The sky — astronomy

The Moon: Earth’s Companion

The Moon is Earth’s only natural satellite, about 384,400 km away on average. It most likely formed after a giant impact between the young Earth and a Mars-sized body roughly 4.5 billion years ago. We always see the same side because the Moon rotates once per orbit, a state called synchronous rotation or tidal locking.

What we know about the Moon

The Moon is Earth’s only natural satellite and our nearest large neighbour in space. Its average distance is about 384,400 kilometres, and its diameter is roughly 3,474 kilometres, a little over a quarter of Earth’s. Surface gravity is about one sixth of what we feel on Earth, which is why the Apollo astronauts moved in long, bouncing strides.

The Moon circles Earth in about 27.3 days relative to the stars. The cycle of phases is slightly longer, about 29.5 days, because Earth also moves around the Sun in the meantime. You can follow the sequence in our article on the Moon’s phases.

Its surface is covered with impact craters and dark plains called maria (Latin for “seas”). They are not water but solidified basaltic lava from ancient volcanic activity. The Moon has only an extremely thin exosphere (a tenuous envelope of gas) and no global magnetic field today. With no thick atmosphere to even things out, temperatures swing sharply: daytime ground can be hotter than boiling water, while the night drops far below freezing.

Reflectors left by the Apollo missions let scientists bounce laser pulses off the Moon and measure its distance very precisely. These measurements show that it is drifting away from Earth by about 3.8 centimetres a year.

How the Moon probably formed

The most widely accepted explanation is the giant-impact hypothesis. About 4.5 billion years ago, the young Earth is thought to have collided with a Mars-sized body, often called Theia. Debris thrown into orbit gradually gathered together and became the Moon.

Several clues support this picture. Apollo samples have oxygen isotope ratios very similar to Earth’s mantle, and the Moon’s small metallic core fits a scenario in which mostly outer layers were ejected. Details are still debated, such as how much of the Moon’s material came from Theia and how much from Earth, so it is fair to call this the leading model rather than a closed case.

Synchronous rotation: why we see one side

The Moon spins on its axis in exactly the time it takes to orbit Earth. This is called synchronous rotation, or tidal locking. As a result, the same hemisphere always faces us.

Gravity explains it. Long ago the Moon rotated faster. Earth’s gravity raised tidal bulges in the Moon, and friction as those bulges shifted slowly braked its spin until it matched its orbital period. Many other moons in the Solar System are locked in the same way.

The far side is not permanently dark. It gets sunlight just like the near side, which is why “far side” is a better name than “dark side”. The first images of it were taken by the Soviet probe Luna 3 in 1959. A slight wobble called libration means that, over time, we can see a little more than half of the surface, about 59 percent.

The Moon and the tides

Ocean tides arise mainly because the Moon’s gravitational pull differs across Earth. Water bulges toward the Moon and on the opposite side of the planet, so many coasts experience two high tides and two low tides each lunar day, which is a bit longer than 24 hours. The Sun also raises tides, but weaker ones than the Moon.

Around new and full Moon the Sun, Earth and Moon are roughly aligned, the two effects add up, and we get stronger spring tides. Around the quarter phases they partly cancel, giving weaker neap tides. Actual tide heights also depend on coastline shape, sea depth and weather. For the general physics, see our article on the gravity of the planets.

Tidal friction is also why the Moon slowly recedes and why Earth’s day is lengthening, by a very small amount.

The Moon in culture and calendars

The repeating phases are among the oldest and easiest sky cycles to observe, so many calendars have been built on them. The word “month” itself comes from the Moon in English and in many other languages. The Islamic calendar is purely lunar, while the Hebrew and traditional Chinese calendars are lunisolar. In Christian tradition the date of Easter is set by a rule tied to the full Moon after the March equinox.

The Moon also fills myths, poetry and art around the world. To learn what happens when Earth’s shadow falls on the Moon, read about the lunar eclipse, and for why it can look unusually large near perigee, see the supermoon. If you are curious about how the Moon is interpreted as a cultural tradition rather than as science, there is a separate article on the Moon in astrology. This page stays with astronomy: the Moon is a world with its own history, composition and motion, all of which we can measure.

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