Except for a few days of the month, the Moon is always visible in the sky somewhere. It has been Earth's constant companion for over 4 billion years and, according to one theory, may have even once been part of our planet until a passsing Mars-size object struck Earth a glancing blow, blasting a piece of it into space. Since then, the airless Moon has taken quite a beating. Its asphalt-colored surface is pitted with countless craters big and small. They and the many prominent faults and craggy mountain ranges make for hours of fascinating scrutiny with a telescope or binoculars.

The Moon lies at a mean distance of 384,000 kilometers and has a diameter of a little over 3,400 kilometers. As moon–planet ratios go in the solar system, our Earth–Moon system is an oddity, with the Moon being rather large for a planet our size. Consequently, a body at the distance and with the mass of the Moon creates some gravitational effects on Earth in the form of flowing tides. The Moon's pull may also have some very minor influence on enhancing earthquakes, although this is not well accepted. Some people claim a full Moon sways behavior. But this is more for the realm of social psychology. Essentially, other than tidal effects and providing light and beauty, the Moon does not overtly affect the Earth.

On the other hand, if we didn't have a Moon, life on Earth would probably not exist, at least not in the form it does today.