Concept

Moon

Overview

The Moon is the only natural satellite of Earth. It orbits around Earth at an average distance of 384,399 kilometers (238,854 mi), a distance roughly 30 times the width of Earth. It completes an orbit (lunar month) in relation to Earth and the Sun (synodically) every 29.5 days. The Moon and Earth are bound by gravitational attraction, which is stronger on the sides facing each other. The resulting tidal forces are the main driver of Earth's tides, and have pulled the Moon to always face Earth with the same near side. This tidal locking effectively synchronizes the Moon's rotation period (lunar day) to its orbital period (lunar month).

In geophysical terms, the Moon is a planetary-mass object or satellite planet. Its mass is 1.2% that of the Earth, and its diameter is 3,474 km (2,159 mi), roughly one-quarter of Earth's (about as wide as the contiguous United States). Within the Solar System, it is larger and more massive than any known dwarf planet, and the fifth-largest and fifth–most massive moon, as well as the largest and most massive in relation to its parent planet. Its surface gravity is about one-sixth of Earth's, about half that of Mars, and the second-highest among all moons in the Solar System after Jupiter's moon Io. The body of the Moon is differentiated and terrestrial, with only a minuscule hydrosphere, atmosphere, and magnetic field. The lunar surface is covered in regolith dust, which mainly consists of the fine material ejected from the lunar crust by impact events. The lunar crust is marked by impact craters; some younger ones feature bright ray-like streaks. The Moon was volcanically active until 1.2 billion years ago, surfacing lava mostly on the thinner near side of the Moon, filling ancient craters, which through cooling formed the today prominently visible dark plains of basalt called maria ('seas'). The origin of the Moon is not clear, although it is thought to have been formed out of material from Earth that was ejected by a giant impact of a Mars-sized body 4.51 billion years ago, not long after Earth's formation.

From a distance, the day and night phases of the lunar day are visible as the lunar phases, and when the Moon passes through Earth's shadow a lunar eclipse is observable. The Moon's apparent size in Earth's sky is about the same as that of the Sun, which causes it to cover the Sun completely during a total solar eclipse. The Moon is the brightest celestial object in Earth's night sky because of its large apparent size, while the reflectance (albedo) of its surface is comparable to that of asphalt. About 59% of the surface of the Moon is visible from Earth owing to the different angles at which the Moon can appear in Earth's sky (libration), making parts of the far side of the Moon visible.

The Moon has been an important source of inspiration and knowledge in human history, having been crucial to cosmography, mythology, religion, art, time keeping, natural science and spaceflight. The first spaceflights to an extraterrestrial body were to the Moon, starting in 1959 with the flyby of Luna 1 (sent by the Soviet Union), and the intentional impact of Luna 2, followed in 1966 by the first soft landing (by Luna 9) and orbital insertion (by Luna 10). Humans first arrived in orbit with Apollo 8 (sent by the United States) on December 24, 1968, and then on the surface with Apollo 11 on July 20, 1969, making the Moon the only celestial body beyond Earth that humans have visited. By 1972, six Apollo missions had landed twelve people on the Moon and stayed up to three days. Renewed robotic exploration of the Moon, in particular to confirm the presence of water on the Moon, has fueled plans to return humans to the Moon, starting with the Artemis program scheduled for the late 2020s.

Overview adapted from Wikipedia (CC BY-SA 4.0; see the article history for authors).

In the original texts

Passages from public-domain books where Moon is discussed.

Anaximenes believes that the mouth of the wheel, about which the moon is turned, being stopped is the cause of an eclipse. Berasus, that it proceeds from the turning of the dark side of the lunar orb towards us. Heraclitus, that it is performed just after the manner of a boat turned upside downwards.
Plutarch, Complete Works of Plutarch — Volume 3
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At conjunction, when the Moon occupies a position between the Sun and the Earth, the Moon is illuminated by the Sun’s rays on her half towards the Sun which is turned away from the Earth, and the other half, with which she regards the Earth, is covered with darkness, and so in no degree illumines the Earth’s surface. When the Moon has slightly separated from the Sun, straightway she is partly illumined on the half…
Galileo Galilei, The sidereal messenger of Galileo Galilei
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The orbit of the Moon is not precisely in the same Plane with that of the Earth; but makes a very small angle with it. The points of intersection with those two Planes, are called, the Nodes of the Moon. These Nodes of the Moon are in continual motion, and in eighteen or nineteen years, revolve backwards, from east to west, through all the different points of the Ecliptic. For the Moon, after having finished her…
Adam Smith, The Essays of Adam Smith
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In the heavens, the Sun and Moon are, by interpreters of dreams, put for the persons of Kings and Queens; but in sacred Prophecy, which regards not single persons, the Sun is put for the whole species and race of Kings, in the kingdom or kingdoms of the world politic, shining with regal power and glory; the Moon for the body of the common people, considered as the King's wife; the Stars for subordinate Princes and…
Isaac Newton, Observations upon the Prophecies of Daniel, and the Apocalypse of St. John
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The more equal the globules of Water or Ice are to one another, the more Crowns of Colours will appear, and the Colours will be the more lively. The Halo at the distance of 22-1/2 Degrees from the Moon is of another sort. By its being oval and remoter from the Moon below than above, I conclude, that it was made by Refraction in some sort of Hail or Snow floating in the Air in an horizontal posture, the refracting…
Isaac Newton, Opticks
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Philosophers who discussed Moon

Related ideas

Opposing perspectives

Geocentrism

Place in the tradition

Part of Cosmology.

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