Eridanus
Eridanus, the river of the sky that begins at Orion's feet and winds down into the southern heavens. It is the river into which the boy who fell from the runaway chariot of the Sun plunged.
Science
The scienceEridanus is the "river of the sky" that begins near Rigel, Orion's bright foot-star, and flows long and winding down below the southern horizon. It is the sixth-largest constellation in the sky, and its chief star Achernar (meaning "the end of the river"), at the river's end (its southern mouth), is the ninth-brightest star in the night sky. This star spins so wildly fast that its equator bulges enormously, making it famous as the most flattened "rugby ball" of a star in the sky. Its southern end lies at such low declination, however, that from Korea only the river's upper reaches can be seen.
Mythology
How it became a constellationEridanus is said to be the river into which Phaethon, son of the sun god Helios, fell to his death. Phaethon begged his father to let him drive the chariot of the Sun for just one day, but he could not master the fierce sky-horses and, straying from the heavenly path, ran wild. As the chariot dipped too close to the ground, the earth caught fire and deserts were born; seeing this, Zeus struck the chariot with a thunderbolt to save the world. Wrapped in flames, Phaethon fell from the sky and drowned in the waters of the Eridanus, and this river that mourned him rose into the sky to become a constellation.
Observing Guide
how to observe🔭 How to find it
The "River of the Sky" begins at the star Cursa, right next to Rigel (Orion's bright foot star), then winds southwest and flows in a long stream down below the southern horizon. Use Rigel to find the river's starting point, then follow the zigzagging current of faint stars southward. Note, though, that the river's southern mouth — and its end-star Achernar — lies below the horizon from the US, Canada, and UK, so northern observers can't trace it all the way to the end (from Australia the whole river rides high).
✨ What to look for
Eridanus is vast but not rich in bright deep-sky objects. The most notable is the barred spiral galaxy NGC 1300, famous as a textbook example of bar structure, though it's faint and needs a large telescope. Near the Orion boundary lies the reflection nebula "Witch Head Nebula" (IC 2118), which reflects Rigel's light and resembles a witch's profile. Epsilon Eridani (ε Eri), a nearby Sun-like star, also lies in this direction.
📍 Where to see it (US · Canada · UK · Australia)
Eridanus winds from Cursa beside Orion (−5°) all the way down to brilliant Achernar at −57°. From the US, Canada, and the UK only the northern reaches of the river are visible on winter evenings (December–February); Achernar, below −57°, never rises, so northern observers never see the river's end. From Australia the entire celestial river is on display during the southern summer, with Achernar riding high in the south.