The Moon is Earth's natural satellite and our easiest celestial neighbor to observe in detail. Its appearance changes throughout the month because we see different portions of its sunlit half as it orbits Earth.
Why phases happen
The Moon does not change shape. The phase depends on the angle among the Sun, Earth and Moon. A full cycle of phases takes about 29.5 days. From new Moon through waxing crescent, first quarter, waxing gibbous, full Moon, waning gibbous, last quarter and waning crescent, the cycle repeats.
Best time to see surface detail
A full Moon is bright, but its directly illuminated face can look surprisingly flat. Near first or last quarter, shadows are long along the terminator—the boundary between lunar day and night—and craters can stand out beautifully.
What you are looking at
The Moon's dark areas, called maria, are ancient plains of cooled lava. The brighter highlands are older and heavily cratered. Binoculars can reveal strong contrast between these regions without requiring a telescope.
Keep an observation record
Note the date, approximate phase, cloud conditions and a quick sketch. Repeat a few nights later and compare the location of the shadow boundary.