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The Moon Illusion

"Wow, the Moon's even bigger than that tree"

You’ve probably all seen it before, a huge Full Moon sitting on the horizon. Time and again I have had people ask me why the Moon is so much bigger some times than others, and the answer is: it isn’t, really.

The Moon orbits the Earth in an elliptical orbit, meaning that it is not always the same distance from the Earth. The closest the Moon ever gets to Earth (called perigee) is 364,000km, and the furthest is ever gets (apogee) is around 406,000km (these figures vary a bit).

So the percentage difference in distance between the average perigee and the average apogee is ~10%. That is, if the Full Moon occurs at perigee it can be up to 10% closer (and therefore larger) than if it occurred at apogee.

This is quite a significant difference, and so it is worth pointing out that the Moon does appear to be different sizes at different times throughout the year.

But that’s NOT what causes the Moon to look huge on the horizon. Such a measly 10% difference in size cannot account for the fact that people describe the Moon as “huge” when they see it low on the horizon.

What’s really causing the Moon to look huge on such occasions is the circuitry in your brain. It’s an optical illusion, so well known that it has its own name: the Moon Illusion.

If you measure the angular size of the Full Moon in the sky it varies between 36 arc minutes (0.6 degrees) at perigee, and 30 arc minutes (0.5 degrees) at apogee, but this difference will occur within a number of lunar orbits (months), not over the course of the night as the Moon rises. In fact if you measure the angular size of the Full Moon just after it rises, when it’s near the horizon, and then again hours later once it’s high in the sky, these two numbers are identical: it doesn’t change size at all.

So why does your brain think it has? There’s no clear consensus on this, but the two most reasonable explanations are as follows:

1. When the Moon is low on the horizon there are lots of objects (hills, houses, trees etc) against which you can compare its size. When it’s high in the sky it’s there in isolation. This might create something akin to the Ebbinghaus Illusion, where identically sized objects appear to be different sizes when placed in different surroundings.

Ebbinghaus Illusion

Ebbinghaus Illusion - the two orange circles are exactly the same size

2. When seen against nearer foreground objects which we know to be far away from us, our brain thinks something like this: “wow, that Moon is even further than those trees, and they’re really far away. And despite how far away it is, it still looks pretty big. That must mean the Moon is huge!”.

These two factors combine to fool our brains into “seeing” a larger Moon when it’s near the horizon compared with when its overhead, even when our eyes – and our instruments – see it as exactly the same size.

  1. March 13, 2011 at 22:53

    how often does this happen? does it occur in a certain season of the year?

    • March 14, 2011 at 07:17

      The Moon illusion can occur any time that the Full Moon is low on the horizon, so every month there will be a couple of days when this is the case in the evening or morning, as the full Moon rises or sets.

      However you’re more likely to see one in summer, I think, for two reasons: 1. The Full Moon is always much lower in the sky in summer than in winter, and so will spend more time near the horizon (in just the same way that the Sun is always much lower in the sky in winter than in summer), and 2. You will probably spend more time outside at sunset in summer than in winter, simply because it’s warmer, and so you’ll be more likely to see the Moon.

  2. john
    March 16, 2011 at 09:39

    I think them orange dots look the exact same size

    • March 16, 2011 at 11:02

      Thanks John. The illusion doesn’t work for everyone. Studies have shown that someone’s ability to see these kinds of illusions is related to the size of their Primary Visual Cortex, an area of the brain used in vision. See here: http://www.nature.com/neuro/journal/v14/n1/full/nn.2706.html

      I’d be interested to know if you have ever seen the full moon looking much bigger than at other times. i.e. are you able to see the Moon Illusion?

      • Eddie
        February 28, 2012 at 12:42

        Quick question, do you know what date and time would be good to go see a full moon low on the horizon. I live in cocoa beach Orlando Florida. I would like to take my girlfriend to the beach at night to see something that incredible .

      • March 5, 2012 at 20:33

        Hi Eddie, sorry for the delay. You’ll need to check a website: http://timeanddate.com/astronomy/ and under the Moon section put in Orlando. Under “modify parameters” select the month you want to view it and click “show”. That’ll tell you moonrise times throughout the month, and in the last column “phase” look for when it’s a full Moon. The moonrise either side of that would be good.

        In general this illusion is most often seen in summer, when full moons never get too high in the sky.

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