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About Moon a astronomical body that orbits Earth | Solar System

Moon

The Moon is an astronomical body that orbits Earth as its only natural satellite. It is the fifth-largest satellite in the Solar System, and the largest among planetary satellites relative to the size of the planet that it orbits (its primary). The Moon is, after Jupiter's satellite Io, the second-densest satellite in the Solar System among those whose densities are known.

The Moon is thought to have formed about 4.51 billion years ago, not long after Earth. The most widely accepted explanation is that the Moon formed from the debris left over after a giant impact between Earth and a Mars-sized body called Theia. New research of Moon rocks, although not rejecting the Theia hypothesis, suggests that the Moon may be older than previously thought.

The Moon is in synchronous rotation with Earth, and thus always shows the same side to Earth, the near side. The near side is marked by dark volcanic maria that fill the spaces between the bright ancient crustal highlands and the prominent impact craters. After the Sun, the Moon is the second-brightest regularly visible celestial object in Earth's sky. Its surface is actually dark, although compared to the night sky it appears very bright, with a reluctance just slightly higher than that of worn asphalt. Its gravitational influence produces the ocean tides, body tides, and the slight lengthening of the day.

Moon 
moon
Full moon seen from the North America continent of Earth in 2010
Designations
Adjectives
  • Lunar
  • selenic
Orbital characteristics
Epoch J2000
Perigee362600 km
(356400370400 km)
Apogee405400 km
(404000406700 km)
Semi-major axis
384399 km  (0.00257 AU)
Eccentricity0.0549
Orbital period
27.321661 d
(27 d 7 h 43 min 11.5 s)
Synodic period
29.530589 d
(29 d 12 h 44 min 2.9 s)
Average orbital speed
1.022 km/s
Inclination5.145° to the ecliptic
Longitude of ascending node
Regressing by one revolution in 18.61 years
Argument of perigee
Progressing by one
revolution in 8.85 years
Satellite ofEarth
Physical characteristics
Mean radius
1737.4 km  
(0.2727 of Earth's)
Equatorial radius
1738.1 km  
(0.2725 of Earth's)
Polar radius
1736.0 km  
(0.2731 of Earth's)
Flattening0.0012
Circumference10921 km  (equatorial)
Surface area
3.793×107 km2  
(0.074 of Earth's)
Volume2.1958×1010 km3  
(0.020 of Earth's)
Mass7.342×1022 kg  
(0.012300 of Earth's)
Mean density
3.344 g/cm3
0.606 × Earth
Surface gravity
1.62 m/s2  (0.1654 g)
Moment of inertia factor
0.3929±0.0009
Escape velocity
2.38 km/s
Sidereal rotation period
27.321661 d  
Equatorial rotation velocity
4.627 m/s
Axial tilt
  • 1.5424° to ecliptic
  • 6.687° to orbit plane
  • 24° to Earth's equator 
North pole right 
ascension
  •      17h 47m 26s
  •       266.86°
North pole declination
65.64°
Albedo0.136
Surface temp.minmeanmax
Equator100 K220 K390 K
85°N 150 K230 K
Apparent magnitude
  • −2.5 to −12.9
  • −12.74  (mean full moon)
Angular diameter
29.3 to 34.1 arcminutes
Atmosphere
Surface pressure
  • 10−7 Pa (1 picobar)  (day)
  • 10−10 Pa (1 femtobar)   
    (night)
Composition by volume
  • He
  • Ar
  • Ne
  • Na
  • K
  • H
  • Rn
The Moon's average orbital distance is 384,402 km (238,856 mi), or 1.28 light-seconds. This is about thirty times the diameter of Earth. The Moon's apparent size in the sky is almost the same as that of the Sun, since the star is about 400 times the lunar distance and diameter. Therefore, the Moon covers the Sun nearly precisely during a total solar eclipse. This matching of apparent visual size will not continue in the far future because the Moon's distance from Earth is gradually increasing.

The Moon was first reached in September 1959 by the Soviet Union's Luna 2, an unmanned spacecraft, followed by the first successful soft landing by Luna 9 in 1966. The United States' NASA Apollo program achieved the only manned lunar missions to date, beginning with the first manned orbital mission by Apollo 8 in 1968, and six manned landings between 1969 and 1972, with the first being Apollo 11 in July 1969. These missions returned lunar rocks which have been used to develop a geological understanding of the Moon's origin, internal structure, and the Moon's later history. Since the 1972 Apollo 17 mission the Moon has been visited only by unmanned spacecraft.

