document_title: Milky Way

question_text: where does the sun lie within the milky way galaxy

question_tokens: where does the sun lie within the milky way galaxy

root

text: Milky Way

text: Appearance

text: The "Milky Way" can be seen as a hazy band of white light some 30 degrees wide arcing across the sky.[39] Although all the individual naked-eye stars in the entire sky are part of the Milky Way,[40][41] the light in this band originates from the accumulation of unresolved stars and other material located in the direction of the galactic plane. Dark regions within the band, such as the Great Rift and the Coalsack, are areas where light from distant stars is blocked by interstellar dust. The area of the sky obscured by the Milky Way is called the Zone of Avoidance.

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text: The Milky Way has a relatively low surface brightness. Its visibility can be greatly reduced by background light such as light pollution or stray light from the Moon. The sky needs to be darker than about 20.2 magnitude per square arcsecond in order for the Milky Way to be seen.[42] It should be visible when the limiting magnitude is approximately +5.1 or better and shows a great deal of detail at +6.1.[43] This makes the Milky Way difficult to see from any brightly lit urban or suburban location, but very prominent when viewed from a rural area when the Moon is below the horizon.[nb 2] The new world atlas of artificial night sky brightness shows that more than one-third of Earth's population cannot see the Milky Way from their homes due to light pollution.[44]

text: As viewed from Earth, the visible region of the Milky Way's Galactic plane occupies an area of the sky that includes 30 constellations.[45] The center of the Galaxy lies in the direction of the constellation Sagittarius; it is here that the Milky Way is brightest. From Sagittarius, the hazy band of white light appears to pass around to the Galactic anticenter in Auriga. The band then continues the rest of the way around the sky, back to Sagittarius. The band divides the night sky into two roughly equal hemispheres.

text: The Galactic plane is inclined by about 60 degrees to the ecliptic (the plane of Earth's orbit). Relative to the celestial equator, it passes as far north as the constellation of Cassiopeia and as far south as the constellation of Crux, indicating the high inclination of Earth’s equatorial plane and the plane of the ecliptic, relative to the Galactic plane. The north Galactic pole is situated at right ascension 12h 49m, declination +27.4° (B1950) near β Comae Berenices, and the south Galactic pole is near α Sculptoris. Because of this high inclination, depending on the time of night and year, the arc of the Milky Way may appear relatively low or relatively high in the sky. For observers from approximately 65 degrees north to 65 degrees south on Earth's surface, the Milky Way passes directly overhead twice a day.

text: Size and mass

text: The Milky Way is the second-largest galaxy in the Local Group, with its stellar disk approximately 100,000 ly (30 kpc) in diameter, and, on average, approximately 1,000 ly (0.3 kpc) thick.[4][5] As a guide to the relative physical scale of the Milky Way, if the Solar System out to Neptune were the size of a US quarter (24.3 mm (0.955 in)), the Milky Way would be approximately the size of the continental United States.[46] A ring-like filament of stars wrapping around the Milky Way may belong to the Milky Way itself, rippling above and below the relatively flat galactic plane.[31] If so, that would mean a diameter of 150,000–180,000 light-years (46–55 kpc).[47]

text: Estimates of the mass of the Milky Way vary, depending upon the method and data used. At the low end of the estimate range, the mass of the Milky Way is 5.8×1011 solar masses (M☉), somewhat less than that of the Andromeda Galaxy.[48][49][50] Measurements using the Very Long Baseline Array in 2009 found velocities as large as 254 km/s (570,000 mph) for stars at the outer edge of the Milky Way.[51] Because the orbital velocity depends on the total mass inside the orbital radius, this suggests that the Milky Way is more massive, roughly equaling the mass of Andromeda Galaxy at 7×1011 M☉ within 160,000 ly (49 kpc) of its center.[52] In 2010, a measurement of the radial velocity of halo stars found that the mass enclosed within 80 kiloparsecs is 7×1011 M☉.[53] According to a study published in 2014, the mass of the entire Milky Way is estimated to be 8.5×1011 M☉,[54] which is about half the mass of the Andromeda Galaxy.[54]

text: Much of the mass of the Milky Way appears to be dark matter, an unknown and invisible form of matter that interacts gravitationally with ordinary matter. A dark matter halo is spread out relatively uniformly to a distance beyond one hundred kiloparsecs from the Galactic Center. Mathematical models of the Milky Way suggest that the mass of dark matter is 1–1.5×1012 M☉.[10][11][55] Recent studies indicate a range in mass, as large as 4.5×1012 M☉ [56] and as small as 8×1011 M☉.[57]

text: The total mass of all the stars in the Milky Way is estimated to be between 4.6×1010 M☉[58] and 6.43×1010 M☉.[10] In addition to the stars, there is also interstellar gas, comprising 90% hydrogen and 10% helium by mass,[59] with two thirds of the hydrogen found in the atomic form and the remaining one-third as molecular hydrogen.[60] The mass of this gas is equal to between 10%[60] and 15%[59] of the total mass of the galaxy's stars. Interstellar dust accounts for an additional 1% of the total mass of the gas.[59]

text: Contents

text: The Milky Way contains between 200 and 400 billion stars[61][62] and at least 100 billion planets.[63] The exact figure depends on the number of very-low-mass stars, which are hard to detect, especially at distances of more than 300 ly (90 pc) from the Sun. As a comparison, the neighboring Andromeda Galaxy contains an estimated one trillion (1012) stars.[64] Filling the space between the stars is a disk of gas and dust called the interstellar medium. This disk has at least a comparable extent in radius to the stars,[65] whereas the thickness of the gas layer ranges from hundreds of light years for the colder gas to thousands of light years for warmer gas.[66][67]

text: The disk of stars in the Milky Way does not have a sharp edge beyond which there are no stars. Rather, the concentration of stars decreases with distance from the center of the Milky Way. For reasons that are not understood, beyond a radius of roughly 40,000 ly (13 kpc) from the center, the number of stars per cubic parsec drops much faster with radius.[68] Surrounding the galactic disk is a spherical Galactic Halo of stars and globular clusters that extends further outward but is limited in size by the orbits of two Milky Way satellites, the Large and Small Magellanic Clouds, whose closest approach to the Galactic Center is about 180,000 ly (55 kpc).[69] At this distance or beyond, the orbits of most halo objects would be disrupted by the Magellanic Clouds. Hence, such objects would probably be ejected from the vicinity of the Milky Way. The integrated absolute visual magnitude of the Milky Way is estimated to be around −20.9.[70][71][a]

text: Both gravitational microlensing and planetary transit observations indicate that there may be at least as many planets bound to stars as there are stars in the Milky Way,[34][72] and microlensing measurements indicate that there are more rogue planets not bound to host stars than there are stars.[73][74] The Milky Way contains at least one planet per star, resulting in 100–400 billion planets, according to a January 2013 study of the five-planet star system Kepler-32 with the Kepler space observatory.[35] A different January 2013 analysis of Kepler data estimated that at least 17 billion Earth-sized exoplanets reside in the Milky Way.[75] On November 4, 2013, astronomers reported, based on Kepler space mission data, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of Sun-like stars and red dwarfs within the Milky Way.[76][77][78] 11 billion of these estimated planets may be orbiting Sun-like stars.[79] The nearest such planet may be 4.2 light-years away, according to a 2016 study.[80] Such Earth-sized planets may be more numerous than gas giants.[34] Besides exoplanets, "exocomets", comets beyond the Solar System, have also been detected and may be common in the Milky Way.[81]

text: 360-degree panorama view of the Milky Way (an assembled mosaic of photographs) by ESO. The galactic centre is in the middle of the view, with galactic north up.

