planet-moon
The Moon is Earth's only natural satellite, a giant rocky world that orbits us once every 27 days. The Moon is much bigger than most planet's moons (relative to its planet), which makes Earth's moon system unusual in the Solar System. The Moon has shaped life on Earth in countless ways: it gives us the tides, it slows Earth's spin, it stabilises our seasons, and it has inspired more myths, calendars, songs and stories than any other object in the sky. It is also the only world beyond Earth that humans have ever set foot on.
- Diameter3,474 kmAbout a quarter the size of Earth
- Distance from Earthapprox. 384,400 kmAround 30 Earth-diameters away
- Orbits Earth in27.3 daysBut 29.5 days between full Moons
- Surface gravity1.62 m/s²About 17% of Earth's
- Surface temperature-173 to 127 °CBrutal day-night swing
- People who have walked there12All American Apollo astronauts, 1969-1972
What is the Moon made of?
The Moon is a rocky world made mostly of silicate rocks, similar to Earth's crust but much drier. It has a small iron core, a thick rocky mantle, and a hard crust covered in a thin layer of broken-up dust called regolith. The regolith is the fine grey powder Neil Armstrong stepped onto in 1969. It is so sticky and abrasive that it caused real problems for the Apollo astronauts: it got into everything, scratched their suits and even made some of them sneeze and cough when it floated around inside the lander.
How the Moon formed
The leading theory of how the Moon was born is called the giant impact hypothesis. About 4.5 billion years ago, when the Earth had just formed, a Mars-sized object often nicknamed Theia crashed into the young Earth at high speed. The collision was enormous: it melted huge parts of both worlds, blasted a vast cloud of debris into orbit around Earth and merged Theia's heavier metals into Earth's core.
Over the next few thousand years, the cloud of orbiting debris slowly clumped together under gravity to form the Moon. This story explains lots of strange features of the Moon: why it is so big, why it has so little iron compared to Earth, why its rocks have almost the same chemistry as Earth's, and why the Earth-Moon system spins as fast as it does. There are some details still being argued, but the giant impact theory has been the best answer for over 30 years.
Why we always see the same side
Look at the Moon any night of the year and you always see the same face. The dark patches form the same pattern: the Man in the Moon, or a rabbit, depending on which culture you grew up in. The reason is that the Moon is tidally locked to Earth. It rotates on its axis at exactly the same rate as it orbits, so the same side always faces us.
The other side is called the far side (often wrongly called the "dark side", although it actually gets the same amount of sunlight as the near side). Nobody saw the far side until 1959, when the Soviet probe Luna 3 sent back the first photos. The far side is dramatically different: it has almost no large dark patches (called maria), and its crust is much thicker than the near side. We still do not fully know why.
The phases of the Moon
The Moon goes through a regular cycle of phases: new Moon, crescent, first quarter, gibbous, full Moon, then back through gibbous, last quarter, crescent and new again. The whole cycle takes 29.5 days, the basis of the original meaning of the word "month".
Phases happen because we see different amounts of the Moon's sunlit half as it orbits Earth. The Moon does not change shape and is not "in shadow" during phases (except during an eclipse). It is just that we see a different angle of its lit-up side.
- New Moon: the Moon is between Earth and the Sun; we see the dark side.
- Crescent: a thin sliver visible, growing or shrinking.
- Quarter (sometimes called half Moon): we see half of the lit-up side.
- Gibbous: more than half but not quite full.
- Full Moon: the Earth is between the Sun and Moon; we see the whole lit-up side.
Tides: how the Moon pulls our oceans
The Moon's gravity pulls on every part of the Earth, but more strongly on the side facing the Moon than the side facing away. This difference makes the Earth slightly stretched along the Earth-Moon line. Solid land barely moves, but the oceans flow freely and pile up on the two sides, creating high tides on the side facing the Moon AND the side facing away from the Moon. As the Earth rotates, every coast moves through these two bulges every day, getting two high tides and two low tides every 24 hours.
The Sun also creates tides, but only about half as strong as the Moon's. When the Sun and Moon line up (at new and full Moon), their pulls combine and we get unusually big spring tides. When the Sun and Moon are at right angles (at the quarters), their pulls partly cancel and we get smaller neap tides.
Eclipses
When the Sun, Moon and Earth all line up perfectly, you get an eclipse. There are two main kinds.
- A solar eclipse happens when the Moon passes directly between Earth and the Sun, blocking part or all of the Sun's light. In a total solar eclipse, the Moon completely covers the Sun for a few minutes and you can see the Sun's outer atmosphere (the corona) glowing around the dark disc.
- A lunar eclipse happens when Earth passes between the Sun and the Moon, casting Earth's shadow onto the Moon's surface. In a total lunar eclipse, the Moon turns a deep blood-red colour because the only light reaching it is sunlight bent through Earth's atmosphere (effectively the combined light of all the sunsets and sunrises happening on Earth at the same time).
Eclipses do not happen every month because the Moon's orbit is tilted slightly compared to Earth's orbit around the Sun, so usually the three bodies do not perfectly line up. When they do, eclipses are predictable centuries in advance.
Going to the Moon
The Moon is the only world beyond Earth that humans have ever visited in person. The American Apollo programme sent 6 successful crewed missions to the Moon between 1969 and 1972. The first landing was Apollo 11 on 20 July 1969, when Neil Armstrong took his "small step for [a] man" onto the Moon's surface. Twelve astronauts in total walked on the Moon (all American men, all between 1969 and 1972).
No human has been back to the Moon since 1972, but that is about to change. NASA's Artemis programme plans to return astronauts (including the first woman) to the Moon later this decade. China, India, Japan and several private companies all have active or planned lunar programmes too. Many of these missions are looking at the Moon's south pole, where water ice trapped in permanently shadowed craters could one day support a long-term base.
Deeper dive: how the Moon stabilises Earth's climate
The Moon does more than give us tides and inspire poets. It plays an important role in keeping Earth's climate stable enough for life over long timescales.
The Earth spins on a tilted axis (currently about 23.5 degrees off vertical), which is what gives us seasons. Without the Moon, that tilt would wobble all over the place over millions of years, sometimes flipping by as much as 60 to 80 degrees. A planet whose tilt swings that wildly would have wild climate swings too: the poles getting alternately roasted and frozen, the equator alternately ice or jungle, and most life having no chance to adapt. Mars (which has only two tiny moons) has had its tilt wobble that much over its history.
The Moon, being so massive, holds Earth's axial tilt steady within a narrow few degrees. That gives us mild and predictable seasons that change in roughly the same way every year, allowing ecosystems to settle into long-term patterns and complex life to evolve. Without the Moon, it is possible that Earth would have had a much wilder climate history, and life might not have had time to develop the way it did. Some scientists think the unusually large Moon (caused by the random Theia impact 4.5 billion years ago) may be one of the lucky breaks that made Earth such a friendly place to live.
For Earth itself see planet Earth. For other planets in our Solar System, browse the Solar System page. For Moon missions in detail see the Apollo missions.