Moon

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

The Moon is smaller than Earth, with a diameter of about 3,474 kilometres—roughly 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 marked by 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 return humans to the Moon again 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 and going back to even older roots. 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, and scientists think it was created when a Mars-sized object named Theia crashed into the early Earth. This collision threw material into orbit, which eventually came together to form the Moon. This idea explains many things about the Moon and Earth, but scientists are still learning more about its early history.

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

Physical characteristics

The Moon is a very slightly scalene ellipsoid due to tidal stretching, with its long axis displaced 30° from facing the Earth. Its shape is more elongated than current tidal forces can account for, indicating that the Moon solidified when it orbited at half its current distance to the Earth.

The Moon is the fifth largest natural satellite of the Solar System. It is smaller than Mercury but considerably larger than the largest dwarf planet, Pluto. The Moon's diameter is about 3,500 km, more than one-quarter of Earth's, with the face of the Moon comparable to the width of mainland Australia, Europe or the contiguous United States. The whole surface area of the Moon is about 38 million square kilometers, comparable to that of the Americas.

The Moon has a solid iron-rich inner core with a radius possibly as small as 240 kilometres and a fluid outer core primarily made of liquid iron with a radius of roughly 300 kilometres. Around the core is a partially molten boundary layer with a radius of about 500 kilometres. The Moon is the second-densest satellite in the Solar System, after Io. However, the inner core of the Moon is small, with a radius of about 350 kilometres or less.

On average the Moon's surface gravity is 1.62 m/s², about half of the surface gravity of Mars and about a sixth of Earth's. The Moon's gravitational field is not uniform. The details of the gravitational field have been measured through tracking the Doppler shift of radio signals emitted by orbiting spacecraft.

The Moon has an external magnetic field of less than 0.2 nanoteslas, or less than one hundred thousandth that of Earth. The Moon does not have a global dipolar magnetic field and only has crustal magnetization likely acquired early in its history when a dynamo was still operating.

The Moon has an atmosphere consisting of only an exosphere, which is so tenuous as to be nearly vacuum, with a total mass of less than 10 tonnes. The surface pressure of this small mass is around 3 × 10⁻¹⁵ atm. Its sources include outgassing and sputtering, a product of the bombardment of lunar soil by solar wind ions.

The lunar surface is exposed to temperature differences ranging from 120 °C to −171 °C depending on the solar irradiance. Because of the lack of atmosphere, temperatures of different areas vary particularly upon whether they are in sunlight or shadow. Parts of many craters, particularly the bottoms of many polar craters, are permanently shadowed.

The topography of the Moon has been measured with laser altimetry and stereo image analysis. Its most extensive topographic feature is the giant far-side South Pole–Aitken basin, some 2,240 km in diameter, the largest crater on the Moon and the second-largest confirmed impact crater in the Solar System. At 13 km deep, its floor is the lowest point on the surface of the Moon.

The main features visible from Earth by the naked eye are dark and relatively featureless lunar plains called maria, which are vast solidified pools of ancient basaltic lava. Although similar to terrestrial basalts, lunar basalts have more iron and no minerals altered by water.

A major geologic process that has affected the Moon's surface is impact cratering, with craters formed when asteroids and comets collide with the lunar surface. There are estimated to be roughly 300,000 craters wider than 1 km on the Moon's near side.

Lunar swirls are enigmatic features found across the Moon's surface. They are characterized by a high albedo, appear optically immature, and often have a sinuous shape. They are located in places with enhanced surface magnetic fields and many are located at the antipodal point of major impacts.

Liquid water cannot persist on the lunar surface. When exposed to solar radiation, water quickly decomposes through a process known as photodissociation and is lost to space. However, since the 1960s, scientists have hypothesized that water ice may be deposited by impacting comets or possibly produced by the reaction of oxygen-rich lunar rocks and hydrogen from solar wind, leaving traces of water which could possibly persist in cold, permanently shadowed craters at either pole on the Moon.

In years since, signatures of water have been found to exist on the lunar surface. The presence of usable quantities of water on the Moon is an important factor in rendering lunar habitation as a cost-effective plan; the alternative of transporting water from Earth would be prohibitively expensive.

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 travels around Earth in a path that is not a perfect circle but slightly stretched out. 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, but 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 cultures used the Moon's phases to track time and created myths around it. Early drawings and writings often showed the Moon as a symbol of gods and natural forces.

In the 17th century, scientists began using telescopes to study the Moon. They discovered mountains and craters on its surface. This changed how people saw the Moon, showing it wasn't perfectly smooth but had rough terrain.

Later, space missions brought us closer to understanding the Moon. In 1959, the Soviet Union sent the first spacecraft to fly by the Moon. The United States followed with its Apollo missions, landing 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 our closest neighbor in space, uncovering more secrets about its past and potential for future exploration.

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, providing 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, affecting 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 and used them to track time. Early tools like notched bones, dating back thousands of years, may have been used to count the days between Moon phases. This way of keeping time led to the idea of a month, which is based on the Moon’s cycle. Many languages reflect this connection, with words for “month” and “Moon” often sharing similar roots.

The Moon has also shaped calendars. For example, the Islamic calendar is based entirely on the Moon’s phases, with months beginning 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 throughout history. 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 even 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 has also been a topic in books and stories, especially science fiction. With space travel, people have actually visited the Moon, which has added 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 causing changes in mood or actions. However, studies have shown there is no strong proof that the Moon directly influences human biology, even though it has a big impact on our culture and history.