Visible at latitudes between +90° and −60°. Best visible at 21:00 (9 p.m.) during the month ofMarch.
Cancer is one of the twelveconstellations of thezodiac and is located in theNorthern celestial hemisphere. Its name isLatin forcrab and it is commonly represented as one. Cancer is a medium-size constellation with an area of 506square degrees and its stars are rather faint, its brightest starBeta Cancri having anapparent magnitude of 3.5. It contains ten stars with knownplanets, including55 Cancri, which has five: onesuper-Earth and fourgas giants, one of which is in thehabitable zone and as such has expected temperatures similar to Earth. At the (angular) heart of this sector of our celestial sphere isPraesepe (Messier 44), one of the closestopen clusters to Earth and a popular target for amateur astronomers.
Cancer is a medium-sized constellation that is bordered byGemini to the west,Lynx to the north,Leo Minor to the northeast,Leo to the east,Hydra to the south, andCanis Minor to the southwest. The three-letter abbreviation for the constellation, as adopted by theInternational Astronomical Union in 1922, is "Cnc".[3]
The official constellation boundaries, as set by Belgian astronomerEugène Delporte in 1930, are defined by a polygon of 3 main and 7 western edgework forming sides (illustrated in infobox). 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 ofLeo,Gemini andCanis Minor. Under city skies, Cancer is invisible to the naked eye.
Cancer is the dimmest of thezodiacal constellations, having only two stars above the fourth magnitude.[1] The German cartographerJohann Bayer used the Greek lettersAlpha throughOmega to label the most prominent stars in the constellation, followed by the letter A, then lowercase b, c and d.[4] Within the constellation's borders, there are 104 stars brighter than or equal toapparent magnitude 6.5.[a][6]
The constellation Cancer as it can be seen by the naked eye.
Also known as Altarf or Tarf,[7]Beta Cancri is the brightest star in Cancer atapparent magnitude 3.5.[8] Located 290 ± 30light-years from Earth,[9] it is abinary star system, its main component anorange 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.[10] 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 14red dwarf companion located 29 arcseconds away that takes 76,000 years to complete an orbit.[8]Altarf represents a part of Cancer's body.
At magnitude 3.9 isDelta Cancri, also known as Asellus Australis.[11] Located 131±1 light-years from Earth,[9] 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.[11] Its common name means "southern donkey".[1] 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."[citation needed] Delta Cancri also makes it easy to findX 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,[12] that is 35 times as luminous as of the Sun.[13] It is located 181 ± 2 light-years from Earth.[9]
Iota Cancri is a wide double star. The primary is a yellow-hued G-type bright giant star of magnitude 4.0,[14] located 330 ± 20 light-years from Earth.[9] 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 awhite 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.[14]
Located 181 ± 2light-years from Earth,[9]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 amateurtelescopes. Its common name means "the claw".[1] 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.[15]
Zeta Cancri or Tegmine ("the shell") is amultiple star system that contains at least four stars located 82 light-years from Earth. The two brightest components are a binary star with anorbital 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.[1]
Ten star systems have been found to have planets. Rho1 Cancri or55 Cancri (or Copernicus[7]) 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.
YBP 1194 is a sunlike star in the open clusterM67 that has been found to have three planets.
