When we walk outside at night and look skyward, it's usually the Moon, our planet's natural satellite that first catches our eye. The fifth largest moon in the solar system – after Jupiter's Ganymede, Callisto, and Io, and Saturn's Titan – it is the second moon in the solar system in relation to its parent body, Earth. (Charon is the largest moon in the solar system relative to its parent body, Pluto.)

The Moon's name originated from the Old English mōna, which arose before 725 AD, and followed a still older proto-Germanic name. Prior to that, the names for the Moon derived from the Latin luna and from the Greek selene. (A quick aside on nomenclature: note that Earth's name is Earth, not the Earth, just as you wouldn't say the Saturn or the Jupiter. But the Moon's proper name is the Moon, differentiating it from many other moons in the solar system. If you catch someone saying, “I'm glad to live on the Earth,” tell them to stop it. There is enough foolishness going on in this world already. In addition, the media also often use a lowercase m for the moon. The proper name is the Moon.)

The Moon is a pretty innocuous body, going about its business, orbiting Earth, and showing its phases each month, from the dark “new,” through the “first quarter” to “full” to “last quarter,” wreaking havoc only to deep-sky observers who wish the Moon would go away and not spoil their dark-sky views of galaxies, clusters, and nebulae. The Moon stretches an impressive 3,476 kilometers across at the equator, which means you could just about wedge its width between New York and Phoenix.

Certainly, the Moon's “face” has been a familiar sight in the sky for the whole history of humankind. It is tidally locked with Earth such that, although it wobbles slightly, only one face covered with darkish maria (“seas”) is visible to us. Even our earliest ancestors must have looked skyward night after night (or day after day!), wondering what this bright and changing light in the sky was. Amazingly, however, we have come to terms with how the Moon must have formed only in the last generation.