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text: Structure

text: The Milky Way consists of a bar-shaped core region surrounded by a disk of gas, dust and stars. The mass distribution within the Milky Way closely resembles the type Sbc in the Hubble classification, which represents spiral galaxies with relatively loosely wound arms.[1] Astronomers began to suspect that the Milky Way is a barred spiral galaxy, rather than an ordinary spiral galaxy, in the 1990s.[84] Their suspicions were confirmed by the Spitzer Space Telescope observations in 2005[85] that showed the Milky Way's central bar to be larger than previously thought.

text: Galactic quadrants

text: A galactic quadrant, or quadrant of the Milky Way, refers to one of four circular sectors in the division of the Milky Way. In actual astronomical practice, the delineation of the galactic quadrants is based upon the galactic coordinate system, which places the Sun as the origin of the mapping system.[86]

text: Quadrants are described using ordinals—for example, "1st galactic quadrant",[87] "second galactic quadrant",[88] or "third quadrant of the Milky Way".[89] Viewing from the north galactic pole with 0 degrees (°) as the ray that runs starting from the Sun and through the Galactic Center, the quadrants are as follows:

text:
1st galactic quadrant – 0° ≤ longitude (ℓ) ≤ 90°[90]
2nd galactic quadrant – 90° ≤ ℓ ≤ 180°[88]
3rd galactic quadrant – 180° ≤ ℓ ≤ 270°[89]
4th galactic quadrant – 270° ≤ ℓ ≤ 360° (0°)[87]

text: Galactic Center

text: The Sun is 25,000–28,000 ly (7.7–8.6 kpc) from the Galactic Center. This value is estimated using geometric-based methods or by measuring selected astronomical objects that serve as standard candles, with different techniques yielding various values within this approximate range.[15][16][17][91][92][93] In the inner few kpc (around 10,000 light-years radius) is a dense concentration of mostly old stars in a roughly spheroidal shape called the bulge.[94] It has been proposed that the Milky Way lacks a bulge formed due to a collision and merger between previous galaxies, and that instead it has a pseudobulge formed by its central bar.[95]

text: The Galactic Center is marked by an intense radio source named Sagittarius A* (pronounced Sagittarius A-star). The motion of material around the center indicates that Sagittarius A* harbors a massive, compact object.[96] This concentration of mass is best explained as a supermassive black hole[nb 3][15][97] (SMBH) with an estimated mass of 4.1–4.5 million times the mass of the Sun.[97] The rate of accretion of the SMBH is consistent with an inactive galactic nucleus, being estimated at around 6995100000000000000♠1×10−5 M☉ y−1.[98] Observations indicate that there are SMBH located near the center of most normal galaxies.[99][100]

text: The nature of the Milky Way's bar is actively debated, with estimates for its half-length and orientation spanning from 1 to 5 kpc (3,000–16,000 ly) and 10–50 degrees relative to the line of sight from Earth to the Galactic Center.[92][93][101] Certain authors advocate that the Milky Way features two distinct bars, one nestled within the other.[102] However, RR Lyrae variables do not trace a prominent Galactic bar.[93][103][104] The bar may be surrounded by a ring called the "5-kpc ring" that contains a large fraction of the molecular hydrogen present in the Milky Way, as well as most of the Milky Way's star formation activity. Viewed from the Andromeda Galaxy, it would be the brightest feature of the Milky Way.[105] X-ray emission from the core is aligned with the massive stars surrounding the central bar[98] and the Galactic ridge.[106]

text: In 2010, two gigantic spherical bubbles of high energy emission were detected to the north and the south of the Milky Way core, using data from the Fermi Gamma-ray Space Telescope. The diameter of each of the bubbles is about 25,000 light-years (7.7 kpc); they stretch up to Grus and to Virgo on the night-sky of the southern hemisphere.[107][108] Subsequently, observations with the Parkes Telescope at radio frequencies identified polarized emission that is associated with the Fermi bubbles. These observations are best interpreted as a magnetized outflow driven by star formation in the central 640 ly (200 pc) of the Milky Way.[109]

text: Later, on January 5, 2015, NASA reported observing an X-ray flare 400 times brighter than usual, a record-breaker, from Sagittarius A*. The unusual event may have been caused by the breaking apart of an asteroid falling into the black hole or by the entanglement of magnetic field lines within gas flowing into Sagittarius A*.[83]

text: Spiral arms

text: Outside the gravitational influence of the Galactic bars, the structure of the interstellar medium and stars in the disk of the Milky Way is organized into four spiral arms.[110] Spiral arms typically contain a higher density of interstellar gas and dust than the Galactic average as well as a greater concentration of star formation, as traced by H II regions[111][112] and molecular clouds.[113]

text: The Milky Way's spiral structure is uncertain, and there is currently no consensus on the nature of the Milky Way's spiral arms.[82] Perfect logarithmic spiral patterns only crudely describe features near the Sun,[112][114] because galaxies commonly have arms that branch, merge, twist unexpectedly, and feature a degree of irregularity.[93][114][115] The possible scenario of the Sun within a spur / Local arm[112] emphasizes that point and indicates that such features are probably not unique, and exist elsewhere in the Milky Way.[114] Estimates of the pitch angle of the arms range from about 7° to 25°.[65][116] There are thought to be four spiral arms that all start near the Milky Way's center.[117] These are named as follows, with the positions of the arms shown in the image at right:

text:
Color
Arm(s)

cyan
3-kpc Arm (Near 3 kpc Arm and Far 3 kpc Arm) and Perseus Arm

purple
Norma and Outer arm (Along with extension discovered in 2004[118])

green
Scutum–Centaurus Arm

pink
Carina–Sagittarius Arm

There are at least two smaller arms or spurs, including:

orange
Orion–Cygnus Arm (which contains the Sun and Solar System)

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text: Two spiral arms, the Scutum–Centaurus arm and the Carina–Sagittarius arm, have tangent points inside the Sun's orbit about the center of the Milky Way. If these arms contain an overdensity of stars compared to the average density of stars in the Galactic disk, it would be detectable by counting the stars near the tangent point. Two surveys of near-infrared light, which is sensitive primarily to red giants and not affected by dust extinction, detected the predicted overabundance in the Scutum–Centaurus arm but not in the Carina–Sagittarius arm: the Scutum-Centaurus Arm contains approximately 30% more red giants than would be expected in the absence of a spiral arm.[116][119] This observation suggests that the Milky Way possesses only two major stellar arms: the Perseus arm and the Scutum–Centaurus arm. The rest of the arms contain excess gas but not excess old stars.[82] In December 2013, astronomers found that the distribution of young stars and star-forming regions matches the four-arm spiral description of the Milky Way.[120][121][122] Thus, the Milky Way appears to have two spiral arms as traced by old stars and four spiral arms as traced by gas and young stars. The explanation for this apparent discrepancy is unclear.[122]

text: The Near 3 kpc Arm (also called Expanding 3 kpc Arm or simply 3 kpc Arm) was discovered in the 1950s by astronomer van Woerden and collaborators through 21-centimeter radio measurements of HI (atomic hydrogen).[123][124] It was found to be expanding away from the central bulge at more than 50 km/s. It is located in the fourth galactic quadrant at a distance of about 5.2 kpc from the Sun and 3.3 kpc from the Galactic Center. The Far 3 kpc Arm was discovered in 2008 by astronomer Tom Dame (Harvard-Smithsonian CfA). It is located in the first galactic quadrant at a distance of 3 kpc (about 10,000 ly) from the Galactic Center.[124][125]

