november: perigee and apogee

When Nicholas Copernicus suggested that the planets, including the Earth, revolve around the Sun, and that the Moon alone revolves around the Earth, most disagreed with him. It is easy to think, with the benefit of hindsight, that people were stupid to not accept his theories, but this would be unfair. The problem was that the observed positions of the planets just did not match his predictions.

It took Johannes Kepler, many years later, to discover what was happening: whilst Copernicus thought the planets travelled in circular orbits, Kepler realised that their paths were ellipses.

The clues were there, though. The Moon's elliptical orbit means that every month it passes through both perigee and apogee. The apparent size of the Moon's disc at these two times can be dramatically different, as this illustration, based on Galileo images, shows.

This effect has nothing to do with the Moon illusion, which is a purely psychological phenomenon.

Image: Galileo Project, JPL, NASA.