Both the Moon's natural prominence in the earthly sky and its regular cycle of phases as seen from Earth have provided cultural references and influences for human societies and cultures since time immemorial. Such cultural influences can be found in language, lunar calendar systems, art, and mythology.

Name and etymology

The usual English proper name for Earth's natural satellite is "the Moon", which in nonscientific texts is usually not capitalized. The noun moon is derived from Old English m┼Źna, which (like all Germanic language cognates) stems from Proto-Germanic, which comes from Proto-Indo-European  "moon", "month", which comes from the Proto-Indo-European root *meh₁- "to measure", the month being the ancient unit of time measured by the Moon. Occasionally, the name "Luna" is used. In literature, especially science fiction, "Luna" is used to distinguish it from other moons, while in poetry, the name has been used to denote personification of Earth's moon.

The modern English adjective pertaining to the Moon is lunar, derived from the Latin word for the Moon, luna. The adjective selenic (usually only used to refer to the chemical element selenium) is so rarely used to refer to the Moon that this meaning is not recorded in most major dictionaries. It is derived from the Ancient Greek word for the Moon, from which is however also derived the prefix "seleno-", as in selenography, the study of the physical features of the Moon, as well as the element name selenium. Both the Greek goddess Selene and the Roman goddess Diana were alternatively called Cynthia. The names Luna, Cynthia, and Selene are reflected in terminology for lunar orbits in words such as apolune, pericynthion, and selenocentric. The name Diana comes from the Proto-Indo-European *diw-yo, "heavenly", which comes from the PIE root *dyeu- "to shine," which in many derivatives means "sky, heaven, and god" and is also the origin of Latin dies, "day".

Formation

The Moon formed 4.51 billion years ago, some 60 million years after the origin of the Solar System. Several forming mechanisms have been proposed, including the fission of the Moon from Earth's crust through centrifugal force (which would require too great an initial spin of Earth), the gravitational capture of a pre-formed Moon (which would require an unfeasibly extended atmosphere of Earth to dissipate the energy of the passing Moon), and the co-formation of Earth and the Moon together in the primordial accretion disk (which does not explain the depletion of metals in the Moon). These hypotheses also cannot account for the high angular momentum of the Earth–Moon system.

The prevailing hypothesis is that the Earth–Moon system formed after an impact of a Mars-sized body (named Theia) with the proto-Earth (giant impact). The impact blasted material into Earth's orbit and then the material accreted and formed the Moon.

The Moon's far side has a crust that is 50 km (31 mi) thicker than that of the near side. This is thought to be because the Moon fused from two different bodies.

This hypothesis, although not perfect, perhaps best explains the evidence. Eighteen months prior to an October 1984 conference on lunar origins, Bill Hartmann, Roger Phillips, and Jeff Taylor challenged fellow lunar scientists: "You have eighteen months. Go back to your Apollo data, go back to your computer, do whatever you have to, but make up your mind. Don't come to our conference unless you have something to say about the Moon's birth." At the 1984 conference at Kona, Hawaii, the giant impact hypothesis emerged as the most consensual theory.

Before the conference, there were partisans of the three "traditional" theories, plus a few people who were starting to take the giant impact seriously, and there was a huge apathetic middle who didn't think the debate would ever be resolved. Afterward, there were essentially only two groups: the giant impact camp and the agnostics.

Giant impacts are thought to have been common in the early Solar System. Computer simulations of giant impacts have produced results that are consistent with the mass of the lunar core and the angular momentum of the Earth–Moon system. These simulations also show that most of the Moon derived from the impactor, rather than the proto-Earth. However, more recent simulations suggest a larger fraction of the Moon derived from the proto-Earth. Other bodies of the inner Solar System such as Mars and Vesta have, according to meteorites from them, very different oxygen and tungsten isotopic compositions compared to Earth. However, Earth and the Moon have nearly identical isotopic compositions. The isotopic equalization of the Earth-Moon system might be explained by the post-impact mixing of the vaporized material that formed the two, although this is debated.

The impact released a lot of energy and then the released material re-accreted into the Earth–Moon system. This would have melted the outer shell of Earth, and thus formed a magma ocean. Similarly, the newly formed Moon would also have been affected and had its own lunar magma ocean; its depth is estimated from about 500 km (300 miles) to 1,737 km (1,079 miles).

While the giant impact hypothesis might explain many lines of evidence, some questions are still unresolved, most of which involve the Moon's composition.

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