Earth has just one Moon, which is about 2,160 miles (3,475 kilometers) across and less than a third as wide as Earth, although Earth has about 80 times as much mass as the Moon. On average the Moon is about 239,000 miles (384,000 kilometers) from Earth, and it goes around Earth in a path called an orbit. Its distance from Earth changes as it moves on an oval orbit, from about 363,300 kilometers (225,700 miles) at its closest on average to about 405,500 kilometers (252,000 miles) at its farthest, and the Moon is also slowly drifting away from Earth, by about 1.5 inches (3.8 centimeters) each year.

The Moon does not make its own light. It shines because it reflects sunlight. The Moon's shape does not really change during the month. What changes is how much of its sunlit side we can see. The Sun always lights half of the Moon, but how much of that lit half we can see from Earth changes during the month. At a full Moon, we can see all of the Moon's sunlit side, and at a new Moon, we can hardly see the Moon at all.

The Moon's phases happen because the Moon, Earth and Sun change positions as the Moon orbits Earth. The Moon goes through eight phases, from new Moon to full Moon and back again, and at first quarter we see half of the Moon's lit side, which is why it is called a quarter Moon. While the lit part we see is growing, the Moon is waxing, and while it is shrinking, the Moon is waning. The Moon takes about 27.3 days to circle Earth once, yet about 29.5 days to go from new Moon to new Moon.

We always see the same side of the Moon from Earth, because the Moon spins exactly once for every trip it makes around Earth, an arrangement called tidal locking. People sometimes call the far side of the Moon the dark side, but that is misleading, because the far side gets sunlight too. Even though the same face of the Moon always points toward Earth, slight rocking and wobbling motions called librations let observers on Earth see as much as about 59 percent of its surface over time. Until a Soviet spacecraft flew past in 1959, nobody had seen the far side of the Moon.

A lunar eclipse happens when Earth's shadow falls on the Moon, and during a total lunar eclipse the Moon can look red or orange. A solar eclipse happens when the Moon passes between the Sun and Earth, which can only happen at a new Moon. We do not get an eclipse every month because the Moon's orbit is tilted compared with Earth's orbit around the Sun.

The Moon's gravity pulls on Earth's oceans and is the main cause of the tides, and ocean water bulges out on the side of Earth nearest the Moon and on the side farthest from it, and these bulges are the high tides. The Sun is about 390 times farther away than the Moon, so its pull on the tides is about half as strong as the Moon's. Spring tides, the biggest tides, happen at new Moon and full Moon, when the Sun, Moon and Earth line up, while neap tides, which are moderate, happen around the first and third quarter Moon, when the Sun and Moon are at right angles to each other.

Words to know

• waxing: growing
• waning: shrinking
• eclipse: shadow event
• neap: moderate tide