Gemini (constellation)

Last updated
Gemini
Constellation
Gemini IAU.svg
AbbreviationGem
Genitive Geminorum
Pronunciation /ˈɛmɪn/
genitive /ˌɛmɪˈnɒrəm/
Symbolismthe Twins, Castor and Pollux
Right ascension 7h
Declination +20°
QuadrantNQ2
Area 514 sq. deg. (30th)
Main stars 8, 17
Bayer/Flamsteed
stars
80
Stars with planets 8
Stars brighter than 3.00m4
Stars within 10.00 pc (32.62 ly)4
Brightest star Pollux (β Gem) (1.15 m )
Messier objects 1
Meteor showers Geminids
Rho Geminids
Bordering
constellations
Lynx
Auriga
Taurus
Orion
Monoceros
Canis Minor
Cancer
Visible at latitudes between +90° and −60°.
Best visible at 21:00 (9 p.m.) during the month of February.

Gemini is one of the constellations of the zodiac and is located in the northern celestial hemisphere. It was one of the 48 constellations described by the 2nd century AD astronomer Ptolemy, and it remains one of the 88 modern constellations today. Its name is Latin for twins , and it is associated with the twins Castor and Pollux in Greek mythology. Its old astronomical symbol is Gemini symbol (Moskowitz, fixed width).svg (♊︎).

Contents

Location

The constellation Gemini as it can be seen with the unaided eye, with added connecting lines. GeminiCC.jpg
The constellation Gemini as it can be seen with the unaided eye, with added connecting lines.
AFGL 5180 - Through the Clouds. Through the Clouds.jpg
AFGL 5180 - Through the Clouds.
An animation of the constellation Gemini (center), "the twins", shows two parallel stick figures. Gemini is associated with the myth of Castor and Polydeuces (also known as Pollux), collectively known as the Dioscuri. Gemini.gif
An animation of the constellation Gemini (center), "the twins", shows two parallel stick figures. Gemini is associated with the myth of Castor and Polydeuces (also known as Pollux), collectively known as the Dioscuri.

Gemini lies between Taurus to the west and Cancer to the east, with Auriga and Lynx to the north, Monoceros and Canis Minor to the south, and Orion to the south-west.

In classical antiquity, Cancer was the location of the Sun on the northern solstice (June 21). During the first century AD, axial precession shifted it into Gemini. In 1990, the location of the Sun at the northern solstice moved from Gemini into Taurus, where it will remain until the 27th century AD and then move into Aries. The Sun will move through Gemini from June 21 to July 20 through 2062. [4]

Gemini is prominent in the winter skies of the northern Hemisphere and is visible the entire night in December–January. The easiest way to locate the constellation is to find its two brightest stars Castor and Pollux eastward from the familiar V-shaped asterism (the open cluster Hyades) of Taurus and the three stars of Orion's Belt (Alnitak, Alnilam, and Mintaka). Another way is to mentally draw a line from the Pleiades star cluster located in Taurus and the brightest star in Leo, Regulus. In doing so, an imaginary line that is relatively close to the ecliptic is drawn, a line which intersects Gemini roughly at the midpoint of the constellation, just below Castor and Pollux.

When the Moon moves through Gemini, its motion can easily be observed in a single night as it appears first west of Castor and Pollux, then aligns, and finally appears east of them.

Features

Stars

The constellation contains 85 stars of naked eye visibility. [5] [6]

The brightest star in Gemini is Pollux, and the second-brightest is Castor. Castor's Bayer designation as "Alpha" arose because Johann Bayer did not carefully distinguish which of the two was the brighter when he assigned his eponymous designations in 1603. [7] Although the characters of myth are twins, the actual stars are physically very different from each other.

α Gem (Castor) is a sextuple star system 52 light-years from Earth, which appears as a magnitude 1.6 blue-white star to the unaided eye. Two spectroscopic binaries are visible at magnitudes 1.9 and 3.0 with a period of 470 years. A wide-set red dwarf star is also a part of the system; this star is an Algol-type eclipsing binary star with a period of 19.5 hours; its minimum magnitude is 9.8 and its maximum magnitude is 9.3.

β Gem (Pollux) is an orange-hued giant star of magnitude 1.14, 34 light-years from Earth. Pollux has an extrasolar planet revolving around it, as do two other stars in Gemini, HD 50554, and HD 59686.

