1911 Encyclopædia Britannica/Cancer (constellation)

CANCER (“The Crab”), in astronomy, the fourth sign of the zodiac, denoted by the symbol ♋︎. Its name may be possibly derived from the fact that when the sun arrives at this part of the ecliptic it apparently retraces its path, resembling in some manner the sidelong motion of a crab. It is also a constellation, mentioned by Eudoxus (4th century B.C.) and Aratus (3rd century B.C.); Ptolemy catalogued 13 stars in it, Tycho Brahe 15 and Hevelius 29. Its most interesting objects are: a large loose cluster of stars, known as Praesepe or the Beehive, visible as a nebulous patch to the naked eye, and ξ Cancri, a remarkable multiple star, composed of two stars, of magnitudes 5 and 5.7, revolving about each other in 60 years, and a third star of magnitude 5.5 which revolves about these two in an opposite direction in a period of 17½ years; from irregularities in the motion of this star, it is supposed to be a satellite of an invisible body which itself revolves about the two stars previously mentioned, in a period of 600 to 700 years.