Earth's Nearest Neighbour
The Moon (Latin: Luna) is Earth's only natural satellite and the fifth largest satellite in the Solar System. The average centre-to-centre distance from the Earth to the Moon is 384,403 km, about thirty times the diameter of the Earth. The common centre of mass of the system (the barycentre) is located about 1,700 kma quarter the Earth's radiusbeneath the surface of the Earth. The Moon makes a complete orbit around the Earth every 27.3 days (the orbital period), and the periodic variations in the geometry of the EarthMoonSun system are responsible for the lunar phases that repeat every 29.5 days (the synodic period).
The Moon's diameter is 3,474 km, a little more than a quarter of that of the Earth. Thus, the Moon's surface area is less than a tenth that of the Earth (about a quarter the Earth's land area, approximately as large as Russia, Canada, and the United States combined), and its volume is about 2 percent that of Earth. The pull of gravity at its surface is about 17 percent of that at the Earth's surface.
The Moon is the only celestial body to which humans have traveled and upon which humans have performed a manned landing. The first artificial object to pass near the Moon was the Soviet Union's Luna 1, the first artificial object to impact the lunar surface was Luna 2, and the first photographs of the normally occluded far side of the Moon were made by Luna 3, all in 1959. The first spacecraft to perform a successful lunar soft landing was Luna 9, and the first unmanned vehicle to orbit the Moon was Luna 10, both in 1966. The United States (U.S.) Apollo program achieved the only manned missions to date, resulting in six landings between 1969 and 1972. Human exploration of the Moon ceased with the conclusion of the Apollo program, although a few robotic landers and orbiters have been sent to the Moon since that time. Several countries have announced plans to return humans to the surface of the Moon in the 2020s.The proper English name for Earth's natural satellite is, simply, the Moon (capitalized). Moon is a Germanic word, related to the Latin mensis (month). It is ultimately a derivative of the Proto-Indo-European root me-, also represented in measure (time), with reminders of its importance in
measuring time in words derived from it like Monday, month and menstrual. The related adjective is lunar, as well as an adjectival prefix seleno- and suffix -selene (from selene, the Ancient Greek word for the Moon). In English, the word moon exclusively meant "the Moon" until 1665, when it was extended to refer to the recently-discovered natural satellites of other planets. Subsequently, these objects were given distinct names in order to avoid confusion. The Moon is occasionally referred to by its Latin name Luna, primarily in science fiction.
Sides of the Moon|
The Moon is in synchronous rotation, which means that it rotates about its axis in about the same time it takes to orbit the Earth. This results in it keeping nearly the same face turned towards the Earth at all times. The Moon used to rotate at a faster rate, but early in its history, its rotation slowed and became locked in this orientation as a result of frictional effects associated with tidal deformations caused by the Earth.
variations (libration) in the angle from which the Moon is seen allow about 59%
of its surface to be seen from the Earth (but only half at any instant). See notes
on 'Libration' below.
Lunar crater Daedalus on the Moon's far sideThe Moon's surface is marked by impact craters which form when asteroids and comets collide with the lunar surface. There are about half a million craters with diameters greater than 1 km on the moon. Since impact craters accumulate at a nearly constant rate, the number of craters per unit area superposed on a geologic unit can be used to estimate the age of the surface (see crater counting). The lack of an atmosphere, weather and recent geological processes ensures that many of these craters have remained relatively well preserved in comparison to those on Earth.
The largest crater on the Moon, which also has the distinction of being one of the largest known craters in the Solar System, is the South Pole-Aitken basin. It is on the far side, between the South Pole and equator, and is some 2,240 km in diameter and 13 km in depth. Prominent impact basins on the near side include Imbrium, Serenitatis, Crisium, and Nectaris.
Blanketed atop the Moon's crust is a highly comminuted (broken into ever smaller particles) and "impact gardened" surface layer called regolith. Since the regolith forms by impact processes, the regolith of older surfaces is generally thicker than for younger surfaces. In particular, it has been estimated that the regolith varies in thickness from about 35 m in the maria, and by about 1020 m in the highlands. Beneath the finely comminuted regolith layer is what is generally referred to as the megaregolith. This layer is much thicker (on the order of tens of kilometres) and comprises highly fractured bedrock
In astronomy libration (from the Latin verb librare "to balance, to sway", cf. libra "scales") refers to the various orbital conditions which make it possible to see more than 50% of the moon's surface over time, even though the front of the Moon is tidally locked to always face towards Earth. By extension, libration can also be used to describe the same phenomenon for other orbital bodies that are nominally locked to present the same face. As the orbital processes are repetitive, libration is manifested as a slow rocking back and forth (or up and down) of the face of the orbital body as viewed from the parent body, much like the rocking of a pair of scales about the point of balance.
In the specific case of the Moon's librations, this motion permits a terrestrial observer to see slightly differing halves of the Moon's surface at different times. This means that a total of 59% of the Moon's surface can be observed from Earth.
There are three types of libration:
Libration in longitude is a consequence of the Moon's orbit around Earth being somewhat eccentric, so that the Moon's rotation sometimes leads and sometimes lags its orbital position.
in latitude is a consequence of the Moon's axis of rotation being slightly inclined
to the normal to the plane of its orbit around Earth. Its origin is analogous
to the way in which the seasons arise from Earth's revolution about the Sun. |
Diurnal libration is a small daily oscillation due to the Earth's rotation, which carries an observer first to one side and then to the other side of the straight line joining Earth's center to the Moon's center, allowing the observer to look first around one side of the Moon and then around the other. This is because the observer is on the surface of the Earth, not at its centre.