Cancer
What is Cancer?
Constellations· Late winter–spring (N) · between Gemini and Leo, Beehive Cluster. The faintest constellation of the zodiac. It is known less for its stars than for the Beehive Cluster, M44, which glows at its centre like a small patch of mist in the gap between Gemini and Leo.
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Stars of the figure
| Star | Magnitude |
|---|---|
| Tarf | 3.5 |
| Asellus Australis | 3.9 |
| Acubens | 4.3 |
| Asellus Borealis | 4.7 |
The 4 brightest of the 5 stars used in the figure (4 lines). Lower magnitude means brighter.
Deep-sky objects near this constellation
Constellation basics
| Season | late winter to spring evenings, February–April |
|---|---|
| Brightest star | Altarf (3.5) |
| How to find | the dim gap between Pollux and Regulus, with the hazy Beehive in the middle |
| Area rank | 31st of 88 |
Figures are representative approximations. Source: compiled from NASA and IAU public data.
The crab whose name lives on in the Tropic of Cancer
The northern tropic on maps is called the Tropic of Cancer. It is the farthest latitude north where the Sun can pass directly overhead, at the June solstice. About two thousand years ago the Sun stood in Cancer at that solstice, which is how the line got its name. Because the direction of Earth's axis slowly shifts, the Sun at the June solstice now sits on the border of Taurus and Gemini, yet the geographical name still preserves the sky of two thousand years ago.
Its southern partner, the Tropic of Capricorn, was named for the same reason. The latitude of each tropic matches the tilt of Earth's axis, about 23.4 degrees. That tilt changes slowly over a cycle of about 40,000 years. It is currently decreasing, so both tropics are creeping toward the equator by about 15 metres each year.
The constellation in depth
In Greek myth Cancer is a giant crab sent by Hera. When the hero Heracles fought the many-headed Hydra, the crab bit his foot. It was soon crushed, but Hera rewarded its loyalty with a place in the sky. All its stars are faint, so the figure is not easy to find.
At its centre lies the Beehive Cluster, M44, about a thousand stars some 600 light-years away. The ancient Greeks called it the Manger and saw the two stars beside it as donkeys feeding there. In 1609 Galileo turned his telescope on this hazy light and was the first to see that it was a crowd of stars.
The Beehive is about the same age as the Hyades in Taurus and moves through space in a similar direction. That suggests the two clusters may have been born in the same place. Today they appear far apart in the sky, but long ago they may have been siblings.
Between Pollux in Gemini and Regulus in Leo lies a gap with no bright stars: that is Cancer. On a moonless night the Beehive shows in the middle of the gap as a faint smudge, and binoculars break it into dozens of stars. With the Sky view set to a March evening and deep-sky objects switched on, the cluster's position is marked.
Frequently asked
- When can I see Cancer?
- Cancer is best seen in late winter to spring evenings, February–April.
- How do I find Cancer?
- To find Cancer, look for the dim gap between Pollux and Regulus, with the hazy Beehive in the middle.
- What is the brightest star in Cancer?
- The brightest star in Cancer is Tarf, at magnitude 3.5.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.