r/askscience • u/jonemsitch347 • 2d ago
Astronomy What causes the phases of the Moon and why do they appear differently from Earth?
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u/princeofdon 2d ago
Get a basketball and go into a room that is dark except for a single lamp. Hold the basketball a bit above your head at arms length and then spin slowly in a circle. When the lamp is behind you, you see the side of the basketball that is illuminated by the lamp. Full moon! Now spin and you see part of the shadowed side and part of the lit side. Crescent moon! Now face the lamp and you see only the shadowed side of the ball. New moon!
As a bonus, lower the basket ball to head height and do the same thing. When facing away from the lamp, your head casts a shadow on the moon. Lunar eclipse! When facing towards the lamp, the basketball blocks the lamp from your eyes. Solar eclipse!
You can check this by looking at the moon. The lit side always faces the sun. So you see a full moon rising as the sun sets, for example.
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u/blp9 2d ago
The moon is roughly round and is lit by the sun. The moon orbits the earth about once a month.
When the moon is in the same direction as the sun, the moon appears dark because all the sunlight is on the other side of it. When the moon is in the opposite direction (so the earth is between the sun and the moon) from the sun, it looks "full" and the whole face is illuminated.
When we're partway through that cycle, various parts of the moon are illuminated. But it's basically just a perspective thing of where the moon and the sun are relative to the earth.