Cancer—Latin for “crab”—is the faintest of the 12 zodiac constellations, and at first glance there isn’t much to suggest the shape of a crab among its stars. Even its brightest star is only about magnitude 3.5, making Cancer far less conspicuous than neighboring Gemini and Leo.
But Cancer is more interesting than its faint appearance suggests. Near its center lies the naked-eye Beehive Cluster, while this otherwise quiet-looking region of sky also contains one of the oldest known open clusters and a nearby star with five known planets.
Cancer Hides Between Gemini and Leo
For Northern Hemisphere observers, Cancer is best placed in the evening sky during late winter and early spring, with March providing particularly good viewing opportunities. The constellation can be seen from latitudes between about +90° and −60°, although dark skies make a considerable difference because Cancer’s principal stars are relatively faint.
Rather than trying to recognize the Crab immediately, it’s easier to begin with the brighter constellations on either side of it. Cancer lies between Gemini to the west and Leo to the east, in a comparatively empty-looking stretch of sky. Pollux, one of the two bright stars marking the heads of the Gemini twins, and Regulus, the brightest star in Leo, provide useful landmarks on either side of this region.
Under a reasonably dark sky, look in this region for what appears to be a faint, misty patch of light. This is the Beehive Cluster (M44), which lies near the center of Cancer and is often easier to notice than the constellation’s individual stars. Once you’ve located the Beehive, look for the faint stars around it that form the loose, branching pattern sometimes described as an inverted Y. The Crab itself may never look particularly convincing, but finding the Beehive means you’ve found Cancer.
A Swarm of Stars Fills the Beehive
The hazy patch we’ve just used to locate Cancer isn’t a cloud or a single fuzzy star. It’s an entire open star cluster. Under a dark sky the Beehive Cluster, or Messier 44 (M44), can be seen with the naked eye as a softly glowing patch, but binoculars transform the view by resolving the haze into a scattered field of individual stars.
M44 lies roughly 600 light-years from Earth and is around 600–700 million years old. The cluster contains around a thousand probable members, ranging from numerous faint red dwarfs to brighter, more massive stars, along with several evolved giants. Spread across more than a degree of sky, the Beehive is surprisingly large in apparent size—several times wider than the full Moon—even though its naked-eye appearance remains subtle.
The name Beehive is relatively modern. An older name for the cluster is Praesepe, Latin for a manger or feeding trough. Two stars beside it, Asellus Borealis and Asellus Australis, were imagined as a pair of donkeys feeding from the trough—their names mean the northern and southern little donkey. What initially appears to us as an anonymous patch of haze therefore formed part of a much older picture in the sky.
The cluster was visible long before telescopes revealed what it really was. Ptolemy described the nebulous object in Cancer in antiquity, when its individual stars could not be distinguished with the naked eye. After turning a telescope toward the region in the early 17th century, Galileo was able to resolve the mysterious haze into a multitude of stars. It’s essentially the same transformation a modern observer can experience on a smaller scale: first a misty patch with the naked eye, then a field of stars through optical aid.
The Stars That Trace Cancer
The stars that trace Cancer are much less conspicuous than the Beehive Cluster sitting among them, but several are interesting in their own right.
- Tarf (Beta Cancri) is Cancer’s brightest star, an orange giant roughly 300 light-years from Earth that shines at about magnitude 3.5. Its name derives from an Arabic word meaning “the end,” referring to its traditional position toward one of the Crab’s legs. Despite its Beta designation, Tarf is brighter than Alpha Cancri.
- Acubens (Alpha Cancri) is a multiple-star system roughly 170 light-years away whose brightest component appears white to the eye. Despite carrying the Alpha designation, it is fainter than Tarf at about magnitude 4.2. Its traditional name derives from Arabic and refers to the Crab’s claws.
- Asellus Borealis (Gamma Cancri) and Asellus Australis (Delta Cancri) are the two “little donkeys” we encountered beside the Beehive Cluster. Asellus Borealis is a white star about 160 light-years away, while Asellus Australis is an orange giant roughly 180 light-years distant. Although they appear together beside Praesepe in the sky, they are separate stars at different distances from Earth. Asellus Australis also preserves a much older Babylonian connection through the extraordinary name Arkushanangarushashutu, traditionally translated as “the southeast star in the Crab.”
None of these stars dominates Cancer in the way Aldebaran defines Taurus or Antares draws the eye toward Scorpius. Cancer is almost the reverse: its most recognizable feature isn’t a star at all, but the Beehive Cluster sitting among them.
