May 1999

Why does the Moon look so big when it rises?

You don't need to be an astronomer to see it. Imagine: the distant horizon acquires a mysterious radiance, then a gigantic glowing orb of a Moon ascends above the mountains. The Moon looks absolutely huge! But last night, when you saw it higher up in the sky, it looked like its normal, smallish self. What's going on here?

This is one of my favorite questions, and I've heard it several times while conducting the Nightly Observing Program here at Kitt Peak. People notice the same effect, whether it is the Moon or Sun, rising or setting. When the darn thing is near the horizon, it looks tremendous. First, it's fairly obvious that the Moon is not any farther away from us when it's higher in the sky. In the few hours it takes for the Moon to go from the horizon to high in the sky, the Moon itself doesn't really go anywhere. Instead, it is the Earth's spin that makes the Moon seem to move at all.

(If you want to get technical, in the six hours it takes for the Moon to appear to move from horizon to its highest point, it will move east in its orbit about three degrees. If the Moon appears high overhead, then you are on the part of the Earth that is closest to the Moon. If the Moon appears on the horizon, you are on a part of the Earth that is a bit farther away. How much farther? Less than an Earth radius; maybe one percent of the total distance to the Moon.)

In fact, the Moon's changing size is just an illusion. Really! If you hold your thumb out at arm's length, you will find that the Moon's apparent diameter is about half the width of your thumbnail, regardless of whether the Moon is low or high. Our lead observer, Adam Block, has a suggestion for the next time you see a giant Moonrise: turn around, bend over, and look at the Moon through your legs. Seeing it upside-down completely destroys the illusion (but be careful who you try this around, as you are sure to leave a lasting impression).

So what is the nature of this illusion? I have heard that there are several answers. Here I would like to describe a common one, which is my favorite. Consider this diagram.

It shows a couple of blocks sitting on some train tracks. If you take a quick glance, the upper block just might look a little bigger than the one below it, even though they are exactly the same size. The upper block's position against the train tracks makes it look like it is farther away than the lower block, so it should appear smaller.

But it doesn't.

Instead it appears the same size. Therefore it must really be a bigger block than the lower one. After reaching this conclusion, your brain makes the appropriate adjustments to your perception, and suddenly the upper block actually looks a bit bigger.

Is it possible, then, that the Moon looks bigger when it is near the horizon because it seems to be farther away than when it is overhead? Near the horizon, the Moon seems at least as far away as all those distant mountains. Overhead... well, there's nothing overhead, except maybe some stars, and we have no reference for how far away those are. It can't be that far, can it? Perhaps a lifetime of living indoors convinces us, on some primative neurological level, that the Moon above our heads really shouldn't be farther away than a light on the ceiling.

If so, then we have our answer. We can't really tell if the Moon is a small, close object or a big, far object. The Moon overhead appears small because we believe it should be close by, and the Moon near the horizon looks big because your brain thinks it is far away.

Steve White
Nightly Observing Program
Kitt Peak Visitor Center

P.S. I just found a great link that explains this mystery very well, and provides some interesting details. - S

This picture of the moon rising over the Visitor Center Center Observatory was taken by Bill Shunn and Laura Chavoen during the NOP on May 29, 1999!

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Updated: 5/29/99