Expert: Courtney Seligman - 3/13/2009

Question
Why does the moon look so much larger (in size) when it is rising in the East (when its a full moon) than it does when it is over head?

Thanks

Answer
This effect (called the Moon illusion) is due to the apparent difference in distance between the (distant) horizon and the sky overhead.

When we see things that are far away, they look smaller than things that are closer. This is a well-known fact. But another fact, not so well-known, is that our brains slightly compensate for this effect, by 'seeing' things that are known to be far away as being larger than their actual appearance on the retina. As an example, if you look at distant mountains, they look relatively small, because of their distance; but you still have an impression that they are really big, because you know they are far away. In a photograph taken of the same mountains, the camera doesn't make this compensation, and they look smaller in the photograph, than when you actually look at them.

In a similar way, when we see the Moon overhead, or on the horizon, we take into account its apparent distance. The sky overhead appears to be only a few thousand feet to a few miles in height, because that's how high clouds are, when they are above us. But the distant horizon appears to be miles or even tens of miles away. As a result, when the Moon is overhead, our brains assume that it is closer than when it is on the distant horizon -- not in a conscious way, but in a primal way, which probably goes back to when we needed to estimate how far away lions were, from how big they looked.

Since the Moon is very far away, its size on our retina is about the same whether it is on the horizon or overhead. But since it feels closer overhead and further away on the horizon, and is the same size on our retina (more or less) in either position, our brain compensates for the apparently greater distance when it is on the horizon by 'seeing' it as being much larger. In other words, if it looks just as big to our eyes, but is farther away on the horizon, then our brain insists it must really be larger.

You can see this most effectively if you go to a place where the horizon appears to be even farther away than normal -- out in the vast open spaces of the desert or prairie, or on top of a mountain. If you see the Moon rising on such a horizon, it looks even bigger than it does when seen in a normal landscape.

I should note that this explanation is one of dozens proposed by psychologists over the last century or so, and many people believe it is only partially correct. But based on my experience with the apparent size of the Moon on a planetarium dome, I'm convinced that this is the best explanation. On the small dome of a planetarium, the brain 'sees' the projected 'Moon' as being very close, and perceives it as being very small. To compensate for this, the Moon (and Sun) have to be projected at four to five times their correct size to 'appear' the 'right' size on the dome.

Courtney Seligman
Professor of Astronomy
Long Beach City College