text: A simulation published in 2011 suggested that the Milky Way may have obtained its spiral arm structure as a result of repeated collisions with the Sagittarius Dwarf Elliptical Galaxy.[126]

text: It has been suggested that the Milky Way contains two different spiral patterns: an inner one, formed by the Sagittarius arm, that rotates fast and an outer one, formed by the Carina and Perseus arms, whose rotation velocity is slower and whose arms are tightly wound. In this scenario, suggested by numerical simulations of the dynamics of the different spiral arms, the outer pattern would form an outer pseudoring,[127] and the two patterns would be connected by the Cygnus arm.[128]

text: Outside of the major spiral arms is the Monoceros Ring (or Outer Ring), a ring of gas and stars torn from other galaxies billions of years ago. However, several members of the scientific community recently restated their position affirming the Monoceros structure is nothing more than an over-density produced by the flared and warped thick disk of the Milky Way.[129]

text: Halo

text: The Galactic disk is surrounded by a spheroidal halo of old stars and globular clusters, of which 90% lie within 100,000 light-years (30 kpc) of the Galactic Center.[130] However, a few globular clusters have been found farther, such as PAL 4 and AM1 at more than 200,000 light-years from the Galactic Center. About 40% of the Milky Way's clusters are on retrograde orbits, which means they move in the opposite direction from the Milky Way rotation.[131] The globular clusters can follow rosette orbits about the Milky Way, in contrast to the elliptical orbit of a planet around a star.[132]

text: Although the disk contains dust that obscures the view in some wavelengths, the halo component does not. Active star formation takes place in the disk (especially in the spiral arms, which represent areas of high density), but does not take place in the halo, as there is little gas cool enough to collapse into stars.[18] Open clusters are also located primarily in the disk.[133]

text: Discoveries in the early 21st century have added dimension to the knowledge of the Milky Way's structure. With the discovery that the disk of the Andromeda Galaxy (M31) extends much further than previously thought,[134] the possibility of the disk of the Milky Way extending further is apparent, and this is supported by evidence from the discovery of the Outer Arm extension of the Cygnus Arm[118][135] and of a similar extension of the Scutum-Centaurus Arm.[136] With the discovery of the Sagittarius Dwarf Elliptical Galaxy came the discovery of a ribbon of galactic debris as the polar orbit of the dwarf and its interaction with the Milky Way tears it apart. Similarly, with the discovery of the Canis Major Dwarf Galaxy, it was found that a ring of galactic debris from its interaction with the Milky Way encircles the Galactic disk.

text: The Sloan Digital Sky Survey of the northern sky shows a huge and diffuse structure (spread out across an area around 5,000 times the size of a full moon) within the Milky Way that does not seem to fit within current models. The collection of stars rises close to perpendicular to the plane of the spiral arms of the Milky Way. The proposed likely interpretation is that a dwarf galaxy is merging with the Milky Way. This galaxy is tentatively named the Virgo Stellar Stream and is found in the direction of Virgo about 30,000 light-years (9 kpc) away.[137]

text: Gaseous halo

text: In addition to the stellar halo, the Chandra X-ray Observatory, XMM-Newton, and Suzaku have provided evidence that there is a gaseous halo with a large amount of hot gas. The halo extends for hundreds of thousand of light years, much further than the stellar halo and close to the distance of the Large and Small Magellanic Clouds. The mass of this hot halo is nearly equivalent to the mass of the Milky Way itself.[138][139][140] The temperature of this halo gas is between 1 and 2.5 million K (1.8 and 4.5 million oF).[141]

text: Observations of distant galaxies indicate that the Universe had about one-sixth as much baryonic (ordinary) matter as dark matter when it was just a few billion years old. However, only about half of those baryons are accounted for in the modern Universe based on observations of nearby galaxies like the Milky Way.[142] If the finding that the mass of the halo is comparable to the mass of the Milky Way is confirmed, it could be the identity of the missing baryons around the Milky Way.[142]

text: Sun’s location and neighborhood

text: The Sun is near the inner rim of the Orion Arm, within the Local Fluff of the Local Bubble, and in the Gould Belt, at a distance of 26.4 ± 1.0 kly (8.09 ± 0.31 kpc)[15][16][17] from the Galactic Center. The Sun is currently 5–30 parsecs (16–98 ly) from the central plane of the Galactic disk.[143] The distance between the local arm and the next arm out, the Perseus Arm, is about 2,000 parsecs (6,500 ly).[144] The Sun, and thus the Solar System, is located in the Milky Way's galactic habitable zone.

text: There are about 208 stars brighter than absolute magnitude 8.5 within a sphere with a radius of 15 parsecs (49 ly) from the Sun, giving a density of one star per 69 cubic parsecs, or one star per 2,360 cubic light-years (from List of nearest bright stars). On the other hand, there are 64 known stars (of any magnitude, not counting 4 brown dwarfs) within 5 parsecs (16 ly) of the Sun, giving a density of about one star per 8.2 cubic parsecs, or one per 284 cubic light-years (from List of nearest stars). This illustrates the fact that there are far more faint stars than bright stars: in the entire sky, there are about 500 stars brighter than apparent magnitude 4 but 15.5 million stars brighter than apparent magnitude 14.[145]

text: The apex of the Sun's way, or the solar apex, is the direction that the Sun travels through space in the Milky Way. The general direction of the Sun's Galactic motion is towards the star Vega near the constellation of Hercules, at an angle of roughly 60 sky degrees to the direction of the Galactic Center. The Sun's orbit about the Milky Way is expected to be roughly elliptical with the addition of perturbations due to the Galactic spiral arms and non-uniform mass distributions. In addition, the Sun passes through the Galactic plane approximately 2.7 times per orbit.[146] This is very similar to how a simple harmonic oscillator works with no drag force (damping) term. These oscillations were until recently thought to coincide with mass lifeform extinction periods on Earth.[147] However, a reanalysis of the effects of the Sun's transit through the spiral structure based on CO data has failed to find a correlation.[148]

text: It takes the Solar System about 240 million years to complete one orbit of the Milky Way (a galactic year),[18] so the Sun is thought to have completed 18–20 orbits during its lifetime and 1/1250 of a revolution since the origin of humans. The orbital speed of the Solar System about the center of the Milky Way is approximately 220 km/s (490,000 mph) or 0.073% of the speed of light. The Sun moves through the heliosphere at 84,000 km/h (52,000 mph). At this speed, it takes around 1,400 years for the Solar System to travel a distance of 1 light-year, or 8 days to travel 1 AU (astronomical unit).[149] The Solar System is headed in the direction of the zodiacal constellation Scorpius, which follows the ecliptic.[150]

text: Galactic rotation

text: The stars and gas in the Milky Way rotate about its center differentially, meaning that the rotation period varies with location. As is typical for spiral galaxies, the orbital speed of most stars in the Milky Way does not depend strongly on their distance from the center. Away from the central bulge or outer rim, the typical stellar orbital speed is between 210 and 240 km/s (470,000 and 540,000 mph).[153] Hence the orbital period of the typical star is directly proportional only to the length of the path traveled. This is unlike the situation within the Solar System, where two-body gravitational dynamics dominate, and different orbits have significantly different velocities associated with them. The rotation curve (shown in the figure) describes this rotation. Toward the center of the Milky Way the orbit speeds are too low, whereas beyond 7 kpcs the speeds are too high to match what would be expected from the universal law of gravitation.

