The Copernican system - the most recent of the four periods of lunar geological activity - takes its name from this magnificent crater. Impacts during this period, which began about 3.9 billion years ago, are generally much smaller than earlier ones. Copernicus itself represents the upper size limit for craters formed during the last billion years.

Photo: Thierry Legault. The full size original of this and other high definition lunar images may be seen at his High Resolution CCD Imaging site.

Inconstant Moon