Science Questions

Sat, 9th Sep 2006

Part of the show Hot Nectar, Warming Weather and Birds Missing the Spring

Question

Roger in Clacton asked:

My wife was out on Friday evening and noticed that the Moon was much larger than normal. Is there a reason for this?

Answer

If the Moon was low in the sky, the reason is due to an optical illusion. When the Moon is high up in the sky you've got nothing in your visual world to compare it with size-wise, so your brain attributes it to being very small. There's nothing in the foreground, you're looking at empty sky and the Moon looks very far away. But when the Moon is near the horizon, then you're seeing the Moon close or in contact with things in the foreground such as buildings, pylons, trees or even people. This fools your brain into thinking that because there's something in the foreground that it does know the size of, it compares it with the Moon and assumes that it must be much larger than it is.

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