γ Gem (Alhena) is a blue-white hued star of magnitude 1.9, 105 light-years from Earth.

δ Gem (Wasat) is a long-period binary star 59 light-years from Earth. The primary is a white star of magnitude 3.5, and the secondary is an orange dwarf star of magnitude 8.2. The period is over 1000 years; it is divisible in medium amateur telescopes.

ε Gem (Mebsuta), a double star, includes a primary yellow supergiant of magnitude 3.1, nine hundred light-years from Earth. The optical companion, of magnitude 9.6, is visible in binoculars and small telescopes. [7]

ζ Gem (Mekbuda) is a double star, whose primary is a Cepheid variable star with a period of 10.2 days; its minimum magnitude is 4.2 and its maximum magnitude is 3.6. It is a yellow supergiant, 1,200 light-years from Earth, with a radius that is 60 times solar, making it approximately 220,000 times the size of the Sun. The companion, a magnitude 7.6 star, is visible in binoculars and small amateur telescopes.

η Gem (Propus) is a binary star with a variable component. 380 light-years away, it has a period of 500 years and is only divisible in large amateur telescopes. The primary is a semi-regular red giant with a period of 233 days; its minimum magnitude is 3.9 and its maximum magnitude is 3.1. The secondary is of magnitude 6. [7]

κ Gem is a binary star 143 light-years from Earth. The primary is a yellow giant of magnitude 3.6; the secondary is of magnitude 8. The two are only divisible in larger amateur instruments because of the discrepancy in brightness.

ν Gem is a double star divisible in binoculars and small amateur telescopes. The primary is a blue giant of magnitude 4.1, 550 light-years from Earth, and the secondary is of magnitude 8.

38 Gem, a binary star, is also divisible in small amateur telescopes, 84 light-years from Earth. The primary is a white star of magnitude 4.8 and the secondary is a yellow star of magnitude 7.8. [7]

U Gem is a dwarf nova type cataclysmic variable discovered by J. R. Hind in 1855. [8]

Mu Gem (Tejat) is the Bayer designation for a star in the northern constellation of Gemini. It has the traditional name Tejat Posterior, which means back foot, because it is the foot of Castor, one of the Gemini twins.

Deep-sky objects

Deep sky objects
M35atlas.jpg
Messier 35 and NGC 2158
ESO Very Large Telescope images the Medusa Nebula.jpg
Medusa Nebula
Ngc2392.jpg
NGC 2392
Medusa nebula.jpg
Medusa Nebula

M35 (NGC 2168) is a large, elongated open cluster of magnitude 5, discovered in the year 1745 by Swiss astronomer Philippe Loys de Chéseaux. It has an area of approximately 0.2 square degrees, the same size as the full moon. Its high magnitude means that M35 is visible to the unaided eye under dark skies; under brighter skies it is discernible in binoculars. The 200 stars of M35 are arranged in chains that curve throughout the cluster; it is 2800 light-years from Earth. Another open cluster in Gemini is NGC 2158. Visible in large amateur telescopes and very rich, it is more than 12,000 light-years from Earth. [7]

NGC 2392 is a planetary nebula with an overall magnitude of 9.2, located 4,000 light-years from Earth. [9] In a small amateur telescope, its 10th magnitude central star is visible, along with its blue-green elliptical disk. It is said to resemble the head of a person wearing a parka. [7]

The Medusa Nebula is another planetary nebula, some 1,500 light-years distant. Geminga is a neutron star approximately 550 light-years from Earth. Other objects include NGC 2129, NGC 2158, NGC 2266, NGC 2331 and NGC 2355.

Meteor showers

The Geminids is a bright meteor shower that peaks on December 13–14. It has a maximum rate of approximately 100 meteors per hour, making it one of the richest meteor showers. [7] The Epsilon Geminids peak between October 18 and October 29 and have only been recently confirmed. They overlap with the Orionids, which make the Epsilon Geminids difficult to detect visually. Epsilon Geminid meteors have a higher velocity than Orionids. [10]

