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Name: Marian
Status: student
Grade: other
Country: Australia
Date: Spring 2012

Someone said that our full moon on 7/4/12 was the biggest moon for the year, if this is so, why? How is it that the apparent size of the moon changes?

I am not certain about exact dates. It is true, however, that the Moon's orbit about the Earth is not a perfect circle, but an ellipse. The Moon is closer to the Earth at some points in its orbit than at others. If it is full when it is close, it appears bigger in the sky.

Richard E. Barrans Jr., Ph.D., M.Ed. Department of Physics and Astronomy University of Wyoming


I do not know about 7/4/12 but I have seen reports that the full moon last 3/18 - 19/12 was the largest observed in 20 years (sorry, I did not actually observe this myself). This was because the moon was in perigee - a point in the moon's orbit that takes it closest to the Earth, and this coincided with a full moon. The moon's orbit around the Earth is not circular, it is elliptical. This means that there will be times that it will be a little bit farther or closer depending on the coincidence of the Earth and Moon orbits as well as the occurrence of the full moon. The moon can appear as much as 14% larger during perigee.

Greg (Roberto Gregorius) Canisius College

Dear Marian,

Good question. The Moon does change its apparent size because its distance from, the Earth is constantly changing as it orbits the Earth. If full Moon occurs near when the Moon is closest to Earth, at perigee, then the Moon will appear somewhat larger.

Sincerely David H. Levy

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