The Moon Doesn't Grow at the Horizon. Your Brain Does the Inflating

Feed the Reader

18 hours ago

5 min read

You've probably seen it. You're driving home, or walking the dog, and there it is: a full Moon just clearing the rooftops, huge and golden, looking close enough to hit with a tennis ball.

An hour or two later you look up again, and it's back to normal. A small, bright coin high in the sky.

Here's the thing. Nothing about the Moon changed in between. Take a photo of it at moonrise and another when it's high overhead, with the same zoom setting, and line them up. The Moon is the same width in both. The giant was never there. It was something your brain made.

Two explanations that don't work

A common explanation is the atmosphere. Near the horizon, you're looking at the Moon through a lot more air, and surely all that air acts like a magnifying glass.

It's an old idea. Aristotle suggested something like it more than 2,300 years ago, comparing it to the way things look bigger under water. But it's wrong. The atmosphere doesn't magnify the Moon. If anything, it does the opposite. Light from the Moon's lower edge passes through more air than light from the top, and that bends it slightly, so a Moon sitting on the horizon can look a touch squashed, flatter top to bottom. Squashed, not bigger.

The second guess is that the Moon is closer when it's low. It isn't. When the Moon is overhead, you're standing on the side of the Earth nearest to it. When it's on the horizon, you're off to one side, a little further away. About one and a half per cent further, according to NASA. So the horizon Moon is, if anything, very slightly smaller in your eye.

So the Moon isn't bigger, and it isn't closer. Which leaves one suspect. You.

The size you see isn't measured. It's estimated

To see why, it helps to think about what the eye actually receives.

When you look at the Moon, it takes up a certain slice of your view. Astronomers call this its angular size: roughly, how wide it looks from where you stand. Hold your arm straight out and the full Moon is about the size of a fingernail. That slice stays the same whether the Moon is low or high.

But your brain doesn't report the slice. It reports a size. And to turn one into the other, it has to guess how far away something is. A car that takes up a fingernail's width might be a toy on the table or a real car down the street. Your brain decides which, mostly from clues about distance, and then tells you how big it is.

With the Moon, those distance clues go haywire.

Theory one: a flattened sky

The first plausible explanation came from Ibn al-Haytham, a scholar working in the Islamic world in the eleventh century, in his great book on optics.

His idea goes like this. We don't experience the sky as a perfect dome over our heads. We experience it as a flattened one, like an upturned bowl that's been sat on. The bit overhead feels nearer, and the bit at the horizon feels further away. Part of the reason is that when you look towards the horizon, there's a whole landscape in between, fields and buildings and trees, stretching away into the distance. Overhead, there's nothing.

So when the Moon sits at the horizon, your brain places it on the far-off edge of that bowl. Same slice of your view, but "further away". And something that takes up the same slice while being further away must be bigger. Your brain does the sum and serves up a giant Moon.

It's an elegant idea, and versions of it have been repeated ever since. But it has a snag. Ask people which Moon looks closer, and many say the big horizon one. That's the opposite of what the theory needs. One psychologist who has studied the illusion calls the apparent-distance explanation popular but inadequate. Another researcher still finds the flattened-sky version of the idea more promising.

Theory two: the railway tracks

The second family of explanations leans on the scenery itself.

There's a well-known illusion called the Ponzo illusion. Draw two lines that converge towards a point, like railway tracks disappearing into the distance. Then place two identical bars across them, one near the narrow end and one near the wide end. The bar near the narrow end looks longer, even though the two are exactly the same.

Your brain reads the converging lines as depth, assumes the upper bar is further away, and inflates it.

The horizon gives the Moon the same treatment. Clouds, planes and ships all shrink as they get closer to the horizon, because they're further away. The Moon doesn't. So, one suggestion goes, the brain compensates by inflating it. Trees, rooftops and hills along the skyline may add to the effect, giving your brain something to compare the Moon against.

It's a popular explanation. But it can't be the whole thing either, because NASA notes that astronauts in orbit, with no rooftops or trees in the way, see the illusion too. As one NASA account puts it, there's likely more going on.

Try it yourself

The lovely thing about the Moon illusion is that you don't have to take anyone's word for it.

Next time you see a big moonrise, hold your arm out straight and cover the Moon with a fingernail. Then do the same thing later, when the Moon is high. It's the same fingernail both times.

Or roll a sheet of paper into a tube and look at the Moon through it, so you can't see the horizon or anything else around it. Cut off from its scenery, the giant Moon shrinks back to normal.

And if you don't mind looking a bit odd, NASA suggests one more. Turn your back to the Moon, bend over and look at it upside down, between your legs. Many people find the horizon Moon suddenly looks much smaller.

Still an open case

So which theory is right? Honestly, nobody's completely sure.

There's broad agreement on the important part. The Moon doesn't change size, the atmosphere doesn't magnify it, and the effect happens in our heads. Beyond that, researchers still disagree about exactly how. NASA's own scientists describe it as unsolved, with experiments still being designed to pin it down. The flattened sky, the Ponzo-style scenery and other ideas are best thought of as candidates. It may well be several of them at once.

That's quite something for an effect people have been noticing for more than two thousand years.

What the giant Moon tells us

Here's the bit worth remembering. We tend to think of seeing as a camera: light goes in, a picture comes out. But size isn't something your eyes simply measure. Your brain works it out, by combining what lands in your eye with a best guess about how far away things are.

Most of the time that guess is so good you never notice it's happening. The rising Moon is one of the rare moments when you can catch your brain in the act, making a confident estimate and getting it gloriously wrong.

So the next time a huge Moon hangs over the rooftops, enjoy it. Just remember that the most impressive thing in that view isn't the Moon. It's you.