Crab · pronounced Cancer. Cancer, meaning “Crab,” is an official IAU constellation centred near right ascension 8.5 hours and declination 27°. Its northern position determines where and when it is easiest to observe. It belongs to the classical constellation tradition recorded by Ptolemy in the second-century Almagest. Its modern role is different: the IAU constellation is a precisely bounded area of sky, not only the familiar stick figure. The plotted outline is only a recognition aid; every galaxy, nebula and star inside the official boundary belongs to Cancer, even when it is not joined by a line. Observers should begin with a wide-field chart, identify brighter neighbouring patterns, and then compare the field with a red-light atlas. Binoculars reveal denser star fields than the naked eye, while a telescope is needed for most faint deep-sky targets. Because visibility changes with latitude, season and local horizon, the coordinates and boundary chart are more reliable than mythology alone when planning an observing session.
About Cancer
Cancer is one of the 88 constellations recognized by the International Astronomical Union. Its standard three-letter abbreviation is Cnc, and astronomers use the genitive form Cancri when naming stars associated with it. The constellation's reference position lies around right ascension 8.6 hours and declination +27 degrees.
Its location places it in the northern sky. For a typical mid-latitude observer, this part of the celestial sphere is especially well placed during winter evenings. Exact visibility depends on latitude, local horizon, date and time.
History and mythology of Cancer
Classical origin
It belongs to the classical constellation tradition recorded by Ptolemy in the second-century Almagest. Its modern role is different: the IAU constellation is a precisely bounded area of sky, not only the familiar stick figure.
The modern constellation
In 1922 the International Astronomical Union adopted Cancer among the 88 official constellations. Eugène Delporte’s later boundary work made it a defined region of the celestial sphere, allowing cataloguers to assign objects unambiguously.
How to find Cancer
- Choose a dark site with a clear view toward the northern sky.
- Observe during northern winter and spring evenings, when the constellation is better placed in the evening.
- Centre a chart near RA 8.5h, Dec 27° and identify brighter neighbouring patterns before tracing its official line figure.
- Confirm faint targets with an interactive Aladin or Stellarium view; the IAU boundary extends beyond the connected stars.
Bright stars in Cancer
Altarf
Notable star- Magnitude
- 3.53
- Distance
- about 290 light-years
A documented stellar landmark in Crab; magnitude is apparent visual brightness and distance is rounded.
Asellus Australis
Notable star- Magnitude
- 3.94
- Distance
- about 180 light-years
A documented stellar landmark in Crab; magnitude is apparent visual brightness and distance is rounded.
Acubens
Notable star- Magnitude
- 4.26
- Distance
- about 170 light-years
A documented stellar landmark in Crab; magnitude is apparent visual brightness and distance is rounded.
Asellus Borealis
Notable star- Magnitude
- 4.67
- Distance
- about 158 light-years
A documented stellar landmark in Crab; magnitude is apparent visual brightness and distance is rounded.
Deep-sky objects in Cancer
M44 Beehive
A well-established open cluster within the official Crab sky region and a useful target for appropriately sized optics.
M67
A well-established open cluster within the official Crab sky region and a useful target for appropriately sized optics.
NGC 2775
A well-established spiral galaxy within the official Crab sky region and a useful target for appropriately sized optics.
Arp 82
A well-established interacting galaxies within the official Crab sky region and a useful target for appropriately sized optics.
Interesting facts about Cancer
- Cancer’s traditional name refers to crab, while its official IAU boundary defines a precise region of the sky.
- Altarf, at magnitude 3.53, is a key star in Cancer. A documented stellar landmark in Crab; magnitude is apparent visual brightness and distance is rounded.
- M44 Beehive is a notable open cluster in Cancer. A well-established open cluster within the official Crab sky region and a useful target for appropriately sized optics.
- M67, a open cluster, is another observing target within the constellation.
- It belongs to the classical constellation tradition recorded by Ptolemy in the second-century Almagest.
Cancer lies on the astronomical path of the Sun through the IAU sky regions. The Sun crosses 13 official constellations, including Ophiuchus; this is different from the equal 12-sign structure used by Western tropical astrology.