Starts With A Bang!

The Moon looks huge!!

Have you ever seen the Moon shortly after it rises, and had it look huge to you? Maybe you’ve seen something like this:

Or maybe, if you don’t happen to be living by the Temple of Poseidon in Greece, it looks more like this:

And yet, when the Moon gets higher in the sky, it appears much smaller. Here’s the bizarre thing, though. When I photograph the Moon with a camera, it doesn’t appear any bigger at the horizon than anywhere else in the sky; on film, it looks to be the same size everywhere.

What’s going on? I think it’s pretty clear that your eyes are playing tricks on you, but how? The answer (from NASA) is that we still don’t know. Regardless of where the Moon is, it forms the same size image on your retina.

But there are two possible explanations, and they both have to do with the fact that seeing is relative. Why don’t you listen to both of them, and then tell me which one you like better:

1. We don’t know where the sky is. When we look at the horizon, we think of “the edge of the sky” as being farther away than when we look up. This is called the “flattened sky” model.

2. Trees and temples and whatever’s on the horizon looks like it converges to smaller and smaller sizes on the horizon, so that something that’s the same angular size looks bigger based on its surroundings. Here’s an example with train tracks; both yellow lines are the same size, even though the top one appears larger; this is called Ponzo’s illusion.

Finally, if you don’t like either of these explanations, there are other alternatives here from a retired professor, here from two psychologists, and some further explanation from a physicist.

My take? I think it has to do with the fact that we perceive things based on our surroundings. When the Moon is close to the horizon, we recognize that it’s much farther away to the objects that are seen relative to it, and yet is the same size as a tree or a temple. In the sky, all you have is clouds, if that. There’s nothing relative to compare it to, but at the horizon there is. If you want proof that it isn’t an atmospheric effect, look at the horizon Moon through your fingers like this, so you can’t see anything except the Moon:

and you’ll see that it appears to shrink! Neat, hmm?