More lunar astronomy (Stop me when you've had enough!)

When the phase of the Moon is right (as it was last night) the lunar surface can be seen to be girdled by hundreds of bright rays. On examination, these emanate from a number of obvious craters. These rays consist of ejecta thrown up during the formation of the craters. The Moon's gravity being only a sixth of the Earth's, and the Moon having virtually no atmosphere, this material travelled a considerable distance. In the case of Tycho (which has the most impressive system) one of its rays passes upwards through the Sea of Serenity, through the crater Bessel and keeps going to the pole! Since this ejecta overlies seas, craters and other features, it must have been hurled out by comparatively recent impacts: in the case of Tycho, probably around 105 million years ago (but possibly considerably less!) The rays are even more obvious in negative...