Moon
The Moon is Earth's only natural satellite and the nearest large celestial body to the planet. It orbits at an average distance of about 384,000 kilometres. Its gravity contributes to ocean tides, while its rocky surface preserves billions of years of impact and volcanic history.[2]
Size and physical characteristics
The Moon is about 3,475 km in diameter, slightly more than one quarter of Earth's diameter. Its equator measures approximately 10,920 km around; on an imaginary road encircling the Moon, a car travelling at 100 km/h would complete the journey in about four and a half days without stopping.[4][1]
Its mass is approximately 7.35 × 1022 kg, and its average density is 3.34 g/cm³, substantially lower than Earth's. Surface gravity is about one sixth as strong as on Earth. A person with a mass of 72 kg would retain the same mass but press on the ground with roughly the same force as a 12 kg load on Earth.[4]
Orbit and rotation
The Moon follows an elliptical orbit around Earth at an average speed of about 1 km/s. One revolution relative to distant stars takes 27.3 Earth days. Because the distance between the centres of Earth and the Moon changes, the Moon's apparent size in the sky also varies.[1]
The rotation period is equal to the orbital period. This synchronous rotation keeps approximately the same hemisphere facing Earth. Small apparent oscillations called librations gradually reveal regions beyond the edge of the familiar lunar disc.[5]
The Moon is slowly receding from Earth at a current rate of about 3.8 cm per year. Tidal interaction causes Earth's rotation to slow while transferring part of its angular momentum to the Moon's orbital motion.[6]
Phases and eclipses
The Moon shines by reflected sunlight. As it moves around Earth, the visible fraction of its illuminated hemisphere changes through new moon, crescent, first quarter, full moon and the waning phases. A complete phase cycle takes about 29.5 days. This is longer than the revolution relative to the stars because Earth and the Moon move along their orbit around the Sun during the same interval.[5]
The far side also receives sunlight. Across almost all of its surface, lunar day and night alternate just as they do on the near side.[5]
A solar eclipse occurs when the Moon blocks the Sun for an observer on Earth. During a lunar eclipse, the Moon passes through Earth's shadow. Eclipses do not happen every month because the lunar orbit is inclined to Earth's orbital plane, so the Moon usually passes above or below the line of exact alignment.[7]
Surface and topography
The dark areas of the lunar disc are the maria, broad plains formed by solidified basaltic lava. They received their names long before their true nature was understood. The brighter highlands consist mainly of ancient rocks rich in feldspar.[3]
The surface is covered by regolith, a loose mixture of dust, rock fragments and glassy particles. It forms as meteoroid impacts break up and repeatedly mix the ground. The Moon has no rain or rivers to erode the terrain, allowing many craters to survive for extremely long periods.[8][9]
One of the most conspicuous craters on the near side is Tycho, about 85 km across. Bright rays of ejected material extend outward from it. Its age is estimated at approximately 110 million years, making it much younger than many surrounding regions.[9]
Internal structure and moonquakes
The Moon has a crust, mantle and small metallic core. The mantle consists mainly of silicate rock. Measurements of rotation and gravity, together with seismic waves, indicate that the core has a solid inner part and a liquid outer part.[3]
The lunar interior remains seismically active. Deep moonquakes are associated with tidal deformation caused by Earth. Shallower events can result from movement along faults, meteoroid impacts and thermal expansion of rocks.[10]
As the Moon cools, it slowly contracts. Its brittle crust fractures under compression, producing scarps and thrust faults. Comparisons between Lunar Reconnaissance Orbiter images and seismic data indicate that some of these structures may remain active.[11]
Temperature and exosphere
The sunlit surface can warm to about 127 °C, while temperatures during the lunar night fall to roughly −173 °C. The Moon has no dense atmosphere to transport heat efficiently or moderate these extremes. Its exceptionally thin envelope of gas is known as an exosphere.[2]
Different conditions exist near the poles. The floors of some craters never receive direct sunlight and remain far colder than the ordinary night-time surface.[12]
Water on the Moon
Water ice has been detected on the Moon, including in permanently shadowed polar regions. In 2009, the LCROSS mission examined material thrown up by a deliberate impact in Cabeus crater near the south pole. Observations identified water in the ejected material.[13]
In 2020, the SOFIA observatory confirmed molecular water on the sunlit surface near Clavius crater. The finding concerns small quantities of water associated with lunar soil rather than exposed bodies of liquid water.[14]
Origin
The Moon formed about 4.5 billion years ago. The leading family of hypotheses connects its origin to a major collision between the young Earth and another large body. Material thrown into orbit then accumulated into the satellite. In a widely used form of this model, the impactor—conventionally named Theia—was approximately the size of Mars.[15]
The young Moon was intensely heated. As its magma ocean cooled, low-density minerals rose toward the surface and contributed to the early crust. The composition of lunar rocks and their similarities to terrestrial material provide important evidence for testing models of lunar origin.[15]
Spacecraft exploration
In 1959, Luna 1 became the first spacecraft to fly past the Moon. In 1966, Luna 9 made the first successful soft landing and transmitted images from the surface.[16]
On 20 July 1969, the Apollo 11 lunar module landed in Mare Tranquillitatis with Neil Armstrong and Buzz Aldrin, while Michael Collins remained in lunar orbit. The mission marked the first human landing on another celestial body.[17]
The European SMART-1 spacecraft studied the Moon from orbit between 2004 and 2006, examining the surface and its composition.[18] NASA's Lunar Reconnaissance Orbiter, launched in 2009, has produced detailed maps of topography, illumination and surface temperature.[19]
On 23 August 2023, India's Chandrayaan-3 mission made a soft landing at high southern latitudes. Its lander and rover investigated the properties and composition of the lunar soil.[20]
On 25 June 2024, China's Chang'e 6 mission delivered the first samples from the far side of the Moon to Earth. The material enables direct comparison between the Moon's two hemispheres and provides new evidence about its early history.[21]
See also
References
- ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 1.14 1.15 1.16 Moon Fact Sheet // NASA, National Space Science Data Center.
- ↑ 2.0 2.1 2.2 2.3 Moon Facts // NASA Science.
- ↑ 3.0 3.1 3.2 Moon Composition // NASA Science.
- ↑ 4.0 4.1 Planetary Fact Sheet — Metric // NASA, National Space Science Data Center.
- ↑ 5.0 5.1 5.2 Moon Phases // NASA Science.
- ↑ Tides // NASA Science.
- ↑ Eclipses and the Moon // NASA Science.
- ↑ What Is Lunar Regolith? The Moon's Surface Explained // NASA Science.
- ↑ 9.0 9.1 Moon Craters // NASA Science.
- ↑ Moonquakes // NASA Science.
- ↑ Shrinking Moon Causing Moonquakes and Faults Near Lunar South Pole // NASA. 25 January 2024.
- ↑ Moon Water and Ices // NASA Science.
- ↑ LCROSS // NASA Science.
- ↑ NASA's SOFIA Discovers Water on Sunlit Surface of Moon // NASA. 26 October 2020.
- ↑ 15.0 15.1 Moon Formation // NASA Science.
- ↑ Moon Exploration // NASA Science.
- ↑ Apollo 11 // NASA.
- ↑ SMART-1 overview // ESA.
- ↑ Lunar Reconnaissance Orbiter // NASA Science.
- ↑ Release of Chandrayaan-3 science data to the Scientific Community // ISRO. 2024.
- ↑ Chang'e 6 mission successfully completes the first sample return from the far side of the Moon // CNSA. 25 June 2024.