Cancer
Cancer is the faintest of the 12 zodiac constellations. It has no bright stars, but at its heart lies the Beehive Cluster (Praesepe), faintly visible even to the naked eye.
| Latin name | Cancer |
|---|---|
| Abbreviation | Cnc |
| Brightest star | Altarf (mag 3.5) |
| Best seen | Late winter–spring evenings (Northern Hemisphere) |
| Astrological dates | June 21 – July 22 |
| Sun actually passes | July 21 – August 10 |
| Stars used by Planetica | 1,130 |
How to find it
Cancer is well placed in the evening sky from February to April, and in early March it rides high in the south after dusk. But even its brightest star is only magnitude 3.5, so it's nearly invisible from cities. Choose a moonless night and a spot with little light pollution.
It's easiest to find between two bright neighbors. Look about halfway along a line from Castor and Pollux in Gemini to Regulus in Leo. There, a few faint stars form an upside-down Y: that's Cancer.
In the middle of the Y is a hazy glow that looks like a small scrap of cloud. This is the Beehive Cluster (M44, also called Praesepe). It's faint to the naked eye, but through binoculars dozens of stars sparkle at once. In fact, the best way to find Cancer is to find this cluster first.
The myth: the crab sent by Hera
In Greek mythology, Cancer is a crab linked to the hero Hercules. As one of his Twelve Labors, Hercules had to fight the Hydra, a many-headed water serpent living in the swamps of Lerna. The goddess Hera, who hated Hercules, sent a giant crab named Karkinos to get in his way. The crab pinched Hercules' foot but was quickly crushed underfoot. Hera honored the crab's loyalty by placing it in the sky.
The two stars at the center of Cancer, Asellus Borealis and Asellus Australis, mean "northern donkey" and "southern donkey" in Latin, and Praesepe between them means "manger." One story says that in the war between the gods and the giants, some gods rode in on donkeys, and the giants fled in terror at the donkeys' braying; as a reward, the donkeys were placed in the sky along with their manger.
In the East Asian 28 lunar mansions, the center of Cancer belongs to the Ghost mansion (鬼宿), and the Beehive Cluster was called "Jishiqi" (積尸氣), the "piled-up corpse vapor."
Main stars
| Name | Apparent magnitude | Distance (approx., ly) | Notes |
|---|---|---|---|
| Altarf (Beta) | 3.5 | ~290 | The brightest star in Cancer, an orange giant. |
| Asellus Australis (Delta) | 3.9 | ~130 | Lies so close to the ecliptic that it is sometimes hidden by the Moon. |
| Iota Cancri | 4.0 | ~300 | A double star that a small telescope splits into a yellow star and a bluish star. |
| Acubens (Alpha) | 4.3 | ~170 | The Alpha star, but not the brightest in Cancer. Its name comes from the Arabic for "the claw." |
| Asellus Borealis (Gamma) | 4.7 | ~180 | A white star just north of the Beehive Cluster. |
The Beehive Cluster (M44) is an open cluster about 600 light-years away. Around 1610, Galileo turned his telescope on it and was the first to see that it is made of many stars. M67, near Acubens, is a very old open cluster, several billion years old, and is widely used to study how stars evolve. 55 Cancri (about 41 light-years away), invisible to the naked eye, is known to host five planets.
Birthday sign dates: two different answers
In astrology, Cancer runs from June 21 to July 22. But for most of that period, the real Sun is in Gemini. The Sun actually passes through the Cancer constellation from about July 21 to August 10, a short stay of about 21 days.
The difference starts with precession. Earth's axis slowly turns like a spinning top, once every 26,000 years or so, and as a result the reference points of the seasons move along the ecliptic by about 1 degree every 72 years. The astrological signs divide the ecliptic into 30-degree slices from the vernal equinox point based on the sky of about 2,000 years ago, so today they are almost one constellation out of step with the real sky.
On top of that, the constellation boundaries set by the International Astronomical Union (IAU) in 1930 vary in width. Cancer covers only a narrow stretch of the ecliptic, so the Sun passes through quickly. The dates can vary by about a day from year to year.
In Planetica
If your birthday's astrological sign is Cancer, Planetica places your planetary system next to a real star inside the Cancer constellation. The candidates are stars in the HYG catalog that lie within about 3,000 light-years of Earth and inside the IAU boundaries of Cancer, 1,130 in all. Only a few are visible to the naked eye, but this many stars really do lie in that direction.
One of them is chosen at random, and your system is placed about 1–6 light-years from it. "Twin systems" from people with the same name and birthday all sit in Cancer, but each next to a different star.
Planetica also shows which constellation the Sun was really in on your birthday. For example, someone born on July 5 is a Cancer in astrology, but on that day the Sun was actually in Gemini.
Fun facts
- The Tropic of Cancer is named after this constellation, because about 2,000 years ago the Sun was in Cancer at the June solstice.
- The Beehive Cluster sits near the ecliptic, so you can sometimes see the Moon or a planet pass in front of it.
Other constellations
Create your own planetary system
Enter a name and a birthday to get a one-of-a-kind planetary system, placed next to a real star in your birthday constellation.