It is possible that the single most delightful observing session can happen when there is a penumbral eclipse of the Moon. Such an event is as far removed from a total solar eclipse as one can get, and still have an eclipse of some sort. In a penumbral lunar eclipse, the full moon enters, travels through, and then exits the outer shadow of the Earth. This part of the shadow is called the penumbra, and its effects can range from absolutely nothing to, at best, a slight darkening of one edge, or limb, of the Moon. For almost all observers, that is all. For Thomas Hardy, whose wonderful poem “At a Lunar Eclipse” appears at the start ofChapter 13, the total eclipse he saw in 1903 was preceded by a penumbral shading.

However, for me there is much more. At no other time do the rays surrounding the younger craters, like Tycho and Copernicus, appear so obviously. These big craters are certainly not young by human standards; Tycho was formed about 100 million years ago by the sudden impact on the Moon of a comet or an asteroid, and Copernicus is a bit older, but both are young geologically. The Moon's oldest craters were formed probably by impacts mostly during a period about 3.9 billion years ago, and is remembered today as the period of late heavy bombardment.