Why the Moon Looks Bigger on the Horizon
A rising full Moon looks huge, hanging over the rooftops. A few hours later, high in the sky, it looks small. It's the same size both times.
What was reported
When the full Moon rises, it often looks enormous, much bigger than when it's high overhead. Photographs show it's actually the same size, or even very slightly smaller near the horizon, because it's further away from you.
Thinkers have argued about this for centuries. Ptolemy, in Alexandria, wrote about it, and around 1000 AD the scientist Ibn al-Haytham, working in Cairo, argued that the effect happens not in the sky but in the mind.
Explanations offered
It's an illusion, and scientists still debate exactly how it works. One idea is that the brain judges the horizon to be further away than the sky overhead, and so 'enlarges' the Moon there. Others point to comparisons with trees and buildings. You can make it shrink by looking at it upside down between your legs.
- When
- Puzzled over for more than 2,000 years
- Where
- Cairo, Egypt
- Witnesses
- Everyone who has seen a moonrise
- Status
- Partly explained
Famous cases are real, widely reported events. Each file sets out what witnesses said and the explanations that have been offered, without taking a side.
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