The Moon

The Moon is Earth's only natural satellite and the brightest object in the night sky. It is a rocky, airless world about a quarter of Earth's diameter, orbiting the planet at an average distance of roughly 384,000 kilometres. Lacking any significant atmosphere, its surface preserves a record of impacts as countless craters, alongside dark plains called maria that early observers mistook for seas and bright, rugged highlands.

The Moon shines by reflecting sunlight rather than producing light of its own. As it travels around Earth over about a month, the angle between the Sun, Earth and Moon changes, so observers see a varying portion of its sunlit half. This produces the familiar cycle of phases from new moon to full moon and back. The Moon also keeps the same face turned toward Earth, because it rotates on its axis in the same time it takes to orbit, an effect known as synchronous rotation; the side permanently turned away is often called the far side.

The Moon's gravity, together with that of the Sun, is the main cause of ocean tides, raising the water on the side of Earth facing it and on the opposite side. When the Sun, Earth and Moon line up, eclipses can occur: a lunar eclipse when Earth's shadow falls on the Moon, and a solar eclipse when the Moon passes between Earth and the Sun. The Moon was the first and so far only world beyond Earth visited by human explorers, during the Apollo missions.

Frequently asked questions

Why do we always see the same side of the Moon?

The Moon rotates once on its axis in the same time it takes to orbit Earth, so the same hemisphere always faces us. The hidden hemisphere is called the far side.

What causes the phases of the Moon?

As the Moon orbits Earth, the lit portion we can see changes with the Sun-Earth-Moon angle, cycling from new moon through full moon and back.

How does the Moon affect the tides?

Its gravity pulls on Earth's oceans, raising bulges of water that produce high and low tides as Earth rotates beneath them.