Moon at Apogee and Perigee -- How Big Was That Supermoon?

July 24, 2013


Photographer: David K. Lynch; Dave's Web site
Summary Author: David K. Lynch

The Moon’s orbit around the Earth is not circular, but rather slightly elliptical. As a result, the Earth-Moon distance changes during the lunar month. At closest approach (perigee) it's 225,291 mi (362,570 km) away, while at its farthest (apogee) it's 251,910 mi (405,410 km) distant. Thus, the full Moon appears larger in the sky at different times, from 29.74 arc minutes in diameter at apogee to 34.06 arc minutes at perigee. The distance changes cause the perigee Moon to be about 25 percent brighter than the apogee Moon.

The dates of apogee and perigee aren't in synch with the phases of the Moon, and only occasionally coincide with the new or full Moon. On June 23, 2013 perigee occurred very close to the time of the full Moon. This happens about once a year, but even then, the times of perigee and full Moon don't exactly coincide, differing by usually a few hours.

The picture above shows the effects of the Moon’s elliptical orbit. Just after new Moon, which was close to apogee, the slender crescent and earthshine were photographed. About half a lunar month later at perigee, the Moon was full and noticeably larger. This comparison was easily done because the same camera and telescope were used for both pictures.