The Moon is the only natural satellite of the planet Earth. It is a rocky object with an extremely thin atmosphere measuring 10,921 kilometers in circumference, and is visible from most locations on Earth’s surface. Because of its highly visible location, the Moon features prominently in mythologies and folklore from around Earth. It also has a significant effect on Earth by causing tides with its own gravity. However, the Moon itself is believed to be lifeless; humans and creatures involved in experiments have visited it. The Moon is the fifth-largest satellite in the Solar System, and the largest planetary satellite with respect to the planet it orbits.
Despite claims made by Amanda Kirby, commercial flights to the Moon were over two decades away from reality as of 2001. The Moon is considered to be an international body and cannot be legally claimed by any country under the Outer Space Treaty of 1967.
The name of the Moon comes from the Old English word mona, which ultimately is derived from the Proto-Indo-European root meaning “to measure.” This references the fact that the Moon is used to measure the passage of time by tracking its distinct illumination phases. The Moon is sometimes referred to as “Luna” to differentiate it from other moons in the Solar System; the name Luna is the Latin word for moon.
Anything pertaining to the Moon is called “lunar,” after the Latin name.
The Moon orbits the planet Earth. As of the twenty-first century, it orbits at an average distance of 1.28 light-seconds, or 384,402 kilometers (238,856 miles). The Moon is gradually moving away from Earth, with the distance between the two bodies increasing by 3.78 centimeters (1.48 inches) per terrestrial year. The Moon takes 27.3 days to fully orbit Earth. Because the Moon and Earth are tidally locked, the same side of the Moon always faces Earth; this is called the near side, while the opposite is the far side.
The Moon is a rocky object in the shape of a scalene ellipsoid. It is not perfectly spherical because when it formed, it was much closer to Earth; gravitational interactions stretched it, and now that it is solidified it is unable to settle into a rounder shape. However, its bulge is not easily visible from Earth, making the Moon appear circular. The Moon is 10,921 kilometers (6,786 miles) in circumference at its equator and has a mass of 7.342×1022 kilograms. Its size gives it a gravitational strength of 0.1654g, far less than Earth.
The near side of the Moon, which always faces Earth due to tidal locking, is marked with numerous craters and dark regions called maria or seas. Its largest feature is the South Pole-Aitken Basin, a 2,240-kilometer (1,390-mile) crater reaching depths of 13 kilometers (8.1 miles). This is the Moon’s deepest point, and the highest points are located in the highlands northeast of this basin. The far side of the Moon is less affected by impact craters, and its surface is on average 1.9 kilometers (1.2 miles) higher than the near side. The maria (singular: mare) are wide, flat basins of basaltic lava rock, similar to that found on Earth, but with more iron and no water-altered minerals. Maria cover 31% of the Moon’s near side and 2% of its far side.
Because of the Moon’s extremely thin atmosphere (one picobar of pressure during the day, and only a femtobar of pressure at night) and weak magnetic field (only 0.2 nanoteslas), liquid water cannot persist on the Moon’s surface. Solar radiation instantly causes it to vaporize. However, water ice is found under the lunar surface and within craters whose positions causes their interiors to be permanently shadowed. The atmosphere is too thin to transmit heat efficiently, so anything that is not in direct sunlight will remain frigid. The Moon regularly passes through the shadow of Earth, which causes it to go through a series of illumination phases over the course of 29.5 days. It is said to be a “new moon” when the Moon is fully within Earth’s shadow, so no sunlight reaches its near side.
4.53BYA-3.92BYA: Pre-Nectarian period
It is generally believed that the Moon formed 4.53 billion years ago, making it about sixty million years younger than the Solar System itself. Research has suggested that the early Earth, sometimes called Gaia at this stage, experienced a collision with a smaller planet called Theia. The impact liquefied Gaia and launched molten debris into orbit. The larger blob solidified into Earth, while the smaller, orbiting blob solidified into the Moon.
During its early ages, the Moon’s surface was covered in a magma ocean. Over time, this began to solidify and crystallize into rock. This period of time also saw the Moon heavily beaten by impacts with spaceborne debris such as asteroids and comets. For a period of about 70 million years, volcanic eruptions built up an atmosphere around the Moon which was around twice as thick as the present-day atmosphere of Mars. Without a powerful magnetic field, though, the Moon’s atmosphere was eventually torn away by the solar wind.
3.92BYA-3.85BYA: Nectarian period
By this period of time, the Moon was entirely solidified, but continued to be bombarded by asteroids, comets, and other objects. The Nectaris impacts mark the end of the pre-Nectarian period, making the Moon with a sequence of craters that today form Mare Nectaris. Parts of the lunar highlands were formed by the Nectaris impacts, as debris from the strike was ejected into the atmosphere and collected outside of the basin boundary.
3.85BYA-3.20BYA: Imbrian Period
The Imbrian period is divided into two epochs, the Early and Late Imbrian; the transition between these two epochs occurred about 3.8 billion years ago, making the Early Imbrian a much shorter epoch of time. The beginning of this period is marked by the formation of Mare Imbrium, a lava plain caused by a massive impact. The Inner Solar System was still being bombarded by space debris during the Imbrian, but this was gradually winding down.
Numerous maria created by impacts during the Early Imbrian filled with basalt during the Late Imbrian epoch. This occurred as the Moon’s mantle rose, the overlying rock having been thinned by impacts from space. Much of the near side of the Moon took its current form during the Late Imbrian.
