The first quarter moon

Tonight, the Moon is at First Quarter: that means that roughly half of the side facing the Earth is illuminated by the Sun. It should look something like the figure at left, which is an image taken by Frank Barrett of celestialwonders.com.

Most of the interesting features on the Moon are those near the terminator, the line between the light and dark regions. Near the terminator, the Sun casts long shadows and throws all surface features into strong relief. Some items to check out tonight are:

1. Aristoteles and Eudoxus, two craters in the northern regions. Aristoles is about 87 km in diameter; can you discern the terraced texture of its inner walls? Eudoxus also has terraced walls, but is only 67 km across.

2. Menelaus (named after an Alexandrian geometer, not the Spartan noble of the Illiad) is small enough, only 27 km across, that it has a relatively simple shape: sharp walls which rise 3 km above a smooth floor.

3. Sacrobosco is named after an English mathematician of the thirteenth century. Three large craterlets hide inside it.

4. Maurolycus is large enough, about 114 km in diameter, that we call it a "walled plain" instead of a simple crater. You may be able to see its central peak as well as several smaller craters on its floor.