Click for a full view. It is recommended that you download the image and print a copy for yourself.

You need to compare the size of the moon with the size of shadow. Since Earth is a sphere, you should expect its shadow to be round. Inspect the image carefully, can you see it? For the non-astronomers (including myself), this may necessitate a fair amount of spatial organization in your head. Think about the positions of the Sun, Earth, and Moon during a lunar eclipse. The moon is covered by the shadow of the Earth.

Click here for a diagram of the original eclipse image.

Now, you could estimate the size of the shadow by eye-balling it, but we can be more precise using a little geometry. Click HERE for a quick lesson.

All you need to ask yourself is if the diameter of the shadow is the same size as the diameter of the Earth. Why? Suppose the diameter of the shadow is the same distance as the diameter of the Earth. Can you determine the size of the Moon now? You just determined the diameter of the Moon relative to the diameter of the shadow. If you make this assumption (which is not a correct one) then, knowing the size of the Earth, you can determine the size of the Moon.

One last correction: The size of the shadow is not the same size as the Earth. Why not? Because the shadow tapers off slightly. The shadow of the Earth (as seen in the lunar eclipse image) is actually one Moon diameter SMALLER than the size of the Earth. Click HERE for more information.

Click HERE for my answers to challenge one.

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