Moonrise in Nashville. Photo by Stephen Marr, used by permission.

Feb 25, 2015 Inconstant Moon

Sen—Sometimes a single picture serves multiple purposes. In this case, the photograph above actually illustrates three different and delightful phenomena. Even better, two of them are real, and one isn’t.

The photo was taken by Stephen Marr, and shows the Moon rising over the Nashville airport in October of 2013. He took a series of short snapshots, waiting a few minutes between each one, then simply added them together using Photoshop. The result is quite lovely!

The most obvious thing to notice in the photo is the changing color of the Moon. When it was just over the horizon it was a dull reddish-orange, becoming more yellow as it got higher, and then shone a silvery white. You’ve probably seen this yourself many times; the rising (or setting) Moon can look distinctly reddish. The Sun does this, too.

There are a few steps to this process. The first thing to know is that different wavelengths—colors—of light behave differently. Our atmosphere isn’t really transparent to light. Blue light from the Sun, for example, comes in and smacks into nitrogen molecules, bouncing away in all directions. This effect is called scattering, and has a very strong dependence on color; blue light is about 15 times more prone to scattering than red.

That’s why the sky is blue! Blue light from the Sun gets bounced around, and appears to come from everywhere in the sky, even though it started off coming in directly from the Sun. That colors the sky blue.



The next part of this is that the Earth’s atmosphere surrounds it in a shell a hundred or so kilometers high (it goes up higher, but thins so rapidly that we can call this good enough). That means that when you see an object straight above you, the light from that object passes through less air than when it’s on the horizon. The diagram here shows that; as the air curves around with the Earth, the air you look through gets longer.

So when the Moon (or Sun) is on the horizon, its light is passing through more air, giving the blue light more chance to scatter. Even green light can scatter away, leaving only the redder light to get through. Dust and haze in the air also absorb bluer light, accentuating the effect.

Voila! Red Moonrise. That’s also why the Moon gets brighter as it rises; there’s less junk in the air filtering the light.

But then there’s the Moon shape. Why is it squashed when it’s on the horizon?

That too is due to the air being curved around the Earth. When light enters Earth’s air from space, its direction changes slightly due to refraction. This is the same effect that makes a spoon or straw look bent when it sits in a glass of water.

The amount of bending is essentially zero straight up, but near the horizon it increases rapidly. The light can be bent so much that you can see the Moon even after it’s physically below the horizon! The amount of bending changes so rapidly when the Moon is near the horizon that the light from the bottom part of it bends more than the light from the top, squashing it. As the Moon rises, the effect diminishes, and eventually our satellite becomes more round.

Both those effects are real, and measurable. But I said there were three effects… and the last one is not real. It’s the famous Moon Illusion, when the Moon looks huge when it’s on the horizon, so big you could swear you could touch it.

This really is an illusion, and this photo proves it! Measure it for yourself. The horizontal diameter of the Moon doesn’t change as it rises. Yet it really does look bigger when you see it for yourself.

This illusion is powerful, but an illusion it is. The cause is subtle, but goes like this: We perceive the sky as an inverted bowl, as if the sky at the zenith is closer to us than the horizon. When we see something on the horizon, our brain interprets it as being farther away. But if it’s farther away, and we can see it so plainly, it must be physically huge! At least, that’s what our brains think. It’s similar to the Ponzo Illusion, where your sense of distance is thrown off due to forced perspective.

In fact, that’s why Marr took the photo in the first place. He had read about the Moon Illusion, and wanted to prove for himself that the Moon is the same size as it rises. And so he did.

Science! Sometimes, seeing is believing. And sometimes it isn’t. And sometimes you have to find out for yourself.