The Moon illusion is the name for a widely reported experience: the Moon near the horizon looks larger than the same Moon high in an open sky. It is a report about appearance. It is a different thing from the physical size of the Moon, its width in a photograph, or its slowly changing distance from Earth.
Is the Moon actually bigger near the horizon?
Not in the way it feels. With the same camera and settings, photographs of a low Moon and a high Moon taken close enough in time can show the same horizontal width of the Moon, even when the people taking them report a large difference.
The full Moon spans roughly half a degree of sky. That is about the width of your little fingernail at the end of your outstretched arm, whichever part of the sky it is in.
Why does it look so much larger, then?
This is the part that stays honest. No single perceptual mechanism currently provides an uncontested complete explanation of the natural Moon illusion. Several proposals have support for parts of it.
Distance cues, surrounding context, angle, head and eye position, oculomotor factors, and size-distance processing are among the proposed contributors. The evidence does not justify handing the whole effect to any one of them for all observers and conditions.
It also does not affect everyone the same way. The reported strength of the effect varies across observers and viewing conditions, including how detailed the horizon scene is.
How to test it yourself
- Look at a low Moon with the landscape in view.
- Cover the landscape with your hand or look through a short cardboard tube.
- Hold a fingernail at arm’s length against the Moon, low and then high, and compare.
- Later, when the Moon is high in an empty sky, look again.
Covering the surroundings or measuring equal widths can weaken or expose the effect for some observers. Neither tells you why it happens. The comparator on the main page keeps two drawn discs exactly the same width for the same reason.
What about a supermoon?
Supermoon is an informal name for a full Moon near the closest point of its orbit. The real apparent diameter of the Moon does change with distance, and the closest full Moon of the year can appear up to about 14 percent larger in diameter than the farthest. That is a year-long extreme, and a separate thing from the horizon effect. Read the supermoon versus Moon illusion comparison.