is called apogee. Both of the photographs of the Moon above were made with exactly the same photographic equipment; the difference in size is because the October Moon was 53,000 km closer in its orbit at perigee compared to its apogee.
The Moon also wobbles slightly in its orbit, an effect called libration. The May Moon shows some craters in the north that can't be seen in the October Moon, showing that the May Moon was tilted slightly forward in the north.
This photograph was published in the Journal of the Royal Astronomical Society of Canada, Aug. 2008 ed.
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