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Published online by Cambridge University Press: 30 October 2009
You don't need a book to tell you to look at the Moon with your telescope. It is certainly the easiest thing in the nighttime sky to find, and it is probably the richest to explore. But it can be even more rewarding to observe the Moon, if you have a few ideas of what to look for.
Getting Oriented: The Moon is rich and complex in a small telescope; under high power, you can get lost in a jumble of craters and all the mare regions seem to meld together. So the first thing to do is to get oriented.
The round edge of the Moon is called the limb. The Moon always keeps almost exactly the same side facing towards the Earth. Its apparent wobbles (called “librations”) are small; craters near the limb always stay near the limb.
The Moon goes through phases, as different sides take turns being illuminated by the Sun. The whole sequence takes about 29 days, the origin of our concept of “month”. This means that, except for Full Moon, the round disk we see will always have one part in sunlight, one part in shadow. The boundary between the sunlit part and the shadow part is called the terminator.
The terminator marks the edge of between day and night on the Moon. An astronaut standing on the terminator would see the Sun rising over the lunar horizon (if the Moon is waxing; or setting, if it's waning).
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