Mythology

Gemini as depicted in Urania's Mirror, a set of constellation cards published in London c.1825. Sidney Hall - Urania's Mirror - Gemini.jpg
Gemini as depicted in Urania's Mirror , a set of constellation cards published in London c.1825.
Sculpture showing Castor and Pollux, the legend behind the third astrological sign in the Zodiac and the constellation of Gemini Grupo de San Ildefonso (Museo del Prado) 03.jpg
Sculpture showing Castor and Pollux, the legend behind the third astrological sign in the Zodiac and the constellation of Gemini

In Babylonian astronomy, the stars Castor and Pollux were known as the Great Twins. The Twins were regarded as minor gods and were called Meshlamtaea and Lugalirra, meaning respectively 'The One who has arisen from the Underworld' and the 'Mighty King'. Both names can be understood as titles of Nergal, the major Babylonian god of plague and pestilence, who was king of the Underworld. [11]

In Greek mythology, Gemini was associated with the myth of Castor and Pollux, the children of Leda and Argonauts both. Pollux was the son of Zeus, who seduced Leda, while Castor was the son of Tyndareus, king of Sparta and Leda's husband. Castor and Pollux were also mythologically associated with St. Elmo's fire in their role as the protectors of sailors. [12] When Castor died, because he was mortal, Pollux begged his father Zeus to give Castor immortality, and he did, by uniting them together in the heavens.

Visualizations

Diagram of H. A. Rey's alternative way to connect the stars of the constellation Gemini. Twins are shown holding hands. Gemini constellation map visualization 1.PNG
Diagram of H. A. Rey's alternative way to connect the stars of the constellation Gemini. Twins are shown holding hands.

Gemini is dominated by Castor and Pollux, two bright stars that appear relatively very closely together forming an o shape, encouraging the mythological link between the constellation and twinship. The twin above and to the right (as seen from the Northern Hemisphere) is Castor, whose brightest star is α Gem; it is a second-magnitude star and represents Castor's head. The twin below and to the left is Pollux, whose brightest star is β Gem (more commonly called Pollux); it is of the first magnitude and represents Pollux's head. Furthermore, the other stars can be visualized as two parallel lines descending from the two main stars, making it look like two figures.

H. A. Rey has suggested an alternative to the traditional visualization that connected the stars of Gemini to show twins holding hands. Pollux's torso is represented by the star υ Gem, Pollux's right hand by ι Gem, Pollux's left hand by κ Gem; all three of these stars are of the fourth magnitude. Pollux's pelvis is represented by the star δ Gem, Pollux's right knee by ζ Gem, Pollux's right foot by γ Gem, Pollux's left knee by λ Gem, and Pollux's left foot by ξ Gem. γ Gem is of the second magnitude, while δ and ξ Gem are of the third magnitude. Castor's torso is represented by the star τ Gem, Castor's left hand by ι Gem (which he shares with Pollux), Castor's right hand by θ Gem; all three of these stars are of the fourth magnitude. Castor's pelvis is represented by the star ε Gem, Castor's left foot by ν Gem, and Castor's right foot by μ Gem and η Gem; ε, μ, and η Gem are of the third magnitude. The brightest star in this constellation is Pollux.

Astronomy

In Meteorologica (1 343b30) Aristotle mentions that he observed Jupiter in conjunction with and then occulting a star in Gemini. This is the earliest-known observation of this nature. [13] A study published in 1990 suggests the star involved was 1 Geminorum and the event took place on 5 December 337 BC. [14]

When William Herschel discovered Uranus on 13 March 1781 it was located near η Gem. [15] In 1930 Clyde Tombaugh exposed a series of photographic plates centred on δ Gem and discovered Pluto. [16]

Equivalents

In Chinese astronomy, the stars that correspond to Gemini are located in two areas: the White Tiger of the West (西方白虎, Xī Fāng Bái Hǔ) and the Vermillion Bird of the South (南方朱雀, Nán Fāng Zhū Què).

In some cultures, the twin in Gemini refers to 'the unborn twin' and represents a spiritual or dual self that exists within. [ citation needed ]

Astrology

As of 2011, the Sun appears in the constellation Gemini from June 21 to July 20. In tropical astrology, the Sun is considered to be in the sign Gemini from May 22 to June 21, and in sidereal astrology, from June 16 to July 16. [17]

See also

Related Research Articles

<span class="mw-page-title-main">Carina (constellation)</span> Constellation in the southern celestial hemisphere

Carina is a constellation in the southern sky. Its name is Latin for the keel of a ship, and it was the southern foundation of the larger constellation of Argo Navis until it was divided into three pieces, the other two being Puppis, and Vela.

