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Why is the shadow on the Moon the shape it is?

Why is it that during some phases of the moon (1st quarter and 3rd quarter) show a crescent shadow and during mid phase the shadow is a straight line?

This is kind of a neat question and it's to do with illuminating spheres and the angles that you look at them with respect to the light source. You could try to an experiment with some help from your friends if you have a ball and a flashlight - it is probably the best way to understand it.

One of you has to be the sun (the one with the flashlight) the other the moon (the one with the ball) and the third person is the Earth( you don't get to hold anything!). If the line between the "Sun" and the "Moon" is at a right angle to the line between the "Moon" and the "Earth" (ie. if the 3 of you stand at 3 corners of a square), half of the "Moon" will be illuminated and the shadow will look like a straight line. When the Sun moves forward or backwards from this postion more or less of the bit of the Moon that can be seen from the Earth will be lit up and the shadow will now be curved.

I hope that explains it OK. Try it out of you can - try to use quite a large ball and do it in a darkened room. I think that it should work OK and it should be fun anyway! It's difficult to work in 3D so playing with models can help! It's a tool many scientists use to understand what's going on.

November 2000, Karen Masters (more by Karen Masters) (Like this Answer)

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