Luna (Loo-nuh)

Overview

Luna (sometimes known simply as The Moon) is Earth's only natural satellite. It is about one-quarter of Earth in diameter. In the Solar System it is the fifth largest natural satellite, larger than any of the known dwarf planets and the largest (and most massive) satellite of a planet relative to its primary planet. It lacks any significant natural atmosphere or magnetic field, requiring many habitation measures both technological and thaumaturgic. Its surface gravity is about one-sixth of Earth's (0.1654 g). Jupiter's moon Io is the only satellite in the Solar System known to have a higher surface gravity and density.

Orbiting Earth at an average distance of 384,400 km (238,900 mi), or about 30 times Earth's diameter, its gravitational influence very slowly lengthens Earth's day and is the main driver of Earth's tides. Luna is tidally locked to Earth, which means that the length of a full rotation of the moon on its own axis causes its same side (the near side) to always face Earth, and the somewhat longer lunar day is the same as the synodic period.

Luna is by size and mass the fifth largest natural satellite of the Solar System. It is smaller than Mercury and considerably larger than the largest dwarf planet of the Solar System, Pluto. While the minor-planet moon Charon of the Pluto-Charon system is larger relative to Pluto, the Moon is the largest natural satellite of the Solar System relative to their primary planets.

Luna’s diameter is about 3,500 km, more than a quarter of Earth's. The whole surface area of Luna is about 38 million square kilometers, slightly less than the area of the Earth continent known as The Americas.

The moon’s mass is 1/81 of Earth's, being the second densest among the planetary moons, and having the second highest surface gravity, after Io, at 0.1654 g.

Luna originally formed as a differentiated body under hydrostatic equilibrium after a Mars-sized object collided with the young developing Earth. Most of Luna’s matter is believed to originate from Theia, but some elements and isotopes show evidence that at least some of Luna’s mass must be from that young Earth.

Geography

Luna’s 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 highest elevations of the Moon's surface are located directly to the northeast, which might have been thickened by the oblique formation impact of the South Pole–Aitken basin. Other large impact basins such as Mare Imbrium, Serenitatis, Crisium, Smythii, and Orientale possess regionally low elevations and elevated rims. The far side of the lunar surface is on average about 1.9 km higher than that of the near side.
The main features visible from Earth by the naked eye are dark and relatively featureless Lunar plains called maria (singular mare; Latin for "seas", as they were once believed to be filled with water) are vast solidified pools of ancient basaltic lava. The majority of these lava deposits erupted or flowed into the depressions associated with impact basins. Several geologic provinces containing shield volcanoes and volcanic domes are found within the near side maria. While Luna’s crater basins are named after oceans and seas, Lake Tranquility is the only real body of water visible from orbit.
The lighter-colored regions of Luna are called terrae, or more commonly highlands, because they are higher than most maria. They have been dated to having formed 4.4 billion years ago, and may represent plagioclase cumulates of the Lunar magma ocean. In contrast to Earth, no major Lunar mountains are believed to have formed as a result of tectonic events. The concentration of maria on the near side likely reflects the substantially thicker crust of the highlands of the Far Side, which may have formed in a slow-velocity impact of a second moon of Earth a few tens of millions of years after the Moon's formation. Alternatively, it may be a consequence of asymmetrical tidal heating when the Moon was much closer to the Earth.

Localized Phenomena

Due to thousands of years of war, excavation, and disasters both manmade and natural Luna has fractured, no longer held together in hydrostatic equilibrium. Instead, Luna’s mass is in several fragments held together by their own gravity, patchwork scaffolding, gravitational enchantments, and an estimated 73 million square meters of duct tape. 
The vast majority of Luna’s mass is contained in its three largest fragments: Selene, Diana, and Cynthia respective to their size. Selene is by far the largest of Luna’s fragments, more massive than Diana and Cynthia combined. A significant mass of particulates too small to be secured floats nearby in Luna's gravitational shadow, forming a prototypical ring around Earth.

History

The most widely accepted hypothesis for Luna's origin posits that it formed 4.51 billion years ago, not long after Earth, out of the debris from an impact between the planet and a hypothesized Mars-sized body called Theia. It then receded to a wider orbit because of tidal interaction with the Earth. The near side of Luna is marked by dark volcanic maria ("seas"), which fill the spaces between bright ancient crustal highlands and prominent impact craters. Most of the large impact basins and mare surfaces were in place some three billion years ago.
Both Luna’s prominence in Earth's sky and its regular cycle of phases have provided cultural references and influences for human societies throughout history. Such influences can be found in language, calendar systems, art, and mythology. The first manned extraterrestrial flights were a series of landings on Luna by the ancient United States Apollo program, which landed twelve humans on the surface in the late 2nd Millenium. No permanent colonies were established on the moon until

Alternative Name(s)

The Moon

Type

Planetoid / Moon

Included Locations

• Middle City