How to Find the Capricornus Constellation

Northern hemisphere summer constellations.
Capricornus can be seen from around the world; here is how it appears in mid to late summer in the northern hemisphere summer skies, looking south.

Carolyn Collins Petersen

The constellation Capricornus makes up a small bent-up looking pattern in the sky near the constellation Sagittarius. The stars of Capricornus are best observed in the northern hemisphere summer (southern hemisphere winter). It's one of the oldest-known constellations in the sky and has long been the celestial "avatar" for a sea goat. 

This chart shows Capricornus as a pair of triangles connected by a long line. Look for it near Sagittarius in the skies of July through late September. Carolyn Collins Petersen 

Finding Capricornus

To locate Capricornus, simply look for the constellation Sagittarius. It's in the southern skies for observers located north of the equator, and higher in the northern sky for folks south of the equator. Capricornus looks very much like a squashed-looking triangle. Some charts, like the one shown here, depict it as two triangles arranged along a long line. It lies along the ecliptic, which is the path the Sun appears to take across the sky throughout the year. The Moon and planets also appear to move roughly along the ecliptic. 

All About Capricornus

The star pattern we call Capricornus was known to the ancients at least as far back as the Middle Bronze Age, some 20 centuries before the Common Era. The Babylonians charted the pattern as the Goat-Fish. The Greeks saw it as Amalthea, the goat that saved the life of the infant god Zeus. Over time, Capricornus was referred to more frequently as a sea goat. In China, on the other hand, the constellation was described as a tortoise, while in the South Pacific it was viewed as a cavern.

The Stars of Capricornus

About 20 stars make up the pattern of Capricornus. The brightest star, α Capricorni, is called Algedi. It's a multiple star system and its closest member is just over a hundred light-years away from us.

The second-brightest star is called β Capricorni, or more familiarly as Dabih. It's a giant yellow-colored star and is about 340 light-years away from us. One of the more peculiar stars in Capricornus is called delta Capricorni, or Deneb Algedi, which refers to the tail of the sea-goat.

The brightest star in the δ Capricorni multiple star system is what's known to astronomers as an eclipsing binary star. That means that one member of the star "eclipses" the other every so often, causing the brighter one to dim a bit. Astronomers are also intrigued by the chemical makeup of this strange star because it doesn't quite match the chemistry of other stars of its type. It also appears to rotate quite rapidly.  

The IAU chart for Capricornus.
The official IAU constellation region of Capricornus shows the central pattern, plus other stars within the constellation outline. IAU/Sky Publishing.  

Deep-Sky Objects in Capricornus

Even though the constellation lies against near the backdrop of the plane of the Milky Way Galaxy, Capricornus doesn't have a lot of easily seen deep-sky objects. Observers with good telescopes can spy out a few very distant galaxies in its boundaries. 

In our own galaxy, Capricornus contains the globular star cluster called M30. This tightly packed spherically shaped collection of stars was first observed and cataloged by Charles Messier back in 1764. It's visible through binoculars, but stargazers with a telescope see more details, and those with even larger instruments can make out individual stars in the cluster. M30 has more than a million times the mass of the Sun in its core, and stars that interact there affect each other in ways that astronomers are still working to understand. It's about 93 light-years across and is fairly close to the center of the Milky Way.

Globular cluster M30.
A Hubble Space Telescope image of the globular cluster Messier 30 (M30) shows many stars tightly packed together at its core. This is the central region of the cluster. NASA/ESA/STScI 

Globular clusters like M30 are companions to the Milky Way and contain very old stars. Some have stars much older than the galaxy itself, which indicates that they formed well before the Milky Way, perhaps more than 11 billion years ago. Globular cluster stars are what astronomers call "metal-poor" because they have very few of the heavier elements beyond hydrogen and helium in their atmospheres. Studying the metallicity of a star is one way to tell its age, because stars that formed early in the history of the universe, as these did, aren't "polluted" with metals made by later generations of stars.