The Moon. Earth's only natural satellite, and one of the largest moons in the Solar System. A body that plays an important role in every mythological system we know of, extending to pre-history. It is so large compared with the size of the Earth that the pair is sometimes treated as a binary planet system.
The Moon probably formed very early in the evolution of the Solar System, when it was just a few tens of millions of years old. The most accepted theory argues that the Earth was struck by another planet, named Theia and about the size of Mars. This melted much of both planets and flung large amounts of debris into orbit, forming a short lived ring system that coalesced into the Moon. Material from the surface of the Moon, including some meteorites as well as samples from the Apollo missions, strongly supports the theory that the Earth and Moon are made of nearly identical materials.
The Moon is tidally locked to the Earth, meaning that it always presents the same hemisphere to us. It also means that the time it takes for the Moon to rotate once on its axis (its day) is the same as the time it takes to make one revolution around the Earth (its year), which is 27.3 days. As the orientation between the Earth, Moon, and Sun change, we see different parts of the Moon in shadow- what we call the lunar phases.
When the Moon passes between the Earth and the Sun we get a solar eclipse. By coincidence, the Moon is both 400 times smaller than the Sun and 400 times closer. So it appears the same size in the sky. This makes a total solar eclipse possible, just blocking the Sun and letting us see the outer atmosphere of the Sun, which is normally overwhelmed by the Sun's glare. When the Earth passes between the Moon and the Sun, the Earth's shadow is cast onto the Moon, which we observe as a lunar eclipse.
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