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2017 Eclipse Blog / FAQ

Moon shadow??

Sherry asked 4 months ago

If the moon’s shadow is only 70miles? I find that difficult to understand because then its shadow can only get bigger not smaller. When an object has light shining on it and behind the object is casting a shadow on the ground, it never gets smaller. It gets bigger or the exact same size as the object. 

1 Answers
Admin Staff answered 4 months ago

Your experience is based on the light source (light bulb) being smaller than the object casting the shadow.  In the case of an eclipse, the Sun (light source) is MUCH larger than the Moon (shadow-casting body), and so the umbra of the Moon is a cone shape that starts out as big as the Moon, but then tapers down to a point as it reaches toward the Earth.

In this animation, you note that the umbra (cone) diminishes to a small area, but there IS in fact a larger “shadow” visible – this is called the penumbra, and it represents the locations where SOME of the Sun is being blocked by the Moon – a partial eclipse!


The geometry is well known, and eclipse calculations have been performed with great precision since the early 1800s.  The eclipse WILL occur as predicted!

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