Around the time of the full moon, Tycho's ray system is illuminated to best effect, and can be traced nearly 1000 miles across the lunar surface. When the Moon is a thin crescent, the light reflected from the Earth (which would appear full as seen from the Moon) creates a similar - but fainter - illumination, and some of the stronger rays can be seen on the dark part of the Moon's disc.

Photo: António Cidadão. His Home Page of Astrophotography includes stereograms, animations, and "Seven Craters a Week" from his forthcoming photographic lunar atlas.