The Moon is actually not physically "so big" but appears larger for a couple of key reasons:
- The Moon is relatively large compared to Earth for a natural satellite, being about one-quarter the diameter of Earth. This is unusual and is due to its unique formation history, where a Mars-sized body collided with early Earth, and debris from this impact coalesced to form the Moon. This makes it much larger relative to its planet than most other moons in the solar system.
- The perception that the Moon is "so big" when seen from Earth is often due to an optical illusion called the "Moon illusion." The Moon looks bigger near the horizon than when it is high in the sky, though photographs show it remains the same size. This illusion is caused by the brain misinterpreting the Moon's size when it is viewed in the context of objects on the horizon, like trees or buildings.
In summary, the Moon is physically large compared to Earth relative to typical planet-moon systems, and the impression of its extreme size in the sky is largely a psychological effect due to an optical illusion. Both factors contribute to why the Moon appears "so big" to human observers.