M67 Cluster Reveals How Stars Grow Old
The Beehive isn’t Cancer’s only important open star cluster. Farther away lies Messier 67 (M67), but the two clusters are very different. Whereas M44 is roughly 600–700 million years old and readily visible as a naked-eye haze under good conditions, M67 is around four billion years old, making its stars similar in age to our 4.6-billion-year-old Sun. It also lies much farther away, roughly 2,500–3,000 light-years from Earth, making it far less conspicuous than the Beehive.
That great age makes M67 particularly interesting to astronomers. Stars in an open cluster formed at roughly the same time and from the same original cloud of material, allowing researchers to compare stars that began life under broadly similar conditions but have since reached different stages of their evolution. M67 is therefore an especially useful laboratory for studying how stars change as they age. Through binoculars it appears mainly as a small hazy patch, while a telescope begins to resolve the cluster into individual stars. You can explore M44, M67 and Cancer’s other notable targets in our guide to deep-sky objects in Cancer.
55 Cancri: A Faint Star That Hides Five Worlds
Not everything interesting in Cancer belongs to a star cluster. About 41 light-years away, 55 Cancri is a nearby star system known to host five planets. The primary star is faintly visible to the naked eye under sufficiently dark skies, but there is nothing obvious in its appearance to reveal the planetary system orbiting it.
The most extraordinary of these worlds is 55 Cancri e, also known as Janssen, a super-Earth roughly eight times Earth’s mass. It orbits so close to its star that a complete year lasts less than 18 hours, while temperatures on its dayside are high enough to melt rock. It is a world profoundly unlike any of the terrestrial planets in our own Solar System.
The planet has also become an important target for the James Webb Space Telescope. Webb observations have produced evidence consistent with an atmosphere around 55 Cancri e, possibly replenished by gases escaping from a molten surface or magma ocean below. Its precise nature is still being investigated. To the naked eye, 55 Cancri is simply another faint point of light; in reality, we’re looking toward a system containing five known worlds.
The Sun’s Journey North Ends at the Tropic of Cancer
Cancer’s name appears on maps of Earth as well as maps of the night sky. The Tropic of Cancer marks the farthest north you can ever stand and have the Sun pass directly overhead. Today it lies at about 23.4° north, because Earth’s rotational axis is tilted by about 23.4°. As Earth travels around the Sun, this tilt causes the overhead midday Sun to move gradually northward and southward during the year, between the Tropic of Cancer in the north and the Tropic of Capricorn in the south. At the June solstice, it reaches its northern limit at the Tropic of Cancer before beginning to move south again. Go any farther north and the Sun can still climb high in the summer sky—and near the North Pole it may not set at all—but it will never be directly above your head.
So why is this line named after such an inconspicuous constellation? When the name became established more than two thousand years ago, the June-solstice Sun appeared against Cancer. This was the northern turning point of the Sun’s annual path across the sky, which is also where the word tropic comes from: the Greek tropē, meaning a turn or change of direction. The northern limit of the overhead Sun therefore became known as the Tropic of Cancer.
But the June-solstice Sun no longer appears in Cancer. Earth’s rotational axis slowly traces a circle in the sky through a motion called precession, shifting the solstices relative to the background constellations over thousands of years. The old geographic name remained even as the astronomical position moved on. In that sense, the Tropic of Cancer is a surviving label for a relationship between Earth and the constellation that existed in the ancient sky.
There is a second, subtler change as well. The Tropic itself isn’t permanently fixed at exactly 23.4° north, because Earth’s axial tilt also changes slowly over time. The tilt varies between about 22.1° and 24.5° over a cycle of roughly 41,000 years, causing the Tropic to migrate slightly north and south across Earth’s surface. So two different things have changed since antiquity: precession has moved the June-solstice point away from Cancer among the stars, while changes in Earth’s axial tilt have slowly shifted the latitude of the Tropic itself.
The Crab That Took on Heracles
In Greek mythology, Cancer’s Crab appears during the second of the Twelve Labors of Heracles, when the hero is sent to kill the many-headed Hydra. The goddess Hera sends a giant crab to attack him during the battle. She had long persecuted Heracles, who was the son of her husband Zeus and the mortal woman Alcmene.
The Crab’s intervention doesn’t last long. It attacks Heracles as he struggles with the Hydra, but he crushes it underfoot and continues the fight. Hera nevertheless rewards the creature for coming to the Hydra’s aid by placing it among the stars as Cancer. Accounts differ in some of the details, but the central story remains the same.
There’s a nice connection between the myth and the real night sky. Hydra borders Cancer, so the two creatures that fought Heracles together in the story remain neighbors among the constellations. Once you’ve found Cancer between Gemini and Leo, the mythology is therefore still reflected in the celestial geography: the little Crab sits beside the great Hydra.