text: If the Milky Way contained only the mass observed in stars, gas, and other baryonic (ordinary) matter, the rotation speed would decrease with distance from the center. However, the observed curve is relatively flat, indicating that there is additional mass that cannot be detected directly with electromagnetic radiation. This inconsistency is attributed to dark matter.[36] The rotation curve of the Milky Way agrees with the universal rotation curve of spiral galaxies, the best evidence for the existence of dark matter in galaxies. Alternatively, a minority of astronomers propose that a modification of the law of gravity may explain the observed rotation curve.[154]

text: Formation

text: The Milky Way began as one or several small overdensities in the mass distribution in the Universe shortly after the Big Bang.[155] Some of these overdensities were the seeds of globular clusters in which the oldest remaining stars in what is now the Milky Way formed. Nearly half the matter in the Milky Way may have come from other distant galaxies.[155] Nonetheless, these stars and clusters now comprise the stellar halo of the Milky Way. Within a few billion years of the birth of the first stars, the mass of the Milky Way was large enough so that it was spinning relatively quickly. Due to conservation of angular momentum, this led the gaseous interstellar medium to collapse from a roughly spheroidal shape to a disk. Therefore, later generations of stars formed in this spiral disk. Most younger stars, including the Sun, are observed to be in the disk.[156][157]

text: Since the first stars began to form, the Milky Way has grown through both galaxy mergers (particularly early in the Milky Way's growth) and accretion of gas directly from the Galactic halo.[157] The Milky Way is currently accreting material from two of its nearest satellite galaxies, the Large and Small Magellanic Clouds, through the Magellanic Stream. Direct accretion of gas is observed in high-velocity clouds like the Smith Cloud.[158][159] However, properties of the Milky Way such as stellar mass, angular momentum, and metallicity in its outermost regions suggest it has undergone no mergers with large galaxies in the last 10 billion years. This lack of recent major mergers is unusual among similar spiral galaxies; its neighbour the Andromeda Galaxy appears to have a more typical history shaped by more recent mergers with relatively large galaxies.[160][161]

text: According to recent studies, the Milky Way as well as the Andromeda Galaxy lie in what in the galaxy color–magnitude diagram is known as the "green valley", a region populated by galaxies in transition from the "blue cloud" (galaxies actively forming new stars) to the "red sequence" (galaxies that lack star formation). Star-formation activity in green valley galaxies is slowing as they run out of star-forming gas in the interstellar medium. In simulated galaxies with similar properties, star formation will typically have been extinguished within about five billion years from now, even accounting for the expected, short-term increase in the rate of star formation due to the collision between both the Milky Way and the Andromeda Galaxy.[162] In fact, measurements of other galaxies similar to the Milky Way suggest it is among the reddest and brightest spiral galaxies that are still forming new stars and it is just slightly bluer than the bluest red sequence galaxies.[163]

text: Age and cosmological history

text: Globular clusters are among the oldest objects in the Milky Way, which thus set a lower limit on the age of the Milky Way. The ages of individual stars in the Milky Way can be estimated by measuring the abundance of long-lived radioactive elements such as thorium-232 and uranium-238, then comparing the results to estimates of their original abundance, a technique called nucleocosmochronology. These yield values of about 12.5 ± 3 billion years for CS 31082-001[165] and 13.8 ± 4 billion years for BD +17° 3248.[166] Once a white dwarf is formed, it begins to undergo radiative cooling and the surface temperature steadily drops. By measuring the temperatures of the coolest of these white dwarfs and comparing them to their expected initial temperature, an age estimate can be made. With this technique, the age of the globular cluster M4 was estimated as 12.7 ± 0.7 billion years. Age estimates of the oldest of these clusters gives a best fit estimate of 12.6 billion years, and a 95% confidence upper limit of 16 billion years.[167]

text: Several individual stars have been found in the Milky Way's halo with measured ages very close to the 13.80-billion-year age of the Universe. In 2007, a star in the galactic halo, HE 1523-0901, was estimated to be about 13.2 billion years old. As the oldest known object in the Milky Way at that time, this measurement placed a lower limit on the age of the Milky Way.[168] This estimate was made using the UV-Visual Echelle Spectrograph of the Very Large Telescope to measure the relative strengths of spectral lines caused by the presence of thorium and other elements created by the R-process. The line strengths yield abundances of different elemental isotopes, from which an estimate of the age of the star can be derived using nucleocosmochronology.[168] Another star, HD 140283, is 14.5 ± 0.7 billion years old.[9][169]

text: The age of stars in the galactic thin disk has also been estimated using nucleocosmochronology. Measurements of thin disk stars yield an estimate that the thin disk formed 8.8 ± 1.7 billion years ago. These measurements suggest there was a hiatus of almost 5 billion years between the formation of the galactic halo and the thin disk.[170] Recent analysis of the chemical signatures of thousands of stars suggests that stellar formation might have dropped by an order of magnitude at the time of disk formation, 10 to 8 billion years ago, when interstellar gas was too hot to form new stars at the same rate as before.[171]

text: The satellite galaxies surrounding the Milky way are not randomly distributed, but seemed to be the result of a break-up of some larger system producing a ring structure 500,000 light years in diameter and 50,000 light years wide.[172] Close encounters between galaxies, like that expected in 4 billion years with the Andromeda Galaxy rips off huge tails of gas, which, over time can coalesce to form dwarf galaxies in a ring at right angles to the main disc.[173]

text: Environment

text: The Milky Way and the Andromeda Galaxy are a binary system of giant spiral galaxies belonging to a group of 50 closely bound galaxies known as the Local Group, surrounded by a Local Void, itself being part of the Virgo Supercluster. Surrounding the Virgo Supercluster are a number of voids, devoid of many galaxies, the Microscopium Void to the "north", the Sculptor Void to the "left", the Bootes Void to the "right" and the Canes-Major Void to the South. These voids change shape over time, creating filamentous structures of galaxies. The Virgo Supercluster, for instance, is being drawn towards the Great Attractor,[174] which in turn forms part of a greater structure, called Laniakea.[175]

text: Two smaller galaxies and a number of dwarf galaxies in the Local Group orbit the Milky Way. The largest of these is the Large Magellanic Cloud with a diameter of 14,000 light-years. It has a close companion, the Small Magellanic Cloud. The Magellanic Stream is a stream of neutral hydrogen gas extending from these two small galaxies across 100° of the sky. The stream is thought to have been dragged from the Magellanic Clouds in tidal interactions with the Milky Way.[176] Some of the dwarf galaxies orbiting the Milky Way are Canis Major Dwarf (the closest), Sagittarius Dwarf Elliptical Galaxy, Ursa Minor Dwarf, Sculptor Dwarf, Sextans Dwarf, Fornax Dwarf, and Leo I Dwarf. The smallest dwarf galaxies of the Milky Way are only 500 light-years in diameter. These include Carina Dwarf, Draco Dwarf, and Leo II Dwarf. There may still be undetected dwarf galaxies that are dynamically bound to the Milky Way, which is supported by the detection of nine new satellites of the Milky Way in a relatively small patch of the night sky in 2015.[177] There are also some dwarf galaxies that have already been absorbed by the Milky Way, such as Omega Centauri.[178]

