Moon

The Moon is Earth's only natural satellite. It orbits our planet at an average distance of about 384,399 kilometres. It takes roughly 29.5 days to go around Earth once. Because of gravitational forces, the Moon always shows the same face to Earth. This is called tidal locking.

The Moon is smaller than Earth. Its diameter is about 3,474 kilometres—one-quarter the size of our planet. Its surface gravity is much weaker, about one-sixth that of Earth. The Moon's surface is covered in dust and has many craters. In the past, volcanic activity created dark areas called maria, which we can still see today.

Humans have been fascinated by the Moon for thousands of years. It has played important roles in mythology, art, and helping us keep track of time. The first spacecraft to reach the Moon were Soviet missions in the late 1950s and 1960s. In 1969, astronauts from the United States landed on the Moon, and twelve people have walked on its surface so far. Today, there are plans to send humans back to the Moon through programs like Artemis.

Names and etymology

The English name for Earth's natural satellite is written as Moon, with a capital M. The word moon comes from old languages, starting from Old English.

In Latin, the Moon is called lūna, and this is where the English word lunar comes from.

In Ancient Greek, the Moon was called selḗnē, and it was also the name of the moon goddess Selene. The Moon has special symbols used in astronomy, like the crescent and decrescent shapes.

Classification

The International Astronomical Union (IAU) calls Earth's satellite "the Moon" with a capital "M". Other natural satellites of planets are called 'moons' with a lower-case "m". Some people have suggested that Earth and the Moon could be considered a double planet system, but most scientists do not agree with this.

Natural history

Main articles: Origin of the Moon and Giant-impact hypothesis

The Moon formed about 50 million years after the Solar System. Scientists think it was created when a Mars-sized object named Theia crashed into the early Earth. This collision threw material into orbit, which came together to form the Moon. This idea helps explain many things about the Moon and Earth. Scientists are still learning more about its early history.

After it formed, the Moon was much closer to Earth and looked larger in our sky. Over time, the Moon has moved farther away. It was once volcanically active, creating dark areas called maria. Its surface has been shaped by many impacts from space rocks. The Moon continues to change slowly. In the far future, it may move so far away that it could break apart.

Physical characteristics

The Moon is not a perfect circle. It is a little stretched out, like an oval. Its shape is a bit longer than we would expect from its current distance from Earth.

The Moon is the fifth biggest natural satellite in the Solar System. It is smaller than Mercury but bigger than Pluto. The Moon's diameter is about 3,500 km, more than one-quarter of Earth's. The face of the Moon is about as wide as mainland Australia, Europe or the contiguous United States. The whole surface area of the Moon is about 38 million square kilometers, about the same size as the Americas.

The Moon has a solid inner core made of iron. Around the core is a partly molten layer. The Moon's surface gravity is about half that of Mars and about a sixth of Earth's. The Moon's gravity is not the same everywhere. Scientists have measured this by tracking radio signals from spacecraft orbiting the Moon.

The Moon has a very thin atmosphere, almost like empty space. It has very little air, with a surface pressure much lower than Earth's. The Moon's surface can get very hot or very cold, depending on whether it is in sunlight or shadow.

The biggest feature on the Moon is a huge crater on the far side, called the South Pole–Aitken basin. It is more than 2,000 km wide.

From Earth, we can see dark areas on the Moon called maria. These are old pools of hardened lava. The Moon's surface has many craters formed when asteroids and comets hit it.

Lunar swirls are strange shapes on the Moon's surface. They are bright and often twisty, and are found where the Moon's magnetic field is stronger.

Liquid water cannot stay on the Moon's surface. But scientists think there might be small amounts of water ice in permanently shadowed craters.

Earth–Moon system

See also: Satellite system (astronomy) and Claimed moons of Earth

Orbit

Main articles: Orbit of the Moon and Lunar theory

The Moon moves around Earth in a path that is not a perfect circle. It is about 385,000 kilometers away from Earth on average. It takes the Moon about 27.3 days to go once around Earth when looking at the stars. Because Earth is also moving around the Sun, it takes about 29.5 days for the Moon to go through all its phases — from a full moon to a new moon and back again.

Because of something called tidal locking, we only ever see one side of the Moon from Earth. However, we can sometimes see a little bit more of the Moon’s surface — about 59% in total — because of a wobbling motion called libration.

Tidal effects

Main articles: Tidal force, Tidal acceleration, Tide, and Theory of tides

Earth and the Moon pull on each other with gravity, which causes tides in Earth’s oceans. The Moon’s gravity pulls more strongly on the water closest to it, creating a bulge on that side. There is also a bulge on the opposite side because as Earth is pulled toward the Moon, the water on the far side gets left behind.

As Earth spins, areas pass through these bulges, causing high tides and low tides about every 12 hours and 25 minutes. The Sun also affects tides, but to a lesser extent than the Moon.

System evolution

The pull between Earth and the Moon is very gradual, but over millions of years it has changed both of them. It has slowed Earth’s spin a little and pushed the Moon a bit farther away — about 38 millimeters farther each year. Scientists have measured this using special mirrors left on the Moon by astronauts. In the very far future, the Sun will become a red giant star and may swallow up Earth and the Moon.

