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 straight overhead. The difference is only a couple of percent.
Test it yourself tonight
- The thumbnail: Hold out your arm when the Moon is low and compare it with your thumbnail. Do the same when the Moon 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 at once.
- 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. An object that looks the same size but feels farther away is read as bigger.
- Comparison with the foreground. At the horizon the Moon sits next to trees, houses and hills we know are large and far away. That makes it seem huge, the same principle as classic optical illusions where two equal lines look different in length.
But the colour is real
That the Moon often looks orange or red near the horizon is no illusion. Its light passes through much more air, which scatters away the blue light, the same thing that makes sunsets red. The air also bends light differently at the bottom and top, so a Moon right at the horizon looks a little squashed.
Want to see the illusion? Look in the calendar for the evening when the full moon rises close to sunset. It climbs big and orange in the twilight.