Cancer is best known among stargazers as the home ofPraesepe (Messier 44), anopen cluster also called theBeehive 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 the 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.[1] 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 objectsGalileo 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 [1213] Asellus Borealis and [1210] Asellus Australis, were eating. The stars represent the donkeys that the godDionysus and his tutorSilenus rode in the war against theTitans. 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."[citation needed]
The smaller, denser open clusterMessier 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.[1]
In the late 1890s,R.H. Allen asserted the following, with no supporting citation:
"Cancer is said to have been the place for theAkkadianSun of the South, perhaps from its position at thewinter solstice in very remote antiquity; but afterwards it was associated with the fourth monthDuzu[broken anchor][araḫ Dumuzu], our June–July, and was known as theNorthern Gate of Sun ..."[19]
Very few of Cancer's stars arevisible to the naked eye, and its brightest stars are only 4th magnitude. Cancer was often considered the "Dark Sign", quaintly described as "black and without eyes".[citation needed]Dante, alluded to its faintness inParadiso, and mentioned it being visible for the whole night when itculminated at midnight in a Northern Hemisphere winter month:
Cancer was the backdrop to the Sun's most northerly position in the sky (thesummer solstice) in ancient times, when the Earth's Sun-facing side was maximally tilted towards the south, in theGregorian calendar kept within a few days of June 21. Equivalently, this is the date when the Sun is directly overhead as far north as23.437° N. The northern-mostparallel where the Sun is directly overhead is still called theTropic of Cancer, even though the corresponding position on the sky now occurs inTaurus, due to theprecession of the equinoxes.[1]
The closeconjunction of Jupiter and Saturn in 1563 – which was observed byTycho 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.
InGreek mythology, Cancer is identified with the crab that appeared whileHeracles fought the many-headedLernaean Hydra. Hercules slew the crab after it bit him in the foot. Afterwards, the goddessHera, an enemy of Heracles, placed the crab among the stars.[21]
Cancer as depicted inUrania's Mirror, a set of constellation cards published in London c.1825.
The modern symbol for Cancer represents the pincers of acrab, but Cancer has been represented as many types of creatures, usually those living in the water, and always those with anexoskeleton.
In the Egyptian records of about 2000 BC it was described as Scarabaeus (Scarab), the sacredemblem 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.[22]
InAncient Greece,Aratus called the crabΚαρκινος (Karkinos), which was followed byHipparchus andPtolemy. TheAlfonsine tables called itCarcinus, a Latinized form of the Greek word.Eratosthenes extended this asΚαρκινος,Ονοι,και Φατνη (Karkinos,Onoi,kai Fatne): the Crab,[the] Asses, and[the] Crib. InAncient Rome,Manilius andOvid called the constellationLitoreus (shore-inhabiting).Astacus andCammarus appear in various classic writers, while it is calledNepa inCicero'sDe Finibus and the works ofColumella,Plautus, andVarro; all of these words signify a crab,[a]lobster, or[a]scorpion.[24]
Athanasius Kircher said that in Coptic Egypt it wasΚλαρια (Klaria), theBestia seu StatioTyphonis (the Power of Darkness).Jérôme Lalande identified this withAnubis, one of the Egyptian divinities commonly associated withSirius.[24]
The Indian languageSanskrit shares a common ancestor with Greek, and the Sanskrit name of Cancer isKarka andKarkata. InTelugu it is"Karkatakam", inKannada"Karkataka" or"Kataka", inTamilKadagam, and inSinhalaKagthaca. The later Hindus knew it asKulira, from the GreekΚολουρος (Kolouros), the term originated byProclus.[24]
As of 2002[update], the Sun appears in the constellation Cancer from July 20 – August 9. Intropical astrology, the Sun is considered to be in thesign ofCancer from June 22 – July 22, and insidereal astrology, from July 16 – August 16. The symbol of the astrological sign (which now covers roughly the constellation ofGemini) is (♋︎).
^Allen, R.H. (1898).Star Names: Their lore and meaning (Thesis). Minneapolis, MN:University of Minnesota. p. 108. Reprinted 1899 byDover Books, New York, NY, and continually to the present.
Quoted nearly verbatim by:
Olcott, W.T. (1911).Star Lore of All Ages: A collection of myths, legends, and facts concerning the constellations of the northern hemisphere. New York, NY: G.P. Putnam. p. 89.Bibcode:1911slaa.book.....O, citating Allen (1898/1899).
Later authors continue to repeat the same quote from Allen (1898/1899) to the present.
Wagman, Morton (2003).Lost Stars: Lost, Missing and Troublesome Stars from the Catalogues of Johannes Bayer, Nicholas Louis de Lacaille, John Flamsteed, and Sundry Others. Blacksburg, Virginia: The McDonald & Woodward Publishing Company.ISBN978-0-939923-78-6.
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