Getting oriented

Setting the stage

The Moon in a small telescope is rich and complex; under high power in a Dob, 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. Since the Moon always keeps the same side facing towards the Earth, 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” (think: “moonth”). This means that, except at 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 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). Because the Sun is so low along the horizon at this time, even the lowest hills will cast long, dramatic shadows.