We have to confess that we occasionally send friends a link to “let me Google that for you” when they ask us something that they could have easily found online. Naturally, if someone asked us how big the moon is, we’d ask Google or another search engine. But not [Prof Matt Strassler]. He’d tell you to figure it out yourself and he would then show you how to do it.
This isn’t a new question. People have been wondering about the moon since the dawn of human civilization. The ancient Greeks not only asked the question, but they worked out a pretty good answer. They knew approximately how big the Earth was and they knew the moon was far away because it is seen over a very wide area. They also knew the sun was even further away because the moon sometimes blocks the sun’s light in an eclipse. Using complex geometry and proto-trigonometry they were able to work out an approximate size of the moon. [Matt’s] method is similar but easier and relies on the moon occluding distant stars and planets.
[Matt] explains that a distant sphere illuminated by a distant light source will cast a shadow on a plane about the size of the sphere. What’s more, is that anyone on the plane in the shadow who can’t see the light source must be in the shadow, which allows you to measure the shadow, and that gives an approximation of the sphere’s size. For this to work, the light source needs to be at least ten times further away from the plane than the sphere is. The further away the light source is, the lower the error in t ..
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