What causes the Moon's phases
Sunlight normally illuminates about half of the Moon. As the Moon orbits Earth, we see different portions of that illuminated half, producing its phases. An ordinary crescent is therefore not caused by Earth's shadow; that shadow is involved in a lunar eclipse, which requires a more specific alignment.
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The lighting stays, but our viewing angle changes
The Sun lights the Moon from one side. As the Moon changes position around Earth, the relationship between the Sun, Moon and observer changes.
Near new Moon, the illuminated side largely faces away from us. Near full Moon, it largely faces towards us. Between those positions, we see a mixture of the sunlit and unlit portions.
A phase is a viewing geometry, not a piece of the Moon being covered. The dividing line between lunar day and night is called the terminator.
- A crescent shows a small part of the illuminated half.
- A quarter Moon presents roughly half its visible face illuminated.
- A full Moon presents nearly the whole visible face illuminated.
The phase cycle takes about 29.5 days because the Earth-Moon system also moves around the Sun while the Moon completes its orbit.