Why the Moon looks huge on the horizon

When the full moon rises above the horizon it looks enormous. A few hours later, high in the sky, it seems much smaller. But the Moon hasn’t shrunk – your brain is being fooled.

The same size all night

The Moon’s apparent diameter is about half a degree, both at the horizon and high in the sky. In fact it is slightly smaller at the horizon: there you are about one Earth radius – 6,400 km – farther from it than when it is overhead. The difference is only a couple of percent.

Test it tonight

• The thumbnail: Hold out your arm when the Moon is low and compare it with your thumbnail. Do the same when it is high. It covers the same amount both times.
• The paper tube: Look at the big horizon Moon through a rolled-up sheet of paper, so you can’t see the landscape around it. It shrinks instantly.
• The photo: Photograph the Moon low and high with the same focal length. In the pictures it is the same size.

Why are we fooled?

No explanation is fully proven, but the most accepted ones are about how the brain judges distance:

• The flattened sky. We perceive the sky as a flattened dome, where the horizon feels farther away than the zenith. Something that looks the same size but feels farther away is read as larger.
• The foreground. At the horizon the Moon sits next to trees, houses and mountains that we know are large and distant. That makes it seem huge – the same principle as classic optical illusions where two equal lines look different.

But the colour is real

The orange or red colour of the Moon near the horizon is not an illusion. Its light passes through much more air, which scatters away the blue – the same thing that makes sunsets red. The air also bends light more at the bottom than at the top, so a Moon right on the horizon looks slightly squashed.

Want to experience the illusion? Check the calendar for the evening when the full moon rises close to sunset – it comes up big and orange in the twilight.