The moon illusion

You may have noticed that sometimes, when the moon rises over the horizon, it seems unusually large.

On several occasions, I've stopped to wonder at it shining behind the skyline—it's always a treat. However, if you stay awake long enough, you might see that later in the night, once the moon has risen higher in the sky, it has reverted to its expected size. This apparent change in size is known as the moon illusion.

Great minds from Aristotle to Isaac Newton have wondered at its cause, and I was surprised to learn that its cause is still debated . The most accepted explanations suggest the illusion arises from comparing the moon as it passes the horizon with known objects, such as buildings and trees, that provide a sense of scale. Higher in the sky, we can only compare the moon to the vastness of space and the sheer size of the sky itself.

To experience the full grandeur of the moon illusion, try a telephoto lens with the moon rising next to a cityscape.

Related Ideas to the Moon Illusion

Also see:

• The ring-segment illusion
• The impossible staircase
• The frequency illusion
• Phases of the moon
• Super moon
• The Chinese Zodiac

I updated this sketch for my book Big Ideas Little Pictures (here's the original)