The
near side of the Moon is the
lunarLunar may refer to:*Relating to the Moon*Lunar , a series of console video games...
hemisphere that is permanently turned towards the
EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
, whereas the opposite side is the
far side of the MoonThe far side of the Moon is the lunar hemisphere that is permanently turned away from the Earth. The far hemisphere was first photographed by the Soviet Luna 3 probe in 1959, and was first directly observed by human eyes when the Apollo 8 mission orbited the Moon in 1968. The rugged terrain is...
. Only one side of the Moon is visible from Earth because the Moon rotates about its spin axis at the same rate that the Moon orbits the Earth, a situation known as
synchronous rotationIn astronomy, synchronous rotation is a planetological term describing a body orbiting another, where the orbiting body takes as long to rotate on its axis as it does to make one orbit; and therefore always keeps the same hemisphere pointed at the body it is orbiting...
or
tidal lockingTidal locking occurs when the gravitational gradient makes one side of an astronomical body always face another; for example, one side of the Earth's Moon always faces the Earth. A tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner. This...
. The Moon is directly illuminated by the
SunThe Sun is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 99.86% of the Solar System's mass....
, and the cyclically varying viewing conditions cause the
lunar phaseA lunar phase or phase of the moon refers to the appearance of the illuminated portion of the Moon as seen by an observer, usually on Earth. The lunar phases vary cyclically as the Moon orbits the Earth, according to the changing relative positions of the Earth, Moon and Sun...
s.
The
near side of the Moon is the
lunarLunar may refer to:*Relating to the Moon*Lunar , a series of console video games...
hemisphere that is permanently turned towards the
EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
, whereas the opposite side is the
far side of the MoonThe far side of the Moon is the lunar hemisphere that is permanently turned away from the Earth. The far hemisphere was first photographed by the Soviet Luna 3 probe in 1959, and was first directly observed by human eyes when the Apollo 8 mission orbited the Moon in 1968. The rugged terrain is...
. Only one side of the Moon is visible from Earth because the Moon rotates about its spin axis at the same rate that the Moon orbits the Earth, a situation known as
synchronous rotationIn astronomy, synchronous rotation is a planetological term describing a body orbiting another, where the orbiting body takes as long to rotate on its axis as it does to make one orbit; and therefore always keeps the same hemisphere pointed at the body it is orbiting...
or
tidal lockingTidal locking occurs when the gravitational gradient makes one side of an astronomical body always face another; for example, one side of the Earth's Moon always faces the Earth. A tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner. This...
. The Moon is directly illuminated by the
SunThe Sun is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 99.86% of the Solar System's mass....
, and the cyclically varying viewing conditions cause the
lunar phaseA lunar phase or phase of the moon refers to the appearance of the illuminated portion of the Moon as seen by an observer, usually on Earth. The lunar phases vary cyclically as the Moon orbits the Earth, according to the changing relative positions of the Earth, Moon and Sun...
s. The unilluminated portions of the Moon can sometimes be dimly seen as a result of
earthshineThe phenomenon known as planetshine occurs when reflected sunlight from a planet illuminates the dark side of one of its moons. Typically, this results in the moon's dark side being bathed in a soft, faint light. The best known example of planetshine is Earthshine, which can be seen from Earth when...
, which is sunlight reflected off the surface of the Earth and onto the Moon. Since the Moon's orbit is both somewhat elliptical and inclined to the
ecliptic planeThe plane of the ecliptic is the plane of the Earth's orbit around the Sun. It is the primary reference plane when describing the position of bodies in the Solar System, with celestial latitude being measured relative to the ecliptic plane. In the course of a year, the Sun's apparent path through...
, librations allow up to 59% of the Moon's surface to be viewed from Earth (but only half at any instant).
Names
The near side of the Moon is characterised by large dark areas that were once believed to be seas by astronomers who first mapped them in the 17th century (notably,
Giovanni Battista RiccioliGiovanni Battista Riccioli , was an Italian astronomer and a Roman Catholic Priest. He was a Jesuit who entered the order in 1614. He was also the first person to measure the rate of acceleration of a freely falling body....
and
Francesco Maria GrimaldiFrancesco Maria Grimaldi, born April 2, 1618 in Bologna and dead on December 28, 1663 in Bologna, was an Italian Jesuit priest, mathematician and physicist who taught at the Jesuit college in Bologna....
). Although it was found out later that the Moon has no water, the term "mare" (plural: maria) are still used. The lighter toned regions are referred to as "terrae", or more commonly, the "highlands"...
Orientation
The image of the Moon here is drawn as is normally shown on maps, that is with north on top and west to the left. Astronomers usually turn the map over to have south on top, as to correspond with the view in most telescopes which also show the image upside down.
Note that west and east on the Moon are where you would expect them, when standing on the Moon.. But when we, on Earth, see the Moon in the sky, then the east — west direction is just reversed. When specifying coordinates on the Moon it should therefore always be mentioned whether geographic (or rather
selenographicSelenographic coordinates are used to refer to locations on the surface of Earth's moon. Any position on the lunar surface can be referenced by specifying two numerical values, which are comparable to the latitude and longitude of Earth...
) coordinates are used or astronomical coordinates.
The actual orientation you see the Moon in the sky or on the horizon depends on your geographic
latitudeLatitude, usually denoted by the Greek letter phi gives the location of a place on Earth north or south of the equator. Lines of Latitude are the imaginary horizontal lines shown running east-to-west on maps that run either north or south of the equator...
on Earth. In the following description a few typical cases will be considered.
- On the north pole, if the Moon is visible, it stands low above the horizon with its north pole up.
- In mid northern latitudes (North America, Europe, Asia) the Moon rises in the east with its northeastern limb up (Mare Crisium), it reaches it highest point in the south with its north on top, and sets in the west with its northwestern limb (Mare Imbrium) on top.
- On the equator, when the Moon rises in the east, its N — S axis appears horizontal and Mare Foecunditatis is on top. When it sets in the west, about 12.5 hours later, the axis is still horizontal, and Oceanus Procellarum is the last area to dip below the horizon. In between these events, the Moon reached its highest point in the zenith
In general terms, the zenith is the direction pointing directly "above" a particular location; that is, it is one of two vertical directions at the location, orthogonal to a horizontal flat surface there...
and then its selenographic directions are lined up with those on Earth.
- In mid southern latitudes (South America, South Pacific, Australia, South Africa) the Moon rises in the east with its southeastern limb up (Mare Nectaris), it reaches it highest point in the north with its south on top, and sets in the west with its southwestern limb (Mare Humorum) on top.
- On the south pole the Moon behaves as on the north pole, but there it appears with its south pole up.
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