The Cancer constellation, one of the twelve zodiac constellations, is named after the crab. This article explores its astronomical features and ancient myths.
The Constellation’s Characteristics
Cancer is the faintest of the twelve zodiac constellations, making it challenging to observe. It occupies a medium-sized area of the sky, about 506 square degrees, and its brightest star, Beta Cancri (Altarf), has an apparent magnitude of 3.5. This relative dimness means that under light-polluted city conditions, Cancer can appear almost invisible, resembling a “hole in the sky” between brighter constellations like Leo and Gemini.
Despite its faintness, Cancer is home to the Beehive Cluster, also known as Praesepe or M44. This open star cluster is one of the closest to Earth (577-610 light-years away) and contains around 1,000 stars. To the unaided eye, especially in dark conditions, the Beehive Cluster appears as a hazy patch of light. Ptolemy, a Greek astronomer, described it as a “nebulous mass in the breast of Cancer.”
The Mythological Foundation
The name “Cancer,” meaning “the crab” in Latin, is from ancient Greek mythology. The most widely accepted myth links the constellation to the second of the Twelve Labors of Heracles (Hercules), a Greek hero.
During this labor, Heracles was tasked with defeating the Lernaean Hydra, a multi-headed serpent, in the swamps of Lerna. As Heracles battled the Hydra, the goddess Hera, his adversary, intervened. She sent a giant crab, often named Karkinos, to distract him. The crab emerged from the swamp and bit Heracles on the foot. Heracles quickly crushed the crab underfoot.
Despite the crab’s minor role and swift defeat, Hera honored its loyalty. She placed the crab among the stars, creating the constellation Cancer, immortalizing the creature in the night sky.
Beyond Greek Myth
While the Greek myth of Karkinos provides the primary origin for the constellation’s name in Western astronomy, other ancient cultures also interpreted this star pattern. In ancient Mesopotamia, the constellation was sometimes associated with a crayfish or a snapping turtle. Egyptian records from around 2000 BCE described it as a scarab beetle, a symbol of resurrection and transformation, which they believed rolled the sun across the sky.
The Beehive Cluster within Cancer also inspired diverse interpretations. Ancient Greeks and Romans saw it as a manger from which two donkeys, represented by the adjacent stars Asellus Borealis and Asellus Australis, were feeding. These donkeys were associated with Dionysus and Silenus, who rode them into battle against the Titans. Despite these various cultural interpretations, the Greek mythological association with the crab ultimately prevailed in Western astronomical nomenclature, making “Cancer” the enduring name.