What you can test yourself
Photograph the Moon near the horizon and again a few hours later, high in the sky, using the same lens. The size in the photos will match to within fractions of a percent, even though the difference looks enormous to your eye.
There is also an old method that needs no camera. If you look at the low Moon through a sheet of paper rolled into a tube, or bend over and look between your legs, the effect disappears, and it immediately looks ordinary.
The reverse is also true: the Moon's angular size near the horizon is even slightly smaller than it is overhead, because it is one Earth radius farther from the observer there. In other words, the illusion runs counter to the measurement, not in its favor.
This shows that the atmosphere is not the cause. Refraction in the air really does flatten the disk near the horizon, but it does not make it larger, so the explanation involving the air is the most popular and the least accurate.
How it is explained
The main explanation involves perceived distance. Near the horizon, the Moon appears behind houses, trees, and hills, so the brain considers it very far away; a distant object of the same angular size should be larger, and perception fills in the size.
There is an objection to this: the illusion remains over an empty sea, where there are no reference points. So the explanation is incomplete, and additional causes are being discussed, right down to the position of the eyes and head.
The shape of the sky itself is also part of the discussion. People perceive the sky not as a hemisphere but as a flattened dome: the horizon seems farther away than the zenith, so objects near the horizon appear larger.
The debate has been going on for more than two thousand years: the phenomenon was described as far back as antiquity. It is a rare case of something simple and familiar to everyone that still cannot be fully explained.
Finish reading in the Miqo app
1 more minute, then a quiz at the end. Find it under “Space → The Moon”
Point your phone camera here to open the App Store or Google Play.
Sources
- The Moon's angular size near the horizon does not exceed its size at the zenith and is even slightly smaller
- the effect is a perceptual illusion. Explanations involving perceived distance and the shape of the visible sky are not considered complete, and atmospheric refraction does not make the Moon appear larger