<span class="mw-page-title-main">Camelopardalis</span> Constellation in the northern celestial hemisphere

Camelopardalis is a large but faint constellation of the northern sky representing a giraffe. The constellation was introduced in 1612 or 1613 by Petrus Plancius. Some older astronomy books give Camelopardalus or Camelopardus as alternative forms of the name, but the version recognized by the International Astronomical Union matches the genitive form, seen suffixed to most of its key stars.

<span class="mw-page-title-main">Centaurus</span> Constellation in the southern celestial hemisphere

Centaurus is a bright constellation in the southern sky. One of the largest constellations, Centaurus was included among the 48 constellations listed by the 2nd-century astronomer Ptolemy, and it remains one of the 88 modern constellations. In Greek mythology, Centaurus represents a centaur; a creature that is half human, half horse. Notable stars include Alpha Centauri, the nearest star system to the Solar System, its neighbour in the sky Beta Centauri, and V766 Centauri, one of the largest stars yet discovered. The constellation also contains Omega Centauri, the brightest globular cluster as visible from Earth and the largest identified in the Milky Way, possibly a remnant of a dwarf galaxy.

<span class="mw-page-title-main">Cygnus (constellation)</span> Constellation in the northern celestial hemisphere

Cygnus is a northern constellation on the plane of the Milky Way, deriving its name from the Latinized Greek word for swan. Cygnus is one of the most recognizable constellations of the northern summer and autumn, and it features a prominent asterism known as the Northern Cross. Cygnus was among the 48 constellations listed by the 2nd century astronomer Ptolemy, and it remains one of the 88 modern constellations.

<span class="mw-page-title-main">Equuleus</span> Constellation in the northern celestial hemisphere

Equuleus is a faint constellation located just north of the celestial equator. Its name is Latin for "little horse", a foal. It was one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and remains one of the 88 modern constellations. It is the second smallest of the modern constellations, spanning only 72 square degrees. It is also very faint, having no stars brighter than the fourth magnitude.

<span class="mw-page-title-main">Leo (constellation)</span> Zodiac constellation straddling the celestial equator

Leo is one of the constellations of the zodiac, between Cancer the crab to the west and Virgo the maiden to the east. It is located in the Northern celestial hemisphere. Its name is Latin for lion, and to the ancient Greeks represented the Nemean Lion killed by the mythical Greek hero Heracles as one of his twelve labors. Its old astronomical symbol is (♌︎). One of the 48 constellations described by the 2nd-century astronomer Ptolemy, Leo remains one of the 88 modern constellations today, and one of the most easily recognizable due to its many bright stars and a distinctive shape that is reminiscent of the crouching lion it depicts. The lion's mane and shoulders also form an asterism known as "The Sickle," which to modern observers may resemble a backwards "question mark."

<span class="mw-page-title-main">Libra (constellation)</span> Zodiac constellation in the southern celestial hemisphere

Libra is a constellation of the zodiac and is located in the Southern celestial hemisphere. Its name is Latin for weighing scales. Its old astronomical symbol is (♎︎). It is fairly faint, with no first magnitude stars, and lies between Virgo to the west and Scorpius to the east. Beta Librae, also known as Zubeneschamali, is the brightest star in the constellation. Three star systems are known to have planets.

<span class="mw-page-title-main">Lepus (constellation)</span> Constellation in the southern celestial hemisphere

Lepus is a constellation lying just south of the celestial equator. Its name is Latin for hare. It is located below—immediately south—of Orion, and is sometimes represented as a hare being chased by Orion or by Orion's hunting dogs.

<span class="mw-page-title-main">Andromeda (constellation)</span> Constellation in the northern celestial hemisphere

Andromeda is one of the 48 constellations listed by the 2nd-century Greco-Roman astronomer Ptolemy, and one of the 88 modern constellations. Located in the northern celestial hemisphere, it is named for Andromeda, daughter of Cassiopeia, in the Greek myth, who was chained to a rock to be eaten by the sea monster Cetus. Andromeda is most prominent during autumn evenings in the Northern Hemisphere, along with several other constellations named for characters in the Perseus myth. Because of its northern declination, Andromeda is visible only north of 40° south latitude; for observers farther south, it lies below the horizon. It is one of the largest constellations, with an area of 722 square degrees. This is over 1,400 times the size of the full moon, 55% of the size of the largest constellation, Hydra, and over 10 times the size of the smallest constellation, Crux.