text: In 2014 researchers reported that most satellite galaxies of the Milky Way actually lie in a very large disk and orbit in the same direction.[179] This came as a surprise: according to standard cosmology, the satellite galaxies should form in dark matter halos, and they should be widely distributed and moving in random directions. This discrepancy is still not fully explained.[180]

text: In January 2006, researchers reported that the heretofore unexplained warp in the disk of the Milky Way has now been mapped and found to be a ripple or vibration set up by the Large and Small Magellanic Clouds as they orbit the Milky Way, causing vibrations when they pass through its edges. Previously, these two galaxies, at around 2% of the mass of the Milky Way, were considered too small to influence the Milky Way. However, in a computer model, the movement of these two galaxies creates a dark matter wake that amplifies their influence on the larger Milky Way.[181]

text: Current measurements suggest the Andromeda Galaxy is approaching us at 100 to 140 km/s (220,000 to 310,000 mph). In 3 to 4 billion years, there may be an Andromeda–Milky Way collision, depending on the importance of unknown lateral components to the galaxies' relative motion. If they collide, the chance of individual stars colliding with each other is extremely low, but instead the two galaxies will merge to form a single elliptical galaxy or perhaps a large disk galaxy[182] over the course of about a billion years.[183]

text: Velocity

text: Although special relativity states that there is no "preferred" inertial frame of reference in space with which to compare the Milky Way, the Milky Way does have a velocity with respect to cosmological frames of reference.

text: One such frame of reference is the Hubble flow, the apparent motions of galaxy clusters due to the expansion of space. Individual galaxies, including the Milky Way, have peculiar velocities relative to the average flow. Thus, to compare the Milky Way to the Hubble flow, one must consider a volume large enough so that the expansion of the Universe dominates over local, random motions. A large enough volume means that the mean motion of galaxies within this volume is equal to the Hubble flow. Astronomers believe the Milky Way is moving at approximately 630 km/s (1,400,000 mph) with respect to this local co-moving frame of reference.[184] The Milky Way is moving in the general direction of the Great Attractor and other galaxy clusters, including the Shapley supercluster, behind it.[185] The Local Group (a cluster of gravitationally bound galaxies containing, among others, the Milky Way and the Andromeda Galaxy) is part of a supercluster called the Local Supercluster, centered near the Virgo Cluster: although they are moving away from each other at 967 km/s (2,160,000 mph) as part of the Hubble flow, this velocity is less than would be expected given the 16.8 million pc distance due to the gravitational attraction between the Local Group and the Virgo Cluster.[186]

text: Another reference frame is provided by the cosmic microwave background (CMB). The Milky Way is moving at 552 ± 6 km/s (1,235,000 ± 13,000 mph)[187] with respect to the photons of the CMB, toward 10.5 right ascension, −24° declination (J2000 epoch, near the center of Hydra). This motion is observed by satellites such as the Cosmic Background Explorer (COBE) and the Wilkinson Microwave Anisotropy Probe (WMAP) as a dipole contribution to the CMB, as photons in equilibrium in the CMB frame get blue-shifted in the direction of the motion and red-shifted in the opposite direction.[187]

text: Etymology and mythology

text: In Babylonia, the Milky Way was said to be the tail of Tiamat, set in the sky by Marduk after he had slain the salt water goddess. It is believed this account, from the Enuma Elish had Marduk replace an earlier Sumerian story when Enlil of Nippur had slain the goddess.

text: Llys Dôn (literally "The Court of Dôn") is the traditional Welsh name for the constellation Cassiopeia. At least three of Dôn's children also have astronomical associations: Caer Gwydion ("The fortress of Gwydion") is the traditional Welsh name for the Milky Way, and Caer Arianrhod ("The Fortress of Arianrhod") being the constellation of Corona Borealis.

text: In western culture the name "Milky Way" is derived from its appearance as a dim un-resolved "milky" glowing band arching across the night sky. The term is a translation of the Classical Latin via lactea, in turn derived from the Hellenistic Greek γαλαξίας, short for γαλαξίας κύκλος (galaxías kýklos, "milky circle"). The Ancient Greek γαλαξίας (galaxias) – from root γαλακτ-, γάλα ("milk") + -ίας (forming adjectives) – is also the root of "galaxy", the name for our, and later all such, collections of stars.[25][188][189][190] In Greek mythology it was supposedly made from the forceful suckling of Heracles, when Hera acted as a wetnurse for the hero.

text: The Milky Way, or "milk circle", was just one of 11 "circles" the Greeks identified in the sky, others being the zodiac, the meridian, the horizon, the equator, the tropics of Cancer and Capricorn, Arctic and Antarctic circles, and two colure circles passing through both poles.[191]

text: Astronomical history

text: In Meteorologica (DK 59 A80), Aristotle (384–322 BC) wrote that the Greek philosophers Anaxagoras (c. 500–428 BC) and Democritus (460–370 BC) proposed that the Milky Way might consist of distant stars.[192] However, Aristotle himself believed the Milky Way to be caused by "the ignition of the fiery exhalation of some stars which were large, numerous and close together"[193] and that the "ignition takes place in the upper part of the atmosphere, in the region of the world which is continuous with the heavenly motions."[194][195] The Neoplatonist philosopher Olympiodorus the Younger (c. 495–570 A.D.) criticized this view, arguing that if the Milky Way were sublunary, it should appear different at different times and places on Earth, and that it should have parallax, which it does not. In his view, the Milky Way is celestial. This idea would be influential later in the Islamic world.[196]

text: The Persian astronomer Abū Rayhān al-Bīrūnī (973–1048) proposed that the Milky Way is "a collection of countless fragments of the nature of nebulous stars".[197] The Andalusian astronomer Avempace (d 1138) proposed the Milky Way to be made up of many stars but appears to be a continuous image due to the effect of refraction in Earth's atmosphere, citing his observation of a conjunction of Jupiter and Mars in 1106 or 1107 as evidence.[195] Ibn Qayyim Al-Jawziyya (1292–1350) proposed that the Milky Way is "a myriad of tiny stars packed together in the sphere of the fixed stars" and that these stars are larger than planets.[198]

text: According to Jamil Ragep, the Persian astronomer Naṣīr al-Dīn al-Ṭūsī (1201–1274) in his Tadhkira writes: "The Milky Way, i.e. the Galaxy, is made up of a very large number of small, tightly clustered stars, which, on account of their concentration and smallness, seem to be cloudy patches. Because of this, it was likened to milk in color."[199]

text: Actual proof of the Milky Way consisting of many stars came in 1610 when Galileo Galilei used a telescope to study the Milky Way and discovered that it is composed of a huge number of faint stars.[200][201] In a treatise in 1755, Immanuel Kant, drawing on earlier work by Thomas Wright,[202] speculated (correctly) that the Milky Way might be a rotating body of a huge number of stars, held together by gravitational forces akin to the Solar System but on much larger scales.[203] The resulting disk of stars would be seen as a band on the sky from our perspective inside the disk. Kant also conjectured that some of the nebulae visible in the night sky might be separate "galaxies" themselves, similar to our own. Kant referred to both the Milky Way and the "extragalactic nebulae" as "island universes", a term still current up to the 1930s.[204][205][206]

text: The first attempt to describe the shape of the Milky Way and the position of the Sun within it was carried out by William Herschel in 1785 by carefully counting the number of stars in different regions of the visible sky. He produced a diagram of the shape of the Milky Way with the Solar System close to the center.[207]

