The moonlight we see on Earth is sunlight reflected off the Moon's grayish-white surface.
The amount of Moon we see changes over the month — lunar phases — because the Moon orbits Earth
and Earth orbits the Sun. Everything is moving.
When sunlight hits off the Moon's far side — the side we can't see without from Earth the aid
of a spacecraft — it is called a new moon.
When sunlight reflects off the near side, we call it a full moon.
The rest of the month we see parts of the daytime side of the Moon, or phases.
These eight phases are, in order, new moon, waxing crescent, first quarter, waxing gibbous,
full moon, waning gibbous, third quarter and waning crescent.
The cycle repeats once a month (every 29.5 days).
The phase of the Moon is defined as the angle between the Moon and the Sun along the ecliptic.