3.20BYA-1.10BYA: Eratosthenian period
This period of time is defined by craters such as Eratosthenes. These craters are not eroded by more recent impacts, since they formed during a period of time in which impacts from space debris were infrequent. Craters formed during the Eratosthenian period typically do not show ray systems of debris blasted out during the impacts, being too old for these to remain. The lava flows that dominated the Imbrian period grew less extreme throughout the Eratosthenian, ceasing by the end of the period.
1.10BYA-present: Copernican period
During the Copernican period, which is still considered ongoing, the Moon has remained a mostly quiet and inactive body. Moonquakes occasionally occur, as do asteroid impacts (one such impact which formed the crater Tycho occurred 180 million years ago during Earth’s Cretaceous period). Craters formed during the Copernican period typically have ray systems, formed by debris being ejected out during the crater’s impact formation. Older craters lose these features as they darken due to space weathering, but younger ray systems are brightly reflective.
During the Copernican period, animal life capable of seeing the Moon evolved on Earth. The Moon, despite having settled down geologically, exerts gravitational pressure on Earth which causes tidal motion. This is most obvious in Earth’s seas, as the water level rises and falls regularly as the Moon orbits Earth. The phases of the Moon, as caused by Earth’s shadow, affect the amount of ambient light available at night. These influences affected the evolution of life on Earth.
The evolution of humans, Earth’s first technological species, brought new purpose to the Moon beyond its ecological effects. Humans intensely studied the Moon for thousands of years, and by the year 1966, vehicles had been landed on the Moon’s surface by the Russian Luna program. Humans first set foot on the Moon in 1969, with astronaut Neil Armstrong of the American Apollo missions being the first to do so. While lunar research decreased during subsequent decades, it has not stopped entirely, as several countries continue to send exploratory vehicles to the Moon.
Commercial spaceflight has not yet, as of the early 2020s, reached the Moon. The concept for this has existed for decades, though: as early as 2001, Amanda Kirby considered the idea that a wealthy person could purchase tickets for the first-ever commercial lunar spaceflight to be a believable lie.
Since prehistoric times, the Moon has been considered with reverence and awe by human beings as the second-brightest object in the sky. Because it is most visible at night, the Moon is often considered as the sun’s opposite, and this duality is mentioned in many cultures. The Moon is an important feature in many religious and spiritual beliefs as well as folklore around the world. In many cultures, the predictable monthly phases of the Moon are used to tell time, and have been since ancient history.
Scientific research into the Moon has also persisted throughout human history. Since the early days of astronomy, humans around the world have used the latest scientific technology to study and map the Moon’s surface, exploring theories behind its formation, nature, and history. The invention of human spaceflight in the 1950s and 1960s culminated with the Apollo program, in which American astronauts physically landed on the moon and walked on its surface to perform research. This continued into the 1970s.
The concept of civilians walking on the Moon has existed for some decades, though it has yet to be realized. As of 2001, the idea that commercial spaceflight to the Moon was around the corner and could be booked by wealthy people was considered a realistic-enough lie by Amanda Kirby to try and use it to pass for a wealthy person. The Moon is considered “international waters” and cannot be claimed by any country under the Outer Space Treaty of 1967, though there are few regulations enforced on the Moon due to the lack of authorities present on its surface. This has allowed research institutions and corporations to act without significant legal oversight, resulting in incidents such as one in 2019 in which live tardigrades were accidentally seeded on the Moon’s surface.
Ever since the Moon and Earth formed from molten rock over four billion years ago, they have exerted gravitational pressure on each other. While the Moon’s surface is an environment hostile to life, the Moon’s gravitational effects on Earth have had an immense effect on life there. Among the most prominent of the Moon’s effects are tides. As the Moon orbits Earth, its gravity pulls Earth’s fluids (and to some degree, its solid surface) toward the Moon. This creates high tides on the side of Earth facing the Moon as well as on the opposite side, while in between, low tides occur. This has a drastic effect on coastal regions, as the ocean rises and falls throughout the day. Organisms living on coasts must adapt to this regular change.
The Moon also reflects light from the sun, becoming a source of light at night. The Moon is often partially blocked by Earth’s shadow, creating lunar phases. Many animals, such as moths and sea turtles, use the phases of the Moon to reproduce; when the Moon is fully illuminated, it provides the maximum amount of light at night. This is capitalized upon by these creatures, who use the Moon’s light to navigate.
Because of its extremely thin atmosphere, weak magnetic field, and extremely cold temperatures, the Moon’s surface is inhospitable to life. During a brief period of time between three and four billion years ago, the Moon’s atmosphere was thick enough to support life, but there is currently no evidence of life evolving there. Throughout the history of human spaceflight, organisms from Earth have been transported to the Moon, including humans, cotton, and tardigrades. A cotton seedling sprouted in a terrarium on the Moon in January 2019, but died overnight due to the extreme cold. In April 2019, an industrial spacecraft crash released thousands of tardigrades onto the Moon’s surface. The animals have probably entered a tun state, hibernating and awaiting conditions more suitable to life. Because of the nature of the Moon’s surface, it is unlikely that the tardigrades will be able to resume normal activity without human intervention.
Behind the Scenes
The Moon has been portrayed in several installments of the Jurassic franchise. It has appeared in Jurassic Park: The Game, Jurassic World, and Jurassic World: Fallen Kingdom. In the Moon’s film appearances, its phase has been portrayed accurately with respect to the date on which the film is supposed to take place. It is incorrectly portrayed in Jurassic Park: The Game as being full on June 11, 1993, when it actually would have been illuminated on its left side only.