text: In 1845, Lord Rosse constructed a new telescope and was able to distinguish between elliptical and spiral-shaped nebulae. He also managed to make out individual point sources in some of these nebulae, lending credence to Kant's earlier conjecture.[208]

text: In 1917, Heber Curtis had observed the nova S Andromedae within the Great Andromeda Nebula (Messier object 31). Searching the photographic record, he found 11 more novae. Curtis noticed that these novae were, on average, 10 magnitudes fainter than those that occurred within the Milky Way. As a result, he was able to come up with a distance estimate of 150,000 parsecs. He became a proponent of the "island universes" hypothesis, which held that the spiral nebulae were actually independent galaxies.[209] In 1920 the Great Debate took place between Harlow Shapley and Heber Curtis, concerning the nature of the Milky Way, spiral nebulae, and the dimensions of the Universe. To support his claim that the Great Andromeda Nebula is an external galaxy, Curtis noted the appearance of dark lanes resembling the dust clouds in the Milky Way, as well as the significant Doppler shift.[210]

text: The controversy was conclusively settled by Edwin Hubble in the early 1920s using the Mount Wilson observatory 2.5 m (100 in) Hooker telescope. With the light-gathering power of this new telescope, he was able to produce astronomical photographs that resolved the outer parts of some spiral nebulae as collections of individual stars. He was also able to identify some Cepheid variables that he could use as a benchmark to estimate the distance to the nebulae. He found that the Andromeda Nebula is 275,000 parsecs from the Sun, far too distant to be part of the Milky Way.[211][212]

text: Mapping

text: The ESA spacecraft Gaia provides distance estimates by determining the parallax of a billion stars and is mapping the Milky Way with four planned releases of maps in 2022.[213][214]


document_title: Wikipedia:Citing Wikipedia

question_text: how to find the publication date of a wikipedia article

text: Wikipedia:Citing Wikipedia

text: Problems with citing Wikipedia

text: As with any source, especially one of unknown authorship, you should be wary and independently verify the accuracy of Wikipedia information if possible. For many purposes, but particularly in academe, Wikipedia may not be an acceptable source;[1] indeed, some professors and teachers may reject Wikipedia-sourced material completely.[2] This is especially true when it is used without corroboration. Most educators and professionals do not consider it appropriate to use tertiary sources such as encyclopedias as a sole source for any information—citing an encyclopedia as an important reference in footnotes or bibliographies may result in censure or a failing grade. However, much of the content on Wikipedia is itself referenced, so an alternative is to cite the reliable source rather than the article itself.

text: A wiki is an unusual medium, and as such doesn't conform well to the usual book-citation formats. Wiki is not paper, so you will need to use an electronic-citation format instead. The exact format will depend upon the citation guide that you are following, but here are a few general principles to consider:

text:
A special citation tool is available to assist you. On the left of every article, there is a "Cite this page" link. Clicking it will bring you to a listing of relevant information, as well as automatically generated citations in several styles. Note that it is still your responsibility to ensure the citation meets all requirements.
You should not cite any particular author or authors for a Wikipedia article, in general. Wikipedia is collaboratively written. However, if you do need to find the list of authors of a particular article, you can check the Page history. Authors are listed only by IP address or chosen user name; you normally cannot verify and often cannot even guess at their identities.
Your citation should normally list both the article title and Wikipedia, The Free Encyclopedia, much as you would for an article in a paper publication. Every article should be a separate citation.
Most citation styles will likely require the full article URL. You can click "Permanent link" in the toolbox at the left of this page. This lets the URL include a unique identifier such that you can tie your reference back to the exact version of the article you are referencing. It may or may not be desirable to adopt this approach, depending upon the context of your reference. This lets you show what you saw and ignore any changes made after you accessed the page. If greater brevity is desired, you can use the regular URL, or optionally just the site URL (e.g. https://en.wikipedia.org/ for an English article), because an article URL can be inferred from an article title.
The citation style may request the full date and time of the article revision you are using. If you use the permanent link feature, this may not be necessary. However, the date and time of the last revision can be found at the bottom of every page (above the copyright notice).

text: Examples of alternatives to citing Wikipedia

text: Some Wikipedia articles (list) have been published in peer reviewed academic literature. In that case, it is possible to cite the published article. eg:

text:
Cerebellum → Wright, Marion; Skaggs, William; Årup Nielsen, Finn (2016). "The Cerebellum". WikiJournal of Medicine. 3 (1). doi:10.15347/wjm/2016.001.
Circular permutation in proteins → Bliven, Spencer; Prlić, Andreas; Wodak, Shoshana (29 March 2012). "Circular Permutation in Proteins". PLoS Computational Biology. 8 (3): e1002445. Bibcode:2012PLSCB...8E2445B. doi:10.1371/journal.pcbi.1002445. PMID 22496628.

text: Examples of how to cite Wikipedia

text: Wikipedia has a tool to generate citations for particular articles. For the cite tool, see Special:Cite, or follow the "Cite this page" link in the toolbox on the left of the page in the article you wish to cite.

text: The following examples assume you are citing the Wikipedia article on Plagiarism, using the version that was submitted on July 22, 2004, at 10:55 UTC, and that you retrieved the article on August 10, 2004, except as otherwise noted.

text: APA style

text: Citation in APA style, as recommended by the American Psychological Association: [3]

text: Plagiarism. (n.d.). In Wikipedia. Retrieved August 10, 2004, from https://en.wikipedia.org/wiki/Plagiarism[4]

text: Note that in APA 5th Edition style, the following rules apply for the reference:

text:
For reference books, which includes encyclopedias, dictionaries, and glossaries, the book title is preceded by the word In. It is not italicized, but the book title following it is.
The book title appears in sentence case. You capitalize the first word, the first word after a colon, and proper nouns.
The URL must go to the exact page that you reference.
No punctuation follows the URL.
The term or article title appears in the author position. Use sentence case for multiple-word terms or titles, where you capitalize the first word, the first word after a colon, and proper nouns.

text: The proper in-text citation is ("Plagiarism," 2004) for a paraphrased passage or ("Plagiarism," 2004, para. #) if you directly quote the material. Note that para. # represents the paragraph number in the page where the information appears. If there are multiple headings on the page, it is also acceptable to place the subheading and then a paragraph number within that heading.

text: For example, proper in-text citation for a direct quote of fewer than 40 words is:

text: "Plagiarism is the use of another person’s work (this could be his or her words, products or ideas) for personal advantage, without proper acknowledgment of the original work" ("Plagiarism," 2004, "Definition," para. 1).

text: If the quoted material is more than 40 words, use the block quote format instead.

text: As another example, the proper in-text citation for a paraphrased passage is:

text: Plagiarism is stealing the works of others ("Plagiarism," 2004).

text: APA Style requires that you provide a separate reference entry for each term you are citing in your paper because 1) you must provide a URL for each term that goes directly to the term, and 2) you must provide the publication date for each term separately. However, if you are discussing the "online encyclopedia" itself, not a term in the encyclopedia, you might need to reference the site itself. The proper citation of Wikipedia, the site, as referenced in APA 5th Edition Style is:

text: Wikipedia: The free encyclopedia. (2004, July 22). FL: Wikimedia Foundation, Inc. Retrieved August 10, 2004, from https://www.wikipedia.org

text: The in-text citation formation would be (Wikipedia, 2004).

