The Constellation Directory


"The River"


By IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg) [CC-BY-3.0], via Wikimedia Commons

Abbreviation: Eri
Genitive: Eridani
Constellation family: Heavenly Waters
Nearest constellations: Caelum, Cetus, Fornax, Horologium, Hydrus, Lepus, Orion, Phoenix, Taurus, and Tucana
Right ascension: 3.92h
Declination: -15.82°
Visible between latitudes: +32° and -90°
Square degrees: 1138
Luminary: Achernar (Alpha Eridani)
Named stars: Achernar, Cursa, Zaurak, Rana, Acamar, Beid, Keid
Notable deep sky objects: NGC 1234, NGC 1535, IC 2118 (NGC 1909, the Witch Head Nebula)


Eridanus is the sixth-largest and the second-longest constellation (after Hydra), and its luminary is the ninth-brightest star in the night sky. It is visible in the Southern Hemisphere and best seen in December.

Eridanus is located in a section of the sky called the water section due to the number of water-related constellations in the surrounding sky: Aquarius, Cetus, Delphinus, Pisces, and Piscis Austrinus.

Some scientists propose the existence of a so-called "Great Void" in Eridanus to explain a cold spot in the cosmic microwave background radiation. Voids in the universe are areas of space where there is no matter present, both normal matter and dark matter. If the void was proved to exist, the Eridanus void could be as large as 1 billion light years across.


Eridanus by Johannes Hevelius

By Johannes Hevelius [Public domain], via Wikimedia Commons

Eridanus is associated with the story of Phaethon, son of Helios, the personification of the Sun. Helios drove a chariot across the sky every day, representing the Sun, and Phaethon wanted to prove to his father that he could also drive the chariot. Helios didn't think Phaethon could manage the chariot, but Phaethon tried to do so anyway.

Unfortunately, the son lost control of the horses that led the chariot and drove too high, freezing the Earth, and too low, scorching the ground. Zeus sent a thunderbolt to strike the chariot and stop him, and Phaethon fell into the river Eridanus.