Why does the Moon have phases?
Moon has phases because it orbits Earth, which causes the
portion we see illuminated to change. The Moon takes 27.3
days to orbit Earth, but the lunar phase cycle (from new
Moon to new Moon) is 29.5 days. The Moon spends the extra
2.2 days "catching up" because Earth travels about 45
million miles around the Sun during the time the Moon
completes one orbit around Earth.
At the new
Moon phase, the Moon is so close to the Sun in the sky that
none of the side facing Earth is illuminated (position 1 in
illustration). In other words, the Moon is between Earth and
Sun. At first quarter, the half-lit Moon is highest in the
sky at sunset, then sets about six hours later (3). At full
Moon, the Moon is behind Earth in space with respect to the
Sun. As the Sun sets, the Moon rises with the side that
faces Earth fully exposed to sunlight (5).
You can create a mockup of the relationship between Sun,
Earth, and Moon using a bright lamp, a basketball, and a
baseball. Mark a spot on the basketball, which represents
you as an observer on Earth, then play with various
alignments of Earth and Moon in the light of your imaginary