About

Cancer is one of the twelve constellations of the zodiac and is located in the Northern celestial hemisphere. 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 ten 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.

History and mythology

Cancer was first recorded by Claudius Ptolemy in the 2nd century CE in The Mathematical Syntaxis (a.k.a. Almagest), under the Greek name Καρκίνος (Karkinos). In the late 1890s, R.H. Allen asserted the following, with no supporting citation: "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 [araḫ Dumuzu], our June–July, and was known as the Northern Gate of Sun ..." Very few of Cancer's stars are visible 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". Dante, alluded to its faintness in Paradiso, and mentioned it being visible for the whole night when it culminated at midnight in a Northern Hemisphere winter month: 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, when the Earth's Sun-facing side was maximally tilted towards the south, in the Gregorian calendar kept within a few days of June 21. Equivalently, this is the date when the Sun is directly overhead as far north as 23.437° N. The northern-most parallel where the Sun is directly overhead is still called the Tropic of Cancer, even though the corresponding position on the sky now occurs in Taurus, due to the precession of the equinoxes. 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 Lernaean 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.