Cancer is the faintest of the 12 zodiac constellations, and at first glance, its stars bear little resemblance to a crab. Even its brightest star reaches only about magnitude 3.5, making Cancer easy to overlook among its more prominent zodiac neighbors. Yet this unassuming constellation contains some surprisingly rewarding sights, including the Beehive Cluster, one of the most impressive open star clusters visible to the naked eye.
Cancer can be seen from latitudes between about +90° and −60°, and for Northern Hemisphere observers, the constellation is best placed in the evening sky during late winter and early spring.
Finding Cancer in the Night Sky
Look for Cancer between the brighter constellations of Gemini to the west and Leo to the east. Pollux in Gemini and Regulus in Leo provide useful landmarks for locating this faint constellation, which occupies a relatively empty-looking stretch of sky. Under reasonably dark skies, look between these stars for the Beehive Cluster (M44), near Cancer’s center. The surrounding faint stars form a loose pattern sometimes described as an inverted Y. The Crab itself may never look particularly convincing, but finding the Beehive means you’ve found Cancer.
Cancer’s Principal Stars
Beyond the Beehive, Cancer has several stars worth a closer look, including an orange giant, a multiple-star system, and two stars with names rooted in ancient mythology.
- 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 two stars flanking the Beehive Cluster, traditionally known as the “little donkeys.” 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.”
The Beehive Cluster (M44)
The Beehive Cluster, also known as Messier 44 (M44), lies roughly 600 light-years away and is around 600–700 million years old. Under a dark sky it 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 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.
In the 2nd century, Ptolemy described the hazy object in Cancer. In the early 17th century, Galileo used his telescope to resolve that haze into individual stars. The name Beehive is relatively modern though. 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.
M67: An Ancient Star Cluster in Cancer
Messier 67 (M67) is one of the oldest well-studied open star clusters, at around four billion years old. Located roughly 2,500–3,000 light-years away, it contains stars nearly as old as our 4.6-billion-year-old Sun. M67 is particularly valuable to astronomers because its stars formed at roughly the same time, allowing researchers to study how stars of different masses evolve and age. Through binoculars, the cluster appears as a faint, hazy patch, while a telescope begins to resolve individual stars.
You can explore 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
Located about 41 light-years away, 55 Cancri appears as a faint point of light to the naked eye; in reality, we’re looking toward a star system containing five known planets. 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.
Cancer Mythology: The Crab and Heracles
In Greek mythology, Cancer appears during the second of the Twelve Labors of Heracles, when the hero battles the many-headed Hydra. Hera, the goddess who persecuted Heracles because he was the son of Zeus and the mortal Alcmene, sends a giant crab to attack him. The Crab’s intervention is short-lived. Heracles crushes it underfoot, but Hera rewards its loyalty by placing it among the stars as Cancer.
There’s a fitting connection between the myth and the night sky: Hydra borders Cancer, so the two creatures that fought Heracles together remain neighbors among the constellations.
Why Is the Tropic of Cancer Named After Cancer?
The Tropic of Cancer, at approximately 23.4° north latitude, marks the northernmost location where the Sun can appear directly overhead at midday. Because Earth’s rotational axis is tilted by about 23.4°, the overhead Sun shifts northward and southward throughout the year, between the Tropics of Cancer and Capricorn. At the June solstice, it reaches its northern limit before beginning to move south again.
But why name this geographic boundary after such a faint constellation? More than 2,000 years ago, the June-solstice Sun appeared against the stars of Cancer. The name tropic comes from the Greek word tropē, meaning a turn, referring to the point where the Sun’s apparent annual movement northward stops and reverses.
Today, the June-solstice Sun appears against Gemini rather than Cancer. This shift is caused by Earth’s slow axial wobble, known as precession, which gradually changes the Sun’s position relative to the background constellations over thousands of years. The ancient geographic name, however, has remained.
The Tropic itself also moves slightly. Earth’s axial tilt varies between about 22.1° and 24.5° over roughly 41,000 years, gradually shifting the Tropic’s latitude. Precession changes which constellation lies behind the solstice Sun, while changes in axial tilt alter where the Tropic lies on Earth.
