Cancer, the Crab, is one of the constellations through which the sun passes in its yearly trek around the sky. Of all the constellations of the Zodiac, the Crab definitely has the faintest stars. We might not think of it at all but for its accidental placement along the sun’s path between two far more prominent members of the solar club: Gemini, the Twins, and Leo, the Lion.
However, Cancer was an important constellation to our forebears.
In ancient times, the sun climbed to its northernmost point when it was in the constellation Cancer. This point in the sky is called the summer solstice, and it used to lie at a point just to the left of the Crab’s eyes. Because the earth wobbles a bit as it spins on its axis, the solstice has moved over into Gemini in modern times. Some constellations have all the luck.
Still, old habits die hard. Along latitude 23.5 degrees north, the sun stands directly overhead on the middle of summer. Thus, we call that latitude the Tropic of Cancer and not the Tropic of Gemini.
Also, Cancer contains a sky aberration of some significance to the ancients. Among its stars, nearly at its heart, is a mysterious fuzzy patch.
The glow from city streetlights makes it very difficult to see the stars of the constellation from anywhere near a city or town. Its fuzzy patch is totally obscured from view.
Next time you’re out in rural Ohio, you might want to give it a look, however.
Cancer is high in the south by 10 p.m. or so. It is southwest of the Big Dipper, about halfway between Leo to the east and Gemini to the west. It looks a bit like an upside down “Y.”
The ancients took advantage of Cancer’s dimness in an odd way. It was often used in weather forecasting. When Cancer disappeared from the sky but the other stars in the sky remained, it meant that a serious storm was brewing. We now know that storm fronts are often preceded by bands of high, thin cirrus clouds. They are often just thick enough to blot out only the faint stars in Cancer but leave the brighter stars of other constellations shining brightly.
To this day, some amateur astronomers use the Crab in that way. When it disappears, stargazers know it is time to quickly load the scopes in the car and head for home. Telescopes don’t like to get rained on.
Before they do, they usually try to take a look at the fuzzy patch, more properly known as the Praesepe, or Beehive. It is just to the left of two close-together stars that form the eyes of the Crab.
The Beehive is one of those reasons why every diehard stargazer keeps a pair of binoculars close at hand. The Praesepe is too large to fit into the field of view of most telescopes. To the unaided eye, it’s just a fuzzy thing. In binoculars, it explodes into at least a dozen stars gravitationally bound into a star cluster.
Most star clusters of stars are relatively young. In long exposure photos, we sometimes see remnants of the hydrogen gas and dust out of which the stars formed.
Eventually, the gas and dust disperse, and the stars in the cluster slowly drift apart. If our own star, the sun, formed in a cluster of stars, for example, there is no sign that we are still embedded in our original cluster.
The Beehive is the rare exception. The gas and dust have dispersed, and the cluster is made up of mature, middle-aged stars like our sun. Yet the stars have stayed together, albeit in a fairly dispersed grouping, perhaps because the stars are close enough and big enough to maintain a strong gravitational attraction to each other.
Fuzzy spots like the Praesepe were something of a mystery to the ancients. Around 270 BC, the poet and stargazer Aratus called it Achlus, the “Little Mist.” By 130 BCE, Hipparchus refers to the cluster as Nephelion, or “Little Cloud,” in his star catalog.
By the height of the Roman Empire, it was finally referred to as the Praesepe, which we somewhat inaccurately translate as Beehive. In fact, the word refers to any enclosed space. A beehive is certainly enclosed, but then again, so are a lot of things.
What did the name really mean to the Romans? The decisive clue comes from two stars huddled close on the north and south sides of the nebula. Their Latin names, still used today, are Asellus Borealis and Asellus Australis, the Northern and Southern Asses. The donkeys in question are standing around the Praesepe, which seems unlikely if the cloud is supposed to represent a beehive.
According to an ancient Greek myth borrowed by the Romans, the donkeys in question were heroes of the great battle between the gods and a race of Giants called the Titans. War erupted when the gods overthrew their Titan predecessors.
When the gods set the donkeys loose, their loud braying made the non-too-bright Giants think that a horrible monster had been unleashed upon them. They scattered like the wind, and the gods were triumphant.
As a reward for their bravery, the gods put the Asses in the sky near the “little cloud” the Romans called the Praesepe. However, along with the other names mentioned above, the Greeks also called the fuzzy thing Phatne, or Manger. The Romans clearly meant the “enclosed space” of the Praesepe to mean “Manger,” a place in which animal feed was placed. The donkeys had been rewarded with what they like best — a reliable place to fill their stomachs.
The myths about Cancer don’t end with the Praesepe. Cancer as a whole is one of several constellations connected with the twelve labors of Hercules, who is just rising now in the northeast. You’ll need to wait until about 11 p.m. to see him peeking above the horizon.
Hercules was commanded to slay the Lernaean Hydra. The Hydra was a gigantic, nine-headed sea monster raised by Hera, the queen of the gods. Each of its nine mouths has breath so bad that it killed anyone who was foolish enough to come too close. As you might guess, the Hydra was not invited to many parties.
Its head can be found just south of Cancer as a little circle of stars, and its thin, serpentine body is stretched out as a long chain of stars to the southeast.
Hercules began to hack away at the nine heads of the Hydra, but they kept growing back.
To add to his woes, Hera, who loved her nasty little pet very dearly, sent the giant crab, Cancer, to pinch him as he tried to kill the Hydra. Hercules first had to kill the crab and then solicit the aid of a companion to burn the bloody stumps with a torch as fast as he could cut the Hydra’s heads off.
You know the rest. The distraught Hera put the sea monster and the crab up in the heavens. And there they sit to this very day, a memorial to a goddess’s lousy choice of household pets.
Tom Burns is the former director of the Perkins Observatory in Delaware.