Half is always lit

About this video

Look at the Moon on a few evenings in a row. One week it is a thin crescent, a week later a half circle, a week after that a whole bright circle. Then it thins again, until for a few nights you can't see it at all. These changing shapes are the Moon's phases: new moon, crescent, half moon, full moon, and back. But the Moon is a ball of rock, and rock doesn't grow and shrink.

The Moon makes no light of its own. The Sun lights it, and the Sun always lights one half of it, the half facing the Sun. The other half is the Moon's night side. So the Moon never really changes: what changes is how much of its lit half we can see from the Earth, and that depends on where the Moon is on its trip around the Earth, about once a month. When it is between the Earth and the Sun, its lit half faces away from us: new moon. A quarter of the way round, we see half of the lit half: a half moon. On the far side of the Earth from the Sun, the whole lit half faces us: full moon. That is also why a full moon rises just as the Sun sets.

Many people think the dark part of the Moon is the Earth's shadow. It isn't: at a half moon the Earth's shadow points away from the Sun, nowhere near the Moon. From one full moon to the next takes about 29 and a half days, and the word month comes from moon. In the southern half of the Earth a growing crescent is lit on its left side. This video goes with the lesson Phases of the Moon in Astronomy Level 1 on Knaph.

Transcript

1. 0:00Night after night, the Moon changes shape. New moon, crescent, half moon, full moon. But the Moon is a ball of rock, and rock doesn't grow and shrink.
2. 0:13The Moon makes no light of its own. The Sun always lights one half of it, the half facing the Sun. The other half is its night side.
3. 0:25The Moon goes around the Earth about once a month. When it is between the Earth and the Sun, its lit half faces away from us. New moon. A quarter of the way round, we see half of its lit half. A half moon. On the far side of the Earth, its whole lit half faces us. Full moon. Then less and less, back to new moon.
4. 0:56Many people think the dark part is the Earth's shadow. It isn't. At a half moon, the Earth's shadow points away from the Sun, nowhere near the Moon. The dark part is the Moon's own night side.
5. 1:10At full moon, the Sun and the Moon are on opposite sides of the Earth. So as the spin carries you into night, the Sun sets on one side, and the full moon rises on the other.
6. 1:24From one full moon to the next takes about twenty-nine and a half days. People counted time this way for thousands of years. The word month comes from moon.
7. 1:38In the southern half of the Earth, the Moon looks the other way round. A growing crescent is lit on the left. The phases still come in the same order.
8. 1:50The Sun always lights half of the Moon. We see more or less of that half.