Where does the shadow on the moon come from?

16 July 2006

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Question

Sitting in my kitchen which faces west, I can see the sun well up in the sky and I can also see the moon. But the moon still had a shadow on it. Where did the shadow come from if I can see the whole disk of the moon?

Answer

The reason for that is that the Moon is at an angle to the light arriving from the Sun, so only part of the lunar surface - the part facing the Sun - is illuminated. Imagine shining a torch at a football - you would see the surface of the ball facing the torch illuminated and a curved edge to the illuminated patch. Behind this the ball would be poorly- or un-lit. The Moon's the same. And because it orbits the Earth, taking 1 month to do so, at different points in its orbit more, or less of the surface is facing the Sun, so the illuminated zone enlarges or shrinks accordingly. When there is a "new" Moon, that moon is between us and the Sun, so none of the surface that we can see is being lit at all, so it's invisible against the dark sky.

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