Capricornus
Capricornus depicts a "sea-goat," a goat in front and a fish behind. Its stars are on the faint side, but once you know its broad triangular shape, you can find it in the southern sky on late summer and autumn evenings.
| Latin name | Capricornus |
|---|---|
| Abbreviation | Cap |
| Brightest star | Deneb Algedi (mag 2.9) |
| Best seen | Late summer–autumn evenings (Northern Hemisphere) |
| Astrological dates | December 22 – January 19 |
| Sun actually passes | January 20 – February 15 |
| Stars used by Planetica | 905 |
How to find it
Capricornus is well placed in the evening sky from August to October. In early September it sits in the south after dusk, but it's low and its stars are only around 3rd magnitude, so you'll need a spot with little light pollution.
A good signpost is the Summer Triangle. Draw a line from Vega, overhead, to Altair, then extend it southward by the same length again. Near the end of that line are Algedi and Dabih, the head of the sea-goat. It lies just left (east) of the Teapot of Sagittarius and right (west) of Aquarius.
The stars of Capricornus form a large, broad triangle, or a shape that curves downward like a smile or the bottom of a boat. The western corner is Algedi and Dabih, and the eastern end is the brightest star, Deneb Algedi. People with sharp eyes can split Algedi into two stars with the naked eye, and binoculars split it easily.
The myth: Pan becomes a sea-goat
Greek mythology links Capricornus to Pan, god of the wild. While the gods were feasting near the Nile, the terrifying monster Typhon suddenly appeared. The gods each turned into animals and fled, but Pan, in a panic, leaped into the river mid-transformation: the part underwater became a fish while the top half stayed a goat. Zeus is said to have placed this comical form in the sky to remember it. In another story, the constellation is Amalthea, the goat who nursed the infant Zeus.
In fact, the goat-fish figure appears in ancient Mesopotamia, far earlier than in Greece. The Babylonians pictured this constellation as a "goat-fish" associated with the god of water and wisdom, and that image was passed on to the Greeks.
In the East Asian 28 lunar mansions, the head of Capricornus belongs to the Ox mansion (牛宿), with Dabih as its determinative star.
Main stars
| Name | Apparent magnitude | Distance (approx., ly) | Notes |
|---|---|---|---|
| Deneb Algedi (Delta) | 2.9 | ~39 | The star in the sea-goat's tail; an eclipsing binary that dims slightly about once a day when its companion passes in front. |
| Dabih (Beta) | 3.1 | ~330 | A yellowish star in the goat's head; a small telescope shows a faint companion. |
| Algedi (Alpha²) | 3.6 | ~110 | The brighter member of a naked-eye double. Its partner, Alpha¹, is much farther away and only lines up in the same direction. |
| Nashira (Gamma) | 3.7 | ~140 | A white star in the tail, right next to Deneb Algedi. |
Capricornus doesn't have many bright nebulae or clusters. Its best-known object is the globular cluster M30, about 27,000 light-years away, which a small telescope shows as a round, fuzzy glow.
Birthday sign dates: two different answers
In astrology, Capricorn runs from December 22 to January 19. But during that period, the real Sun is in Sagittarius. The Sun actually passes through the Capricornus constellation from about January 20 to February 15, about 27 days. In other words, the astrological dates and the real dates are almost a full month apart.
The reason starts with precession. Earth's axis slowly turns like a spinning top, about once every 26,000 years, so the equinox and solstice points drift west along the ecliptic by about 1 degree every 72 years. The astrological signs divide the ecliptic evenly into 30-degree slices from the vernal equinox point based on the reference of about 2,000 years ago, so they're now about one constellation off from today's sky.
Also, the constellation boundaries set by the International Astronomical Union (IAU) in 1930 cover different areas. Capricornus is the smallest of the zodiac constellations. The dates can vary by about a day from year to year.
In Planetica
If your birthday's astrological sign is Capricorn, Planetica places your planetary system next to a real star inside the Capricornus constellation. The candidates are the 905 stars in the HYG catalog that lie within about 3,000 light-years of Earth and inside the IAU boundaries of Capricornus. Because the constellation is small and lies away from the Milky Way, it has the fewest candidates of any zodiac constellation.
One of them is picked 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 Capricornus, 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 January 5 is a Capricorn in astrology, but on that day the Sun was actually in Sagittarius.
Fun facts
- The Tropic of Capricorn is named after this constellation, because about 2,000 years ago the Sun was in Capricornus at the December solstice.
- Deneb Algedi means "the goat's tail" in Arabic.
Other constellations
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