Overview
Eridanus is a long, winding constellation representing a river. It is among the largest of the 88 constellations by area, but most of its stars are faint, so it is easy to overlook. It begins near the bright star Rigel in Orion and meanders south, ending at Achernar, one of the brightest stars in the sky.
Where to find Eridanus
Eridanus is best seen in the evening during the winter months for Northern Hemisphere observers, and in summer evenings for Southern Hemisphere observers. Its northern end starts close to Orion’s foot, near Rigel. From there, the river curves west and south across a fairly empty region of sky.
Where can you see all of it? The northern part is visible from most of the world. Achernar, at the far southern end, is far south of the celestial equator, so it is visible only from tropical and southern latitudes and stays below the horizon for most mid-northern observers. People in the southern United States can catch it very low, and those at higher northern latitudes cannot. A planetarium app will show exactly what is visible from your location.
A dark sky helps. In a city you may see only the brightest stars, so the river will look like a few scattered points.
The main stars of Eridanus
- Achernar (Alpha Eridani): the brightest star in the constellation, and one of the brightest in the whole night sky. It is a hot blue-white star notable for being unusually flattened because it spins fast. Its name comes from an Arabic phrase often translated as “end of the river.”
- Cursa (Beta Eridani): a fainter star near Rigel, marking the start of the river.
- Acamar (Theta Eridani): a double star in the southern part of the river.
- Epsilon Eridani: a nearby star, discussed below.
- 40 Eridani (Omicron-2 Eridani): a triple star system, known for its fictional association with the planet Vulcan in Star Trek.
Is Eridani a real star?
People often search “Eridani” because of Epsilon Eridani, and yes, it is a real star. It is one of the closest stars visible to the naked eye, roughly 10 light-years away, and it is somewhat cooler and dimmer than the Sun. Astronomers have studied it for signs of planets and a surrounding disk of debris, and a planet candidate has been reported around it. Eridani appears in names like this because it is the genitive form of Eridanus in Latin; “Epsilon Eridani” means “the epsilon star of Eridanus.” It is also a frequent setting in science fiction.
What does Eridanus represent?
Eridanus represents a river, but which one is not fixed. Ancient writers linked it with different real rivers, sometimes the Po in Italy and sometimes the Nile. In a well-known myth, Phaethon, the son of the sun god, lost control of his father’s chariot and fell into the river Eridanus after being struck down by Zeus. As with most lore, versions differ. For more tales, see constellations and the stories behind them.
Observing tips
- Start from Orion and Rigel, then look for a faint curve of stars to the west.
- Binoculars bring out many more stars than the unaided eye, and our stargazing for beginners guide explains how to prepare.
- For Achernar, choose a clear southern horizon if you are in the north, or simply look south if you are in the Southern Hemisphere.
- Use a chart, since the faint route is easy to lose.
Eridanus is also a big, sparse area that includes many distant galaxies, though most need a telescope and a dark site (see how to see galaxies with a telescope). If you are still learning your way around, begin with the most famous constellations and come back to the river afterward. It is also among the big but faint constellations worth hunting.
Frequently asked questions
What stars are in Eridanus? The best known are Achernar, Cursa, Acamar and Epsilon Eridani.
Where can you see Eridanus? Most of it is visible from both hemispheres, but Achernar requires a southern location.
Is Eridanus one of the biggest constellations? It ranks among the larger ones by area.