Why does the Moon have phases?
The 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 Sun.
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