text: MLA style

text: Citation in MLA style, as recommended by the Modern Language Association, 8th edition:

text: "Plagiarism." Wikipedia: The Free Encyclopedia. Wikimedia Foundation, Inc. 22 July 2004. Web. 10 Aug. 2004, en.wikipedia.org/w/index.php?title=Plagiarism&oldid=5139350

text: The eighth edition published in 2016 calls for urls to omit “http://” or “https://”.

text: Note that MLA style calls for both the date of publication (or its latest update) and the date on which the information was retrieved.

text: Be sure to double check the exact syntax your institution requires.

text: For citation of Wikipedia as a site, use:

text: Wikipedia: The Free Encyclopedia. Wikimedia Foundation, Inc. 22 July 2004. Web. 10 Aug. 2004.

text: MHRA style

text: Citation in MHRA style, as recommended by the Modern Humanities Research Association:

text: Wikipedia contributors, 'Plagiarism', Wikipedia, The Free Encyclopedia, 22 July 2004, 10:55 UTC, <https://en.wikipedia.org/w/index.php?title=Plagiarism&oldid=5139350> [accessed 10 August 2004]

text: Chicago style

text: Citation in Chicago style:

text: Wikipedia, The Free Encyclopedia, s.v. "Plagiarism," (accessed August 10, 2004), https://en.wikipedia.org/w/index.php?title=Plagiarism&oldid=5139350

text: Note that the Chicago Manual of Style states that "Well-known reference books, such as major dictionaries and encyclopedias, are normally cited in notes rather than bibliographies."

text: CBE/CSE Style

text: Citation in CBE/CSE style, as recommended by the Council of Science Editors:

text: Wikipedia contributors. Plagiarism [Internet]. Wikipedia, The Free Encyclopedia; 2004 Jul 22, 10:55 UTC [cited 2004 Aug 10]. Available from: https://en.wikipedia.org/w/index.php?title=Plagiarism&oldid=5139350.

text: Turabian style

text: The following are examples of how to cite Wikipedia articles according to A Manual for Writers of Term Papers, Theses, and Dissertations, 6th edition, by Kate L. Turabian (Chicago: University of Chicago Press, 1996). ISBN 0226816265 (cloth), ISBN 0226816273 (paper).

text: Note on Turabian style: Please understand that Turabian does not have rules that cover anything like Wikipedia. These examples are based on "reading between the lines" and assimilating rules from various not-so-similar cases that Turabian does cover. If the party to which you are submitting your paper is particularly strict, you might want to find out if they have their own adaptation of Turabian that would apply in this case. Alternately, you could always consult with the party before the deadline to make sure it's acceptable.

text: Notes

text: 1"Plagiarism," in Wikipedia: The Free Encyclopedia; (Wikimedia Foundation Inc., updated 22 July 2004, 10:55 UTC) [encyclopedia on-line]; available from https://en.wikipedia.org/wiki/Plagiarism; Internet; retrieved 10 August 2004.

text: 2Wikipedia contributors, "Marketing."Wikipedia, The Free Encyclopedia,https://en.wikipedia.org/wiki/Marketing (Accessed August 10, 2004)

text: Bibliography

text: Wikipedia: The Free Encyclopedia. Wikimedia Foundation Inc. Updated 22 July 2004, 10:55 UTC. Encyclopedia on-line. Available from https://en.wikipedia.org/wiki/Endangered Species. Internet. Retrieved 10 August 2004.

text: Parenthetical reference

text: ("Plagiarism," Wikipedia: The Free Encyclopedia)

text: (Wikipedia: The Free Encyclopedia, s.v. "Plagiarism")

text: Reference list

text: Plagiarism. 22 July 2003, 10:55 UTC. In Wikipedia: The Free Encyclopedia. Wikimedia Foundation Inc. Encyclopedia on-line. Available from https://en.wikipedia.org/wiki/Plagiarism. Internet. Retrieved 10 August 2004.

text: Legal citation styles

text: The Harvard Journal of Law & Technology has adopted the following format for citations to articles in Wikipedia:

text: [Signal] Wikipedia, [article], https://en.wikipedia/wiki/[article] [(optional other parenthetical)] (as of [date], [time] GMT).

text: Here is an example:

text: See Wikipedia, Bluebook, https://en.wikipedia/wiki/Bluebook (describing history and application of the Bluebook) (as of Mar. 21, 2006, 20:50 GMT).

text: This format reflects Rule 18.2 of the 18th and 19th edition of the Bluebook, but uses "as of" rather than "last updated"/"last visited" in the date parenthetical. This change allows specification of the exact version of the article to which the author is referring.

text: The date and time used should correspond exactly to the latest version listed in the article's Wikipedia history page that states the proposition for which you are citing it. Use of GMT conforms to the timestamp format used in those history entries (e.g., use 24-hour notation to avoid AM/PM).

text: BibTeX entry

text: The additional curly brackets are necessary to prevent the values from being interpreted depending on the style. In BibTeX syntax, author="Wikipedia contributors" indicates an author with the first name "Wikipedia" and the last name "contributors", and may then be formatted as, e.g., "contributors, W.".

text: Some people like adding a field howpublished="\url{https://en.wikipedia.org/w/index.php?title=Plagiarism&oldid=5139350}" which requires \usepackage{url}, but certain BibTeX styles and in particular biblatex will already use the "url" field itself, causing duplicate URLs. The URL field must not contain a \url command (so that bibtex styles can use the URL as link of the article title, and not only a standalone url), and in general the use of LaTeX code within BibTeX should be avoided.

text: AMA style

text: Citation in AMA style, as recommended by the American Medical Association: [5]

text: Wikipedia contributors. Plagiarism. Wikipedia, The Free Encyclopedia. July 22, 2004, at 10:55 UTC. Available at: https://en.wikipedia.org/w/index.php?title=Plagiarism&oldid=5139350. Accessed August 10, 2004.


document_title: Climate of Asia

question_text: the cold dry winds that blow over northern india in winter are called

text: Climate of Asia

text: Temperature[edit]

text: The Southern sections of Asia are mild to hot, while far northeastern areas such as Siberia are very cold. East Asia has a temperate climate. The highest temperature recorded in Asia was 54.0 °C (129.2 °F) at Tirat Zvi, Israel on June 21, 1942.[1] West-central Asia experiences some of the largest diurnal temperature ranges on Earth. The lowest temperature measured was −67.8 °C (−90.0 °F) at Verkhoyansk and Oymyakon, both in Sakha Republic of Russia on February 7, 1892 and February 6, 1933 respectively.[2]

text: Precipitation[edit]

text: A large annual rainfall minimum, composed primarily of deserts, stretches from the Gobi Desert in Mongolia west-southwest through the Taklamakan Desert in Western China, the Thar Desert in Western India and Iranian Plateau into the Arabian Desert in the Arabian Peninsula. Rainfall around the continent is favored across its southern portion from Eastern India and Northeast India across the Philippines, Indochina, Malay Peninsula, Malay Archipelago and Southern China into Korean Peninsula Taiwan and Japan due to the Monsoon advecting moisture primarily from the Indian Ocean into the region.[3] The monsoon trough can reach as far north as the 40th parallel in East Asia during August before moving southward thereafter. Its poleward progression is accelerated by the onset of the summer monsoon which is characterized by the development of lower air pressure (a thermal low) over the warmest part of Asia.[4][5][6] Cherrapunji, situated on the southern slopes of the Eastern Himalaya in Shillong, India is one of the wettest places on Earth, with an average annual rainfall of 11,430 mm (450 in). The highest recorded rainfall in a single year was 22,987 mm (904.9 in) in 1861. The 38-year average at Mawsynram, Meghalaya, India is 11,873 mm (467.4 in).[7] Lower rainfall maxima are found around Turkey and central Russia.