Orientation and appearance

The Moon looks different in the sky depending on where you are on Earth and the time of year. It appears highest in the sky during winter and lowest during summer. At the North and South Poles, the Moon can stay above the horizon for two weeks straight.

The Moon always shows us nearly the same face because it spins in such a way that one side always faces Earth. We call the side we see the "near side" and the hidden side the "far side." The far side is just as lit as the near side, but we never see it from Earth.

The Moon's distance from Earth changes, ranging from about 356,400 kilometers to 406,700 kilometers. This makes the Moon appear up to 14% larger or smaller. When the Moon looks bigger near the horizon, this is called the Moon illusion.

History of scientific understanding and exploration

The Moon has fascinated humans for thousands of years. Ancient people used the Moon's phases to track time and made stories about it. Early drawings and writings often showed the Moon as a symbol of gods and natural forces.

In the 1600s, scientists used telescopes to study the Moon. They found mountains and craters on its surface. This showed the Moon was not perfectly smooth but had rough terrain.

Later, space missions helped us learn more about the Moon. In 1959, the Soviet Union sent the first spacecraft to fly by the Moon. The United States landed the first humans on the Moon in 1969. These missions helped scientists learn about the Moon's surface and history. Since then, many countries have sent probes and rovers to explore the Moon.

Main articles: Exploration of the Moon, List of spacecraft that orbited the Moon, List of missions to the Moon, and List of lunar probes

Human interaction and presence

See also: Human presence in space

The first probes reached the Moon in 1959, just one year after the start of the space age. Since then, many probes and people have traveled to the Moon. Humans first landed on the Moon in 1969 during the Apollo Program, with the last landing happening in 1972.

Today, several orbiters and landers continue to work on the Moon, giving us important information for future missions. There are also plans to build a long-term human presence on the Moon, with the Artemis program leading the way.

Human impact

See also: Space debris, Space sustainability, List of artificial objects on the Moon, Space art § Art in space, Moonbase, Lunar resources § Mining, Tourism on the Moon, and Space archaeology

The Moon’s surface has changed because of human activity. Dust from landings and movements can spread, changing the Moon’s natural state. Scientists and scholars have talked about taking care of the Moon to keep it a good place for science and future visits.

Astronomy from the Moon

Further information: Extraterrestrial sky § The Moon

The Moon is a great place for astronomy. Earth looks three to four times bigger from the Moon than the Moon or Sun looks from Earth. The first pictures of Earth from the Moon were taken in 1966. In 1972, astronauts set up the first telescope there. The Moon’s surface, especially in dark craters, could host special telescopes that are hard to build on Earth.

Living on the Moon

Main article: Lunar habitation

Twelve people have visited the Moon, staying for up to three days at a time. They lived in special landing modules. One big challenge is the lunar dust, which sticks to everything and can be hard to manage. In 2019, a small experiment on the Moon showed that at least one plant seed could grow there.

Legal status

See also: Space law, Politics of outer space, Space advocacy, Colonization of the Moon, Outer Space Treaty, and Moon Treaty

No country owns the Moon. The 1967 Outer Space Treaty says the Moon belongs to all of humanity and should only be used for peaceful purposes. Later agreements tried to control how the Moon’s resources are used, but not many countries have agreed to them. Recently, some countries, like the United States, have made new agreements, such as the Artemis Accords, which some think might change how the Moon is used in the future.

In culture and life

Timekeeping

Further information: Lunar calendar, Lunisolar calendar, and Metonic cycle

Since ancient times, people have watched the Moon’s phases to track time. Early tools, like notched bones, may have helped count the days between Moon phases. This led to the idea of a month, based on the Moon’s cycle. Many languages show this link, with words for “month” and “Moon” often sharing similar roots.

The Moon has shaped calendars. For example, the Islamic calendar is based on the Moon’s phases, with months starting when the first thin crescent Moon is seen.

Cultural representation

Further information: Cultural astronomy, Archaeoastronomy, Lunar deity, Selene, Luna (goddess), Crescent, and Man in the Moon

See also: Nocturne (painting) and Moon magic

The Moon has inspired many cultures and stories. It has been seen as a spirit, a god, or a symbol in many ways. Ancient people made art and wrote stories about the Moon, and some named gods after it.

One common symbol is the crescent Moon, used in many cultures for thousands of years. It appears in ancient writing and art, and is still a symbol today in flags and emblems.

Representation in modern culture

See also: Moon in science fiction and List of appearances of the Moon in fiction

The Moon appears in famous paintings and movies. It is also a topic in books and stories, especially science fiction. With space travel, people have visited the Moon, adding new meaning to its role in our culture.

Lunar effect

Main article: Lunar effect

Some people believe the Moon’s phases affect human behavior, like changes in mood or actions. However, studies show there is no strong proof that the Moon directly influences human biology, even though it has a big impact on our culture and history.