Have you ever noticed that the Moon close to the horizon looks absolutely enormous? You've just witnessed the Moon illusion.
Look for the Moon when it's low down near the horizon and you may observe it's 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.
Possible causes
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 no appreciable difference in the Moon’s apparent diameter, regardless of its height above the horizon.
Break the Moon illusion
There is a away you can remind your brain that the low-down Moon isn't actually as big as it looks.
Try it next time you witness the Moon illusion, when it looks big while low down near the horizon.
Stretch your arm out and you'll still be able to cover the Moon with your thumb, momentarily breaking the illusion.
Or you could try and photograph the Moon, capturing one image when it's close to the horizon and another when it's higher.
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.
Have you ever observed or photographed the Moon illusion? Do you have any theories as to what causes it? Let us know by emailing contactus@skyatnightmagazine.com
