# Thread: How come the shadow earth is casting on the moon is exactly the same size of the moon itself?

1. ## How come the shadow earth is casting on the moon is exactly the same size of the moon itself? var addthis_config = {"data_track_clickback":false};

How is it that earth's shadow covers the moon in such a perfect way? If you think about it, the shadow could have been smaller or bigger. Is it just a coincidence, or is there an astronomical explanation to this?

2. Actually the Earth's shadow is quite a lot bigger than the Moon.

3. The shadow is certainly NOT the same size as the moon! It is actually about 3 times as wide. See the following (from NASA's Eclipse website) for example: http://eclipse.gsfc.nasa.gov/OH/OHfigures/OH2011-Fig03.pdf

4. Short answer? It's not. Check out the link below to see the size of the Earth's shadow at the Moon's distance.

5. It isn't!

The Earth casts a far larger shadow than the moon.

An Earth eclipse on the moon is not what causes the phases of the moon that you see on a monthly basis. These are caused by the angle that the light from the sun appears to hit the moon. Imagine someone walking in a circle around a tennis ball shining a torch on it. The person with the torch (the sun) sees a full moon all the time. But if you are off to one side you will see the light on the ball appear to go through the phases of the moon.

6. Actually, if you REALLY think about it, and check actual data, you realize it's not perfect at all. The Earth's umbra IS larger than the Moon at the distances at which the Moon orbits. The penumbra is FAR larger.

Note that the inclination of the Moon's orbit means that the Moon usually misses Earth's shadow (umbral or penumbral) entirely. Not "perfect" at all.

7. It's not.

8. The Earth's shadow is significantly larger than the moon, as can be seen from the curvature of it as it passes over the face of the Moon, and the fact that a total lunar eclipse lasts several times longer than a solar eclipse.

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