Cancer (“the crab”) is a zodiac constellations, meaning it lies within the ecliptic path that the Sun, Moon and planets travel within each year in the celestial heavens. It is arguably the constellation that looks the least like the object it depicts, and is also the faintest of the 12 zodiacal constellations, with its brightest star, Al Tarf, of just fourth magnitude.
Represents: Crab sent to kill Heracles
In Greek mythology, the constellation Cancer is associated with Heracles, and the giant crab his step-mother, the goddess Hera, sent to harass him during his fight with the many-headed Hydra. The crab was meant to kill Heracles, but instead of killing the hero, it got crushed underfoot, and so Hera placed it in the heavens for its valiant efforts.
Shape: Inverted ‘Y’
Cancer is the 31st biggest constellation, taking up an area of 506 square degrees of the night sky. As far as its shape goes, it looks nothing like a crab, but appears more like an inverted ‘Y’, representing the crab’s back. The famous Beehive Cluster (M44) located in the centre of the constellation also helps in identifying the constellation proper.
Location: A Northern Constellation
Cancer is a northern constellation that can be seen by observers located between latitudes of +90 and -60 degrees. The easiest way to find Cancer is to look for it between the two striking constellations of Gemini to its west, and Leo to its east. A number of other constellations also border Cancer, including Lynx to the north, and Hydra and Canis Minor to the south.
Best Seen: Winter
Cancer is a winter constellations that in the northern hemisphere is visible from late autumn to spring, although best seen in March. In the southern hemisphere, it can be viewed in the summer and autumn months.
Notable Stars: None Above 4th Magnitude
Cancer is the faintest of the 12 zodiac constellations, with all its stars of magnitude 3.5 or fainter, and as such is often hard to identify.
– Al Tarf (Beta Cancri), the constellation’s most luminous star, is an orange K-type giant situated about 290 light years from our solar system that shines with an apparent magnitude of 3.5. It marks the celestial crab’s southern hind foot, with its name deriving from the Arabic for “the End”. Al Tarf is actually a binary system whose primary component is 53 times bigger than the Sun, and around 660 times brighter, while its faint, 14th magnitude companion, Beta Cancri, is a red dwarf star. The pair are separated by around 2,600 AU, and have an orbital period of more than 76,000 years.
– Asellus Australis (Delta Cancri), the second most luminous star in Cancer, is an orange giant (K0III) found about 180 light years away of magnitude 3.94. It is around 10 times bigger than the Sun, with twice its mass, and 53 times its brightness. Asellus Australis also marks the location of the Beehive Cluster (M44, or Praesepe), which is the northern sky’s most prominent open clusters behind the Pleiades and Hyades in Taurus. Since it lies on the ecliptic, though, it can sometimes be occulted by the Moon, and somewhat infrequently, by the planets. Asellus Australis means the “southern donkey colt” in Latin, but the ancient Babylonians referred to it as Arkushanangarushashutu, which is the longest of all star names, and means “the southeast star in the Crab”.
– Acubens (Alpha Cancri), the constellation’s third brightest star, is a multiple system found 174 light years distant of magnitude 4.20. It consists of a white dwarf star that is 3 times larger and 23 times brighter than the Sun that is separated by a faint 11th magnitude companion by 5.3 AU, with the pair orbiting each other every 6 years or so. There is also another binary system 600 AU away which take around 6,300 years to orbit their brighter companions. Acubens derives from the Arabic for “the claws”, and marks the lower end of the crab’s left leg.
Notable Objects: Beehive Cluster
The constellation of Cancer contains a number of deep-sky objects, including the spiral galaxies NGC 2775, NGC 2535, NGC 2536, NGC 2500, and NGC 2608. It also has two Messier objects, namely M44 and M67:
– Beehive Cluster (M44, Praesepe) is a beautiful open cluster about 577 light years away, making it among the closest, most densely populated open clusters to Earth. It is about 600 million years old, and contains at least 1,000 stars, of which about 63% are red dwarfs, with the remainder all Sun-like stars (F, G, K). It has a magnitude of 3.7, and appears to the naked eye as a misty cloud, with 2nd Century AD astronomer Ptolemy calling it “the nebulous mass in the breast of Cancer.” Praesepe refers to a crib or manger out of which cattle fed, with its bright neighboring stars Gamma and Delta Cancri imagined as donkeys feeding from its celestial trough.
– Messier 67 (M67) is one of the oldest known open clusters with an estimated age of between 3.2 and 5 billion years. It contains more than 100 Sun-like stars, and a large number of red giants, providing students of stellar evolution with a “snap-shot” of stars in different stages of evolution. Apart from 30 or so blue stragglers, nearly all the stars in the cluster are roughly of the same age, and located around 2,500 light years away. You can make M67 out with the naked eye, but since its stars are all magnitude 8 to 13, it looks more like a bright patch rather than a collection of stars.
Meteor Showers: The Delta Cancrids
Only one meteor shower, the Delta Cancrids, is associated with Cancer, with this long-duration shower active from December 14th to February 14th, and peaking on the 17th of January each year. Its maximum meteor count, however, rarely exceed 4-6 sightings per hour. The source of the meteors is unknown, but it has been suggested that meteors from this shower share a similar orbit to the asteroid 2001 YB5.
Planets: 10+ Planets
Two stars with confirmed planets have thus far been discovered in Cancer, with 55 Cancri being the most notable with five planets. It is a suspected binary star, consisting of a yellow star and a red dwarf star, with all five planets orbiting the primary, yellow star (55 Cancri A). At least four of its planets are known to be gas giants, and the other a super-earth. The other star with known planets is YBP 1194, located in the open cluster M67.
Astrology: June 22 to July 22
In astrology, the Sun passes into Cancer on 21st June, coinciding with the summer solstice, the northern hemisphere’s longest day after which the days grow shorter. Due to precession, however, the summer solstice now occurs when the Sun enters Taurus, and does not enter Cancer until around one month later. Other astrological associations include the following:
Date of Birth: June 22nd to July 22nd
Sign Ruler: Moon
Birth Stone: Ruby
Color: White, Yellow
Characteristics: sympathetic, caring, intuitive, moody, defensive,
Compatibility: Pisces, Scorpio
Star Lore: Gateway for Incarnation
In ancient star lore, Cancer was known as the “Gate of Men”, since it was believed to be the portal through which souls descended from Heaven to take up their abodes in the bodies of new-born babies. Likewise, Capricorn was known as ‘the Gate of the Gods’, through which the souls of the dead ascended back into Heaven.