A full moon rising over the horizon can look enormous. A few hours later, high in the sky, it looks much smaller. Yet the Moon’s size in the sky barely changes. This is the Moon illusion, and it has puzzled people since ancient times.
Key Takeaways
- The Moon’s angular size is about half a degree whether it is near the horizon or overhead.
- Photographs taken with the same settings show the Moon about the same size in both positions.
- The illusion comes from how the brain judges size and distance.
- Simple tests, like viewing through a tube, can make the effect shrink.
It is not the atmosphere
Earth’s atmosphere does bend light, but this refraction actually squashes the rising Moon slightly vertically rather than enlarging it. The Moon near the horizon is also slightly farther away from you than when it is overhead, because you are viewing it across part of Earth’s radius. If anything, it should look a little smaller.
Take a photo
Photograph the Moon on the horizon and later high in the sky with the same zoom. Compare the images; the Moon will be essentially the same size.
Explanations for the illusion
Scientists have proposed several explanations, and the illusion may involve more than one.
Apparent distance
One idea is that our brains perceive the sky as a flattened dome, with the horizon seeming farther away than overhead. If the brain thinks the horizon Moon is farther away but it takes up the same angle, it interprets it as bigger.
Comparison with objects
Near the horizon, the Moon appears next to trees, buildings and hills, which may make it seem large. Overhead there is nothing to compare it to. However, the illusion also occurs over open oceans, so this is not the whole story.
Eye and posture effects
Some researchers have suggested that the angle of the eyes and head affects perceived size. The evidence is mixed.
Test it yourself
- Look through a tube, such as a rolled-up paper, to isolate the Moon from the landscape.
- Bend over and view the horizon Moon upside down between your legs; many people report the illusion weakens.
- Hold a small object, such as a pencil eraser at arm’s length, and compare it with the Moon in both positions.
The Sun and constellations too
The Sun near the horizon and constellations rising and setting can appear larger in the same way. Never look directly at the Sun.
Supermoons are different
A supermoon is a full moon near perigee, the closest point in its orbit. It can appear up to about 14% larger than a full moon at apogee, which is a real difference, but the horizon illusion is usually more noticeable than the supermoon effect.
Frequently Asked Questions
Does the Moon look orange near the horizon because of the atmosphere? Yes. Light travelling through more atmosphere scatters blue wavelengths, leaving more red and orange.
Do cameras ever show the Moon bigger? Long telephoto lenses can make the Moon look huge next to distant objects, but that is a lens effect.
Has the illusion been solved? Not completely. Researchers still debate the explanations.
The bottom line
The horizon Moon is not bigger. Your brain is doing the enlarging, and a simple tube of paper can prove it.
