|Right ascension||07h 55m 19.7973s–09h 22m 35.0364s|
|Area||506 sq. deg. (31st)|
|Stars with planets||10|
|Stars brighter than 3.00m||0|
|Stars within 10.00 pc (32.62 ly)||2|
|Brightest star||β Cnc (Altarf) (3.53m)|
|Meteor showers||Delta Cancrids|
Leo Minor (corner)
|Visible at latitudes between +90° and −60°.|
Best visible at 21:00 (9 p.m.) during the month of March.
Cancer is one of the twelve constellations of the zodiac and is located in the Northern celestial hemisphere. Its old astronomical symbol is (♋︎). Its name is Latin for crab and it is commonly represented as one. Cancer is a medium-size constellation with an area of 506 square degrees and its stars are rather faint, its brightest star Beta Cancri having an apparent magnitude of 3.5. It contains two stars with known planets, including 55 Cancri, which has five: one super-earth and four gas giants, one of which is in the habitable zone and as such has expected temperatures similar to Earth. At the (angular) heart of this sector of our celestial sphere is Praesepe (Messier 44), one of the closest open clusters to Earth and a popular target for amateur astronomers.
Cancer is a medium-sized constellation that is bordered by Gemini to the west, Lynx to the north, Leo Minor to the northeast, Leo to the east, Hydra to the south, and Canis Minor to the southwest. The three-letter abbreviation for the constellation, as adopted by the International Astronomical Union in 1922, is "Cnc". The official constellation boundaries, as set by Belgian astronomer Eugène Delporte in 1930, are defined by a polygon of 3 main and 7 western edgework forming sides (illustrated in infobox). In the equatorial coordinate system, the right ascension coordinates of these borders lie between 07h 55m 19.7973s and 09h 22m 35.0364s, while the declination coordinates are between 33.1415138° and 6.4700689°. Covering 506 square degrees or 0.921% of the sky, it ranks 31st of the 88 constellations in size. It can be seen at latitudes between +90° and -60° and is best visible at 9 p.m. during the month of March. Cancer Borders the bright constellations of Leo and Gemini and Canis Minor, Under city skies, Cancer is invisible to the naked eye.
Cancer is the second dimmest of the zodiacal constellations, having only two stars above the fourth magnitude. The German cartographer Johann Bayer used the Greek letters Alpha through Omega to label the most prominent stars in the constellation, followed by the letter A, then lowercase b, c and d. Within the constellation's borders, there are 104 stars brighter than or equal to apparent magnitude 6.5.[a]
Also known as Altarf or Tarf, Beta Cancri is the brightest star in Cancer at apparent magnitude 3.5. Located 290 ± 30 light-years from Earth, it is a binary star system, its main component an orange giant of spectral type K4III that is varies slightly from a baseline magnitude of 3.53—dipping by 0.005 magnitude over a period of 6 days. An aging star, it has expanded to around 50 times the Sun's diameter and shines with 660 times its luminosity. It has a faint magnitude 14 red dwarf companion located 29 arcseconds away that takes 76,000 years to complete an orbit. Altarf represents a part of Cancer's body.
At magnitude 3.9 is Delta Cancri, also known as Asellus Australis. Located 131±1 light-years from Earth, it is an orange-hued giant star that has swollen and cooled off the main sequence to become an orange giant with a radius 11 times and luminosity 53 times that of the Sun. Its common name means "southern donkey". The star also holds a record for the longest name, "Arkushanangarushashutu," derived from ancient Babylonian language, which translates to "the southeast star in the Crab." Delta Cancri also makes it easy to find X Cancri, the reddest star in the sky. Known as Asellus Borealis "northern donkey", Gamma Cancri is a white-hued A-type subgiant of spectral type A1IV and magnitude 4.67, that is 35 times as luminous as of the Sun. It is located 181 ± 2 light-years from Earth.
