Constellation, River
Eridanus
The celestial river, winding from the foot of Orion far into the southern sky. Aratus called it Eridanus, the river into which Phaethon fell, and later writers took it to be the Po.
From New York on October 13, 2026, the centre of Eridanus rises at 10:38 PM, is highest at 3:17 AM (27° above the horizon in the S) and sets at 7:57 AM.
In the 3D sky
The sky over New York turned towards Eridanus. Drag to look around.
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Evening visibility over the year
Height of the constellation’s centre above the horizon at 10 pm local time on the 15th of each month, from New York. Above about 30° it is well placed.
Name and history
In Greek myth, Phaethon persuaded his father, the Sun god Helios, to let him drive the chariot of the Sun. He could not control the horses, and the chariot swerved so close to Earth that it set the land on fire. Zeus struck him down with a thunderbolt, and he fell into the river Eridanus. Aratus used that mythical name, while Ptolemy simply called the figure the River. Eratosthenes said it was the Nile, and later Greek and Latin writers took it to be the Po in Italy.
In Ptolemy’s day the river ended at the star now called Acamar. The name Achernar, from Arabic for the end of the river, first belonged to that star. When the river was extended south in the late 16th century, on a globe of 1598 by Petrus Plancius, it reached a brighter star that took over the name Achernar. That star was too far south for Ptolemy to see. Bedouin Arabs saw Achernar and Fomalhaut as a pair of ostriches.
Sources: Ian Ridpath, Star Tales: Eridanus
Brightest stars
| Star | Designation | Magnitude | Distance |
|---|---|---|---|
| Achernar | α Eri | 0.45 | 139 light-years |
| Cursa | β Eri | 2.78 | 89 light-years |
| Acamar | θ1 Eri | 2.88 | 161 light-years |
| Zaurak | γ Eri | 2.97 | 203 light-years |
| Rana | δ Eri | 3.52 | 30 light-years |
| υ4 Eri | υ4 Eri | 3.55 | 178 light-years |
| φ Eri | φ Eri | 3.56 | 154 light-years |
| χ Eri | χ Eri | 3.69 | 58 light-years |
| τ4 Eri | τ4 Eri | 3.70 | 305 light-years |
| Ran | ε Eri | 3.72 | 11 light-years |
Facts
| Area | about 1,142 square degrees (6 of 88) |
|---|---|
| IAU abbreviation | Eri |
| Sky | Southern sky |
| Origin | Listed in Ptolemy’s Almagest, about AD 150, and extended further south in the late 16th century |
| Stars brighter than magnitude 5 | 47 |
| Centre (J2000) | 3.9 h, -23° |
Deep-sky objects
Galaxies, clusters and nebulae brighter than magnitude 9, from the OpenNGC catalogue. Most need binoculars or a telescope and a dark sky.
| Object | Type | Magnitude |
|---|---|---|
| NGC1269 | Galaxy | 8.7 |
Boundaries: International Astronomical Union, via Astronomy Engine. Areas are computed from the boundaries by sampling (within about one percent). Names and lines: Stellarium. Stars: HYG Database. Deep-sky objects: OpenNGC. Calculated with Astronomy Engine, an open-source library verified against the US Naval Observatory and NASA JPL. Data sources
How to use this page
Choose your place and a night at the top. The answer and the 3D view show where the constellation is, when it is highest and whether it rises at all from where you are. The chart shows its stars and lines, and the tables list its brightest stars and the deep-sky objects inside it.
Frequently asked questions
When is a constellation best seen?
When it is highest in a dark sky, which the page gives for the night you pick. Each constellation is best placed in a particular season, because the stars rise about four minutes earlier every night.
Why can I not see a constellation from my place?
Constellations far south of the celestial equator never rise from northern places, and those far north never rise from southern ones. The page says when this is the case.
What are the constellation boundaries?
The International Astronomical Union divides the sky into 88 areas with official boundaries. The stick figures are only a drawing aid, the boundaries say which constellation a point in the sky belongs to.
Related guides
- How to learn the constellationsStart with one bright pattern you cannot miss, use it as a signpost to its neighbours, and come back a month later: the sky will have turned, and you will know a little more of it.
- Altitude and azimuth: finding things in the skyTwo angles tell you exactly where to look: which direction, and how high.
- Light pollution and dark skies: the Bortle scaleFrom a city centre you can count a few dozen stars. From a truly dark site you can count a few thousand, with the Milky Way bright enough to cast a shadow. The Bortle scale puts a number on the difference.
- Binoculars for stargazing: what to buy and what to look atA pair of 7x50 or 10x50 binoculars shows the craters of the Moon, the moons of Jupiter, dozens of star clusters and the Andromeda Galaxy, for a fraction of the cost of a telescope.