chi and h site logo

{ Practical astronomy | Astronomy | The Moon | Perigee and apogee }

Perigee and apogee

Moon near perigee and near apogee
Moon near perigee and near apogee.

The revolution of the Moon around the Earth is not in a circular orbit. Half the orbit, the Moon is closer than average, half the orbit further away. The montage of two images of the Moon illustrates how its apparent size changes. On the left, it is near its closest to the Earth. Half a month later (right), it shows not only the opposite phase, but is also near its furthest distance from Earth. The distance – and hence apparent diameter – varies about 7.5% either way from the average. These two images were in fact not taken during the same month. The image on the right is from 2005-02-18 and the one on the left from 2007-04-13.

When perigee falls close to Full Moon, this is called by some a "supermoon". Due to its larger size, it is 0.15 mag brighter than the average Full Moon. Casual observers will notice neither the increased size nor the increased brightness.

This phenomenon must not be confused with the optical illusion that a Moon on the horizon appears to the human brain larger than a Moon high in the sky. The opposite is true: When the Moon is in the zenith, the observer is 1.7% closer to the Moon than when it rises on the horizon.

Physical parameters:

The phases of the Moon are easy enough to observe. For this project, the images have to be taken with the same camera, same lens and same focal length. The image scales are then the same and the size comparison becomes possible. A variation of the project is to catch both perigee and apogee at Full Moon and show them side by side or as a positive/negative overlay.

Image parameters: