Demystifying the Moon illusion

An apparently supersized Moon low on the horizon fools the eye... but not the camera

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When the full Moon occurs near the horizon, it looks artificially massive. Even if you've been into astronomy for a long time, this can still catch you out, convincing you that that low-down full Moon, or near-to-full Moon, is indeed larger than it should be. But this is false, an illusion conjured up by our brains. This month's Capture shows you how to debunk the illusion.

While our brains have trouble not seeing the Moon as huge when it's near the horizon, a camera isn't fooled at all. This is especially true when you see a low full or gibbous Moon near the horizon and try to capture its apparently massive appearance with a smartphone. The resulting image shows nothing particularly impressive. The camera can't see the illusion, so it doesn't record it.

In order to debunk what you're seeing, you'll need to attempt to capture a regular full Moon high in the sky. Devoid of any horizon references, this full Moon will provide the control image. Then, you'll have to attempt to capture the full or large waning gibbous Moon when it's low down, either just after rising or just before setting. Comparing the two images will give you the ammunition you need to debunk the illusion.

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But are all things equal? When the Moon is high in the sky and due south, this is when it's closest to you on the surface of Earth. The smallest distance occurs when there's a straight line connecting the centre of Earth, you and the Moon. When the Moon is low on the horizon, the distance is slightly greater, by approximately an Earth radius. Consequently, the low 'massive' Moon should appear, if anything, a tiny bit smaller than the high-up 'regular' Moon.