text: In March 2008, El Niño caused a drop in sea surface temperatures around Southeast Asia by an amount of 2 °C. It also caused heavy rains over Malaysia, Philippines and Indonesia.[8]

text: Monsoon[edit]

text: The Asian monsoons may be classified into a few sub-systems, such as the South Asian Monsoon which affects the Indian subcontinent and surrounding regions, and the East Asian Monsoon which affects southern China, Korea and parts of Japan. The southwestern summer monsoons occur from June through September. The Thar Desert and adjoining areas of the northern and central Indian subcontinent heats up considerably during the hot summers, which causes a low pressure area over the northern and central Indian subcontinent. To fill this void, the moisture-laden winds from the Indian Ocean rush into the subcontinent. These winds, rich in moisture, are drawn towards the Himalayas, creating winds blowing storm clouds towards the subcontinent. The Himalayas act like a high wall, blocking the winds from passing into Central Asia, thus forcing them to rise. With the gain in altitude of the clouds, the temperature drops and precipitation occurs. Some areas of the subcontinent receive up to 10,000 mm (390 in) of rain. The moisture-laden winds on reaching the southernmost point of the Indian subcontinent, due to its topography, become divided into two parts: the Arabian Sea Branch and the Bay of Bengal Branch.

text: The Arabian Sea Branch of the Southwest Monsoon first hits the Western Ghats of the coastal state of Kerala, India, thus making the area the first state in India to receive rain from the Southwest Monsoon. This branch of the monsoon moves northwards along the Western Ghats with precipitation on coastal areas, west of the Western Ghats. The eastern areas of the Western Ghats do not receive much rain from this monsoon as the wind does not cross the Western Ghats. The Bay of Bengal Branch of Southwest Monsoon flows over the Bay of Bengal heading towards northeast India and Bengal, picking up more moisture from the Bay of Bengal. The winds arrive at the Eastern Himalayas with large amounts of rain. Mawsynram, situated on the southern slopes of the Eastern Himalayas in Shillong, India, is one of the wettest places on Earth. After the arrival at the Eastern Himalayas, the winds turns towards the west, travelling over the Indo-Gangetic Plain at a rate of roughly 1–2 weeks per state[citation needed], pouring rain all along its way. June 1 is regarded as the date of onset of the monsoon in India, as indicated by the arrival of the monsoon in the southernmost state of Kerala.

text: The monsoon accounts for 80% of the rainfall in India[citation needed]. Indian agriculture (which accounts for 25% of the GDP and employs 70% of the population) is heavily dependent on the rains, for growing crops especially like cotton, rice, oilseeds and coarse grains. A delay of a few days in the arrival of the monsoon can badly affect the economy, as evidenced in the numerous droughts in India in the 1990s. The monsoon is widely welcomed and appreciated by city-dwellers as well, for it provides relief from the climax of summer heat in June.[9] However, the condition of the roads take a battering each year. Often houses and streets are waterlogged and the slums are flooded in spite of having a drainage system. This lack of city infrastructure coupled with changing climate patterns causes severe economical loss including damage to property and loss of lives, as evidenced in the Bombay floods of 2005. Bangladesh and certain regions of India like Assam and West Bengal, also frequently experience heavy floods during this season. And in the recent past, areas in India that used to receive scanty rainfall throughout the year, like the Thar Desert, have surprisingly ended up receiving floods due to the prolonged monsoon season.

text: The influence of the Southwest Monsoon is felt as far north as in China's Xinjiang. It is estimated that about 70% of all precipitation in the central part of the Tian Shan Mountains falls during the three summer months, when the region is under the monsoon influence; about 70% of that is directly of "cyclonic" (i.e., monsoon-driven) origin (as opposed to "local convection").[10]

text: Around September, with the sun fast retreating south, the northern land mass of the Indian subcontinent begins to cool off rapidly. With this air pressure begins to build over northern India, the Indian Ocean and its surrounding atmosphere still holds its heat. This causes the cold wind to sweep down from the Himalayas and Indo-Gangetic Plain towards the vast spans of the Indian Ocean south of the Deccan peninsula. This is known as the Northeast Monsoon or Retreating Monsoon.

text: While traveling towards the Indian Ocean, the dry cold wind picks up some moisture from the Bay of Bengal and pours it over peninsular India and parts of Sri Lanka. Cities like Madras, which get less rain from the Southwest Monsoon, receives rain from this Monsoon. About 50% to 60% of the rain received by the state of Tamil Nadu is from the Northeast Monsoon.[11] In Southern Asia, the northeastern monsoons take place from December to early March when the surface high-pressure system is strongest.[12] The jet stream in this region splits into the southern subtropical jet and the polar jet. The subtropical flow directs northeasterly winds to blow across southern Asia, creating dry air streams which produce clear skies over India. Meanwhile, a low pressure system develops over South-East Asia and Australasia and winds are directed toward Australia known as a monsoon trough.

text: The East Asian monsoon affects large parts of Indochina, Philippines, China, Korea and Japan. It is characterised by a warm, rainy summer monsoon and a cold, dry winter monsoon. The rain occurs in a concentrated belt that stretches east-west except in East China where it is tilted east-northeast over Korea and Japan. The seasonal rain is known as Meiyu in China, Changma in Korea, and Bai-u in Japan, with the latter two resembling frontal rain. The onset of the summer monsoon is marked by a period of premonsoonal rain over South China and Taiwan in early May. From May through August, the summer monsoon shifts through a series of dry and rainy phases as the rain belt moves northward, beginning over Indochina and the South China Sea (May), to the Yangtze River Basin and Japan (June) and finally to North China and Korea (July). When the monsoon ends in August, the rain belt moves back to South China.

text: Tropical cyclones[edit]

text: Tropical cyclones form in any month of the year across the northwest Pacific Ocean, and concentrate around June and November in the northern Indian Ocean. The area just northeast of the Philippines is the most active place on Earth for tropical cyclones to exist. Across the Philippines themselves, activity reaches a minimum in February, before increasing steadily through June, and spiking from July through October, with September being the most active month for tropical cyclones across the archipelago. Activity falls off significantly in November.[13] The most active season, since 1945, for tropical cyclone strikes on the island archipelago was 1993 when nineteen tropical cyclones moved through the country.[14] There was only one tropical cyclone which moved through the Philippines in 1958.[15] The most frequently impacted areas of the Philippines by tropical cyclones are northern Luzon and eastern Visayas.[16] A ten-year average of satellite determined precipitation showed that at least 30 percent of the annual rainfall in the northern Philippines could be traced to tropical cyclones, while the southern islands receive less than 10 percent of their annual rainfall from tropical cyclones.[17]

text: Most tropical cyclones form on the side of the subtropical ridge closer to the equator, then move poleward past the ridge axis before recurving into the main belt of the Westerlies.[18] When the subtropical ridge position shifts due to El Nino, so will the preferred tropical cyclone tracks. Areas west of Japan and Korea tend to experience much fewer September–November tropical cyclone impacts during El Niño and neutral years. During El Niño years, the break in the subtropical ridge tends to lie near 130°E which would favor the Japanese archipelago.[19] During La Niña years, the formation of tropical cyclones, along with the subtropical ridge position, shifts westward across the western Pacific Ocean, which increases the landfall threat to China.[19]