Iota Cancri is a wide double star. The primary is a yellow-hued G-type bright giant star of magnitude 4.0, located 330 ± 20 light-years from Earth. It spent much of its stellar life as a B-type main sequence star before expanding and cooling to its current state as it spent its core hydrogen. The secondary is a white main sequence star of spectral type A3V and magnitude 6.57. Despite having different distances when measured by the HIPPARCOS satellite, the two stars share a common proper motion and appear to be a natural binary system.
Located 181 ± 2 light-years from Earth, Alpha Cancri (Acubens) is a multiple star with a primary component an apparent white main sequence star of spectral type A5 and magnitude 4.26. The secondary is of magnitude 12.0 and is visible in small amateur telescopes. Its common name means "the claw". The primary is actually two very similar white main sequence stars that are 5.3 AU distant from each other and the secondary is two small main sequence stars, most likely red dwarfs, that are 600 AU from the main pair. Hence the system is a quadruple one.
Zeta Cancri or Tegmine ("the shell") is a multiple star system that contains at least four stars located 82 light-years from Earth. The two brightest components are a binary star with an orbital period of 1100 years; the brighter component is a yellow-hued binary pair and the dimmer component is a yellow-hued star of magnitude 6.2. The brighter component is itself a binary star with a period of 59.6 years; its primary is of magnitude 5.6 and its secondary is of magnitude 6.0. This pair is at its greatest separation around 2019.
Ten star systems have been found to have planets. Rho-1 Cancri or 55 Cancri (or Copernicus) is a binary star approximately 40.9 light-years distant from Earth. 55 Cancri consists of a yellow dwarf and a smaller red dwarf, with five planets orbiting the primary star; one low-mass planet that may be either a hot, water-rich world or a carbon planet and four gas giants. 55 Cancri A, classified as a rare "super metal-rich" star, is one of the top 100 target stars for NASA's Terrestrial Planet Finder mission, ranked 63rd on the list. The red dwarf 55 Cancri B, a suspected binary, appears to be gravitationally bound to the primary star, as the two share common proper motion.
Cancer is best known among stargazers as the home of Praesepe (Messier 44), an open cluster also called the Beehive Cluster, located right in the centre of the constellation. Located about 590 light-years from Earth, it is one of the nearest open clusters to our Solar System. M 44 contains about 50 stars, the brightest of which are of the sixth magnitude. Epsilon Cancri is the brightest member at magnitude 6.3. Praesepe is also one of the larger open clusters visible; it has an area of 1.5 square degrees, or three times the size of the full Moon. It is most easily observed when Cancer is high in the sky. North of the Equator, this period stretches from February to May. Ptolemy described the Beehive Cluster as "the nebulous mass in the breast of Cancer." It was one of the first objects Galileo observed with his telescope in 1609, spotting 40 stars in the cluster. Today, there are about 1010 high-probability members, most of them (68 percent) red dwarfs. The Greeks and Romans identified the nebulous object as a manger from which two donkeys, represented by the neighbouring stars  Asellus Borealis and  Asellus Australis, were eating. The stars represent the donkeys that the god Dionysus and his tutor Silenus rode in the war against the Titans. The ancient Chinese interpreted the object as a ghost or demon riding in a carriage, calling it a "cloud of pollen blown from under willow catkins."
The smaller, denser open cluster Messier 67 can also be found in Cancer, 2600 light-years from Earth. It has an area of approximately 0.5 square degrees, the size of the full Moon. It contains approximately 200 stars, the brightest of which are of the tenth magnitude.
OJ 287 is a BL Lacertae object located 3.5 billion light years away that has produced quasi-periodic optical outbursts going back approximately 120 years, as first apparent on photographic plates from 1891. It was first detected at radio wavelengths during the course of the Ohio Sky Survey. Its central supermassive black hole is among the largest known, with a mass of 18 billion solar masses, more than six times the value calculated for the previous largest object.
