Eridanus
What is Eridanus?
Constellations· Winter evenings (N) · a river winding from Rigel to the southern horizon. The celestial river, winding nearly 60 degrees from beside Rigel down past the southern horizon — the sixth-largest constellation. Achernar at its end spins so fast it is the flattest star known, and Epsilon Eridani is a nearby Sun-like star with a planet.
📍 Visible from where you are right now — computed in your browser
bearing · altitude · rise time · best time tonight
Where Eridanus is in your sky right now: https://nowsky.org/solar/sky?sel=con:Eridanus — location and time set automatically, no login.
Stars of the figure
| Star | Magnitude |
|---|---|
| Achernar | 0.5 |
| Epsilon Eridani | 3.7 |
The 2 brightest of the 6 stars used in the figure (5 lines). Lower magnitude means brighter.
Deep-sky objects near this constellation
Constellation basics
| Season | winter evenings, December–January; the southern end is below Korea's horizon |
|---|---|
| Brightest star | Achernar (0.5) |
| How to find | start at Cursa just west of Rigel and follow the faint bends southwest |
| Area rank | 6th of 88 |
Figures are representative approximations. Source: compiled from NASA and IAU public data.
Frequently asked
- When can I see Eridanus?
- winter evenings, December–January; the southern end is below Korea's horizon.
- How do I find Eridanus?
- start at Cursa just west of Rigel and follow the faint bends southwest.
- What is the brightest star in Eridanus?
- Achernar, magnitude 0.5.
- Where is Eridanus in the sky right now?
- Open the sky view for your location and time and it shows the position in the sky right now.
The first white dwarf recognised — 40 Eridani B
Midway along the river of Eridanus, the 4th-magnitude star 40 Eridani is a system of three stars. Component B was found by William Herschel in 1783, and in 1910 analysis of its spectrum showed it to be white and hot yet extremely faint: a hot, small star, the first white dwarf to be recognised as such. Sirius B had been discovered earlier, but its spectrum was confirmed only in 1915.
40 Eridani B packs half the Sun's mass into a body the size of Earth, and because it lies well apart from the bright A star it is the only white dwarf a small telescope can show. A red dwarf, C, orbits beside it. At 16 light-years the system, with its Sun-like primary, has often served fiction as the home of alien civilisations, but its place in astronomy is as the first specimen of a dead star.
The constellation in depth
Eridanus is the river into which Phaethon fell after losing control of the Sun's chariot, identified with the Po or the Nile. Cursa at its source means "footstool of Orion", and Achernar at the far end is Arabic for "river's end". In antiquity the river stopped at the star now called Acamar; it was extended once the far southern sky became known.
Achernar shines with three thousand Suns and rotates at over 250 km per second, making its equator more than 50 percent wider than its poles. Epsilon Eridani, 10.5 light-years away, is a young Sun-like star with a Jupiter-class planet and a dust disk — a science-fiction favourite. Nearby 40 Eridani B was the first white dwarf identified, and beyond the river's southern reach lies the enormous void called the Eridanus Supervoid.
On winter evenings start at Cursa, just west of Rigel, and follow the faint stars southwest as the river bends back and forth. From Korea only the upper course is visible; Achernar stays below the horizon. Set the Sky view to a January evening with constellation lines on to trace the channel, and move the observer to the southern hemisphere to follow it to Achernar.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.