<span class="mw-page-title-main">Monoceros</span> Faint constellation on the celestial equator

Monoceros is a faint constellation on the celestial equator. Its definition is attributed to the 17th-century Dutch cartographer Petrus Plancius. It is bordered by Orion to the west, Gemini to the north, Canis Major to the south, and Hydra to the east. Other bordering constellations include Canis Minor, Lepus, and Puppis.

<span class="mw-page-title-main">Perseus (constellation)</span> Constellation in the northern celestial hemisphere

Perseus is a constellation in the northern sky, named after the Greek mythological hero Perseus. It is one of the 48 ancient constellations listed by the 2nd-century astronomer Ptolemy, and among the 88 modern constellations defined by the International Astronomical Union (IAU). It is located near several other constellations named after ancient Greek legends surrounding Perseus, including Andromeda to the west and Cassiopeia to the north. Perseus is also bordered by Aries and Taurus to the south, Auriga to the east, Camelopardalis to the north, and Triangulum to the west. Some star atlases during the early 19th century also depicted Perseus holding the disembodied head of Medusa, whose asterism was named together as Perseus et Caput Medusae; however, this never came into popular usage.

<span class="mw-page-title-main">Auriga (constellation)</span> Constellation in the northern celestial hemisphere

Auriga is a constellation in the northern celestial hemisphere. It is one of the 88 modern constellations; it was among the 48 constellations listed by the 2nd-century astronomer Ptolemy. Its name is Latin for '(the) charioteer', associating it with various mythological beings, including Erichthonius and Myrtilus. Auriga is most prominent during winter evenings in the northern Hemisphere, as are five other constellations that have stars in the Winter Hexagon asterism. Because of its northern declination, Auriga is only visible in its entirety as far south as -34°; for observers farther south it lies partially or fully below the horizon. A large constellation, with an area of 657 square degrees, it is half the size of the largest, Hydra.

<span class="mw-page-title-main">Circinus</span> Constellation in the southern celestial hemisphere

Circinus is a small, faint constellation in the southern sky, first defined in 1756 by the French astronomer Nicolas-Louis de Lacaille. Its name is Latin for compass, referring to the drafting tool used for drawing circles. Its brightest star is Alpha Circini, with an apparent magnitude of 3.19. Slightly variable, it is the brightest rapidly oscillating Ap star in the night sky. AX Circini is a Cepheid variable visible with the unaided eye, and BX Circini is a faint star thought to have been formed from the merger of two white dwarfs. Two sun-like stars have planetary systems: HD 134060 has two small planets, and HD 129445 has a Jupiter-like planet. Supernova SN 185 appeared in Circinus in 185 AD and was recorded by Chinese observers. Two novae have been observed more recently, in the 20th century.

<span class="mw-page-title-main">Lacerta</span> Constellation in the northern celestial hemisphere

Lacerta is one of the 88 modern constellations defined by the International Astronomical Union. Its name is Latin for lizard. A small, faint constellation, it was defined in 1687 by the astronomer Johannes Hevelius. Its brightest stars form a "W" shape similar to that of Cassiopeia, and it is thus sometimes referred to as 'Little Cassiopeia'. It is located between Cygnus, Cassiopeia and Andromeda on the northern celestial sphere. The northern part lies on the Milky Way.

<span class="mw-page-title-main">Indus (constellation)</span> Constellation in the southern celestial hemisphere

Indus is a constellation in the southern sky first professionally surveyed by Europeans in the 1590s and mapped on a globe by Petrus Plancius by early 1598. It was included on a plate illustrating southern constellations in Bayer's sky atlas Uranometria in 1603. It lies well south of the Tropic of Capricorn but its triangular shape can be seen for most of the year from the Equator. It is elongated from north to south and has a complex boundary. The English translation of its name is generally given as the Indian, though it is unclear which indigenous people the constellation was originally supposed to represent.