Richard Hinckley Allen asserted this paragraph in the late 1890s, without underlying evidence:
"Cancer is said to have been the place for the Akkadian Sun of the South, perhaps from its position
at the winter solstice in very remote antiquity; but afterwards it was associated with the fourth month Duzu,
our June–July, and was known as the Northern Gate of Sun[...]"
Showing but few stars, and its brightest stars being of only 4th magnitude, Cancer was often considered the "Dark Sign", quaintly described as black and without eyes. Dante, alluding to this faintness and its culmination at midnight (thus visibility the whole night) in a Northern Hemisphere winter month, wrote in Paradiso:
Then a light among them brightened,
So that, if Cancer one such crystal had,
Winter would have a month of only a day.
Cancer was the backdrop to the Sun's most northerly position in the sky (the summer solstice) in ancient times, or from a forces perspective, the earth's sun-facing side happens to be maximally tilted towards the south. This position now occurs in Taurus due to the precession of the equinoxes, around June 21. This is also the time that the Sun is directly overhead at 23.437°N (rounded), its precise parallel now known as the Tropic of Cancer.
The close conjunction of Jupiter and Saturn in 1563— which was observed by Tycho Brahe and led him to note the inaccuracy of existing ephemerides and to begin his own program of astronomical measurements—occurred in Cancer not far from Praesepe.
In Greek mythology, Cancer is identified with the crab that appeared while Heracles fought the many-headed Hydra. Hercules slew the crab after it bit him in the foot. Afterwards, the goddess Hera, an enemy of Heracles, placed the crab among the stars.
In the Egyptian records of about 2000 BC it was described as Scarabaeus (Scarab), the sacred emblem of immortality. In Babylonia the constellation was known as MUL.AL.LUL, a name which can refer to both a crab and a snapping turtle. On boundary stones, the image of a turtle or tortoise appears quite regularly and it is believed that this represents Cancer since a conventional crab has not so far been discovered on any of these monuments. There also appears to be a strong connection between the Babylonian constellation and ideas of death and a passage to the underworld, which may be the origin of these ideas in later Greek myths associated with Hercules and the Hydra. In the 12th century, an illustrated astronomical manuscript shows it as a water beetle. Albumasar writes of this sign in Flowers of Abu Ma'shar. A 1488 Latin translation depicts cancer as a large crayfish, which also is the constellation's name in most Germanic languages. Jakob Bartsch and Stanislaus Lubienitzki, in the 17th century, described it as a lobster.
Richard Allen, in Star-names and Their Meanings, lists names for the constellation as follows: In Ancient Greece, Aratus called the crab Καρκινος (Karkinos), which was followed by Hipparchus and Ptolemy. The Alfonsine tables called it Carcinus, a Latinized form of the Greek word. Eratosthenes extended this as Καρκινος, Ονοι, και Φατνη (Karkinos, Onoi, kai Fatne): the Crab, Asses, and Crib. In Ancient Rome, Manilius and Ovid called the constellation Litoreus (shore-inhabiting). Astacus and Cammarus appear in various classic writers, while it is called Nepa in Cicero's De Finibus and the works of Columella, Plautus, and Varro; all of these words signify crab, lobster, or scorpion.
Athanasius Kircher said that in Coptic Egypt it was Κλαρια (Klaria), the Bestia seu Statio Typhonis (the Power of Darkness). Jérôme Lalande identified this with Anubis, one of the Egyptian divinities commonly associated with Sirius.
The Indian language Sanskrit shares a common ancestor with Greek, and the Sanskrit name of Cancer is Karka and Karkata. In Telugu it is "Karkatakam", in Kannada "Karkataka" or "Kataka", in Tamil Karkatan, and in Sinhala Kagthaca. The later Hindus knew it as Kulira, from the Greek Κολουρος (Kolouros), the term originated by Proclus.
As of 2002[update], the Sun appears in the constellation Cancer from July 20 to August 9. In tropical astrology, the Sun is considered to be in the sign Cancer from June 22 to July 22, and in sidereal astrology, from July 16 to August 16.
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