What is the Moon illusion?
Have you ever noticed the Moon sometimes appears huge when it's close to the horizon?
Look for the Moon when it is low to the horizon and you may get the impression that it is unnaturally large – this is the phenomenon known as the Moon illusion, and it appears to be more pronounced around full Moon when the maximum area of its disc is illuminated.
In reality, the Moon has more or less the same apparent diameter of around 0.5º, whether it is looming over the horizon or riding high in the sky.
For more advice, read our guide on how to observe the Moon, or have a go at sketching it with our guide on how to draw the Moon.
What causes the Moon illusion?
One explanation for the Moon illusion arises from our perception of the shape of the celestial sphere above us: instead of a hemisphere, we perceive the sky to be a flattened dome.
Consequently the lower the Moon is in the sky, the farther away and larger it is perceived to be.
When the Moon is high in the sky we conversely perceive it to be closer to us and therefore smaller in apparent size.
Or perhaps when the Moon is lower in the sky, it appears closer to the landscape and foreground objects such as trees or human-made structures, making it seem comparatively bigger.
What ever the reasons for it, few people seem to be immune to the Moon illusion, even though the viewer may be fully aware that for any given evening there is actually no appreciable difference in the Moon’s apparent diameter, regardless of its height above the horizon.
You can check this for yourself the next time you witness the Moon looking unusually big while it's low down towards the horizon.
Stretch your arm out and you'll find that you can still cover the lunar disc comfortably with your thumb, thus - momentarily at least - breaking the illusion.
Or you could try photographing the Moon, capturing one image when it's close to the horizon and another when it's high in the sky.
Keep your zoom settings the same for both photos, then compare them. You'll see that the apparent size of the Moon hasn't changed at all. It's all merely part of the illusion.
Sir Patrick Moore (1923–2012) presented The Sky at Night on BBC TV from 1957–2012. He was the Editor Emeritus of BBC Sky at Night Magazine, President of the British Astronomical Association and Society for Popular Astronomy, and a researcher and writer of over 70 books.