<span class="mw-page-title-main">Hydra (constellation)</span> Constellation straddling the celestial equator

Hydra is the largest of the 88 modern constellations, measuring 1303 square degrees, and also the longest at over 100 degrees. Its southern end borders Libra and Centaurus and its northern end borders Cancer. It was included among the 48 constellations listed by the 2nd century astronomer Ptolemy. Commonly represented as a water snake, it straddles the celestial equator.

<span class="mw-page-title-main">Aquila (constellation)</span> Constellation near the celestial equator

Aquila is a constellation on the celestial equator. Its name is Latin for 'eagle' and it represents the bird that carried Zeus/Jupiter's thunderbolts in Greek-Roman mythology.

<span class="mw-page-title-main">Draco (constellation)</span> Constellation in the northern celestial hemisphere

Draco is a constellation in the far northern sky. Its name is Latin for dragon. It was one of the 48 constellations listed by the 2nd century Greek astronomer Ptolemy, and remains one of the 88 modern constellations today. The north pole of the ecliptic is in Draco. Draco is circumpolar from northern latitudes, meaning that it never sets and can be seen at any time of year.

<span class="mw-page-title-main">Hercules (constellation)</span> Constellation in the northern celestial hemisphere

Hercules is a constellation named after Hercules, the Roman mythological hero adapted from the Greek hero Heracles. Hercules was one of the 48 constellations listed by the second-century astronomer Ptolemy, and it remains one of the 88 modern constellations today. It is the fifth-largest of the modern constellations and is the largest of the 50 which have no stars brighter than apparent magnitude +2.5.

<span class="mw-page-title-main">Eridanus (constellation)</span> Constellation in the southern hemisphere

Eridanus is a constellation in the southern celestial hemisphere. It is represented as a river. One of the 48 constellations listed by the 2nd century astronomer Ptolemy, it remains one of the 88 modern constellations. It is the sixth largest of the modern constellations, and the one that extends farthest in the sky from north to south. The same name was later taken as a Latin name for the real Po River and also for the name of a minor river in Athens.

References

  1. "Through the Clouds". esahubble.org. Retrieved 12 March 2021.
  2. "K12.mi.us". Archived from the original on 2005-09-27. Retrieved 2012-09-11.
  3. Constellation drawings (often but not always) following "The Stars - A new way to see them", H.A. Rey, 1952–1980, ISBN   0-395-24830-2.
  4. "Astrology: Why Your Zodiac Sign and Horoscope Are Wrong". Live Science . 21 September 2017.
  5. Elijah H. Burritt - The geography of the heavens and class book of astronomy: Accompanied by a celestial atlas Huntington, 1840 Retrieved 2012-06-25
  6. E Colbert Astronomy without a telescope: being a guide-book to the visible heavens, with all necessary maps and illustrations George & C.W. Sherwood, 1869 Retrieved 2012-06-27
  7. 1 2 3 4 5 6 7 Ridpath & Tirion 2017, pp. 152–154.
  8. "U Geminorum". AAVSO. Retrieved 7 June 2012.
  9. Levy 2005, p. 126.
  10. Jenniskens, Peter (September 2012). "Mapping Meteoroid Orbits: New Meteor Showers Discovered". Sky & Telescope: 22.
  11. Babylonian Star-lore by Gavin White, Solaria Pubs, 2008, page 125
  12. Ridpath & Tirion 2017, pp. 150–152.
  13. Star Names: Their Lore and Meaning by Richard Hinckley Allen, Dover, 1963, page 229
  14. Cohen, Sheldon M.; Burke, Paul (1990). "New Evidence for the Dating of Aristotle Meteorologica 1-3". Classical Philology. 85 (2): 126–129. doi:10.1086/367188. JSTOR   269702. S2CID   162257274.
  15. Star Names: Their Lore and Meaning by Richard Hinckley Allen, Dover, 1963, page 236
  16. The Cambridge Guide to the Constellations by Michael E. Bakich, Cambridge University Press, 1995, page 210
  17. Robert Dinwiddie; David Hughes; Geraint H. Jones; Ian Ridpath; Carole Stott; Giles Sparrow. The Stars: The Definitive Visual Guide to the Cosmos. DK Publishing. p. 166.

Sources

  1. A Spring Sky Over Hirsau Abbey
  2. The Eskimo Nebula from Hubble
  3. The Medusa Nebula
  4. Open Star Clusters M35 and NGC 2158
  5. NGC 2266: Old Cluster in the NGC