Eridanus
The great river that runs south
Best on December–February evenings · southern sky.
Eridanus is one of the largest constellations in the sky, tracing a long, winding river that spills from the foot of Orion all the way down to the deep southern sky. It stretches so far south that its brilliant end-point, Achernar, is invisible from most of Europe and North America. If you can see both ends of this river, you're well-placed for a real journey across the sky.
How to find it
On winter evenings, start at Rigel — the bright blue-white star at Orion's foot — and look for the chain of stars curving away from it to the west and south. That chain is Eridanus, and Cursa is the first named star you'll meet, sitting just above and to the northwest of Rigel. From northern latitudes you can trace the river partway south before it dips below the horizon; from the southern hemisphere, follow it all the way down to brilliant Achernar near the south celestial pole.
Brightest stars
Achernar, the river's southern end, blazes at magnitude 0.46 — one of the ten brightest stars in the entire sky, blue-white and unmistakable from southern latitudes. For northern observers the river's brightest reachable star is Cursa at magnitude 2.79, followed closely by orange-tinged Zaurak at 2.95.
Worth seeing
Achernar is the crown jewel: a remarkably blue-white beacon so far south it barely clears the horizon from subtropical latitudes, and from the southern hemisphere it rides high as a beacon pointing toward the south celestial pole.
Frequently asked
When is Eridanus visible?
Winter evenings, roughly December through February, when the upper reaches near Orion climb into the southern sky. The far southern end with Achernar is best seen from the southern hemisphere, where it's well placed on those same winter evenings.
What are the brightest stars in Eridanus?
Achernar (magnitude 0.46) is by far the brightest, anchoring the far southern tip of the river. Cursa (2.79) and Zaurak (2.95) are the brightest stars in the northern portion, with Acamar (3.24) and Rana (3.54) completing the main chain.
Which hemisphere is Eridanus best seen from?
The southern hemisphere gets the best of it — from there you can trace the whole river from Orion's foot all the way to brilliant Achernar. Northern observers can follow the upper reaches from Cursa southward, but Achernar stays below the horizon for anyone much north of about 30° latitude.
Nearby constellations
Columba · Orion · Cetus · Taurus · Canis Major · Aries · Puppis · Pisces