The Moon

A ‘moon’ is also referred to as an natural satellite orbiting another body in space.

The Moon (Latin: Luna) is the Earth's only natural satellite. Although not the largest natural satellite in the Solar System, it is, among the satellites of major planets, the largest relative to the size of the object it orbits and, after Jupiter's satellite Io, it is the second most dense satellite known. 

Recent reports: Water molecules have been discovered on the near (sunlit) side of the moon probably in the form of molecules trapped in glass formations formed from the impact of meteoroids on the surface. Ref: Nature Astronomy

The full moon as viewed from the Southern Hemisphere 17th June 2019. image: R.Conan-Davies

The Moon is in synchronous rotation with Earth (sometimes referred to as tidally locked), always showing the same face with its near side marked by dark volcanic maria that fill between the bright ancient crustal highlands and the prominent impact craters. 

Relative brightness

It is the most luminous object in the sky after the Sun. Although it appears a very bright white, its surface is actually dark, with a reflectance just slightly higher than that of worn asphalt. 

Its prominence in the sky and its regular cycle of phases have, since ancient times, made the Moon an important cultural influence on language, calendars, art, and mythology. 

Major geographic locations of the moon. image: Peter Freiman Cmglee Background photograph by Gregory H. Revera/ wikipedia

The Moon's gravitational influence produces the ocean tides and the slight lengthening of the day. 

Orbital characteristics

Apogee (furthest):  405,400 km

Perigee (closest) : 362, 600 km

The Moon's current orbital distance is about thirty times the diameter of Earth, causing it to have an apparent size in the sky almost the same as that of the Sun. 

Scale model distance between Earth and the Moon. image: wikipedia

This allows the Moon to cover the Sun nearly precisely in total solar eclipse. This matching of apparent visual size is a coincidence. The Moon's linear distance from Earth is currently increasing at a rate of 3.82±0.07 cm per year, but this rate is not constant. 

Formation of the moon

The Moon is thought to have formed nearly 4.5 billion years ago, not long after Earth. Although there have been several hypotheses for its origin in the past, the current most widely accepted explanation is that the Moon formed from the debris left over after a giant impact between proto-Earth and a Mars-sized body (named Theia) .

Simplified diagram showing the formation of the Moon. image: wikipedia

A recent supercomputer simulation of the formation of the Moon

Structure/compostion of the moon

The Moon is a differentiated body: it has a geochemically distinct crust, mantle, and core. The Moon has a solid iron-rich inner core with a radius of 240 kilometers and a fluid outer core primarily made of liquid iron with a radius of roughly 300 kilometers. Around the core is a partially molten boundary layer with a radius of about 500 kilometers. 

Model of internal structure the moon. image: Kelvinsong/wikipedia

 This structure is thought to have developed through the fractional crystallization of a global magma ocean shortly after the Moon's formation 4.5 billion years ago. 

 Crystallization of this magma ocean would have created a mafic(dark coloured) mantle from the precipitation and sinking of the minerals olivine, clinopyroxene, and orthopyroxene; after about three-quarters of the magma ocean had crystallised, lower-density plagioclase minerals could form and float into a crust on top.

Human encounters

The Moon is the only celestial body other than Earth on which humans have currently set foot. The Soviet Union's Luna programme was the first to reach the Moon with unmanned spacecraft in 1959; the United States' NASA Apollo program achieved the only manned missions to date, beginning with the first manned lunar orbiting mission by Apollo 8 in 1968, and six manned lunar landings between 1969 and 1972, with the first being Apollo 11. 

Neil Armstrong became the first person to walk on the Moon as the commander of the American mission Apollo 11 by first setting foot on the Moon at 02:56 UTC on 21 July 1969.

These missions returned over 380 kg of lunar rocks, which have been used to develop a geological understanding of the Moon's origin, the formation of its internal structure, and its subsequent history.

Post Apollo missions

After the Apollo 17 mission in 1972, the Moon has been visited by only unmanned spacecraft. Of these, orbital missions have dominated: Since 2004, Japan, China, India, the United States, and the European Space Agency have each sent lunar orbiters, which have contributed to confirming the discovery of lunar water ice in permanently shadowed craters at the poles and bound into the lunar regolith. The post-Apollo era has also seen two rover missions: the final Soviet Lunokhod mission in 1973, and China's ongoing Chang'e 3 mission, which deployed its Yutu rover on 14 December 2013.

The jurisdiction of the moon

Future manned missions to the Moon have been planned, including government as well as privately funded efforts. The Moon remains, under the Outer Space Treaty, free to all nations to explore for peaceful purposes.

source adapted from: Moon. (2014, September 8). In Wikipedia, The Free Encyclopedia. Retrieved 06:25, September 23, 2014, from