ERIDANUS "The River" pronounced eh-RID-a-nas

Chart showing Eridanus

This constellation meanders from Taurus (the Bull) in the northern sky, to Hydrus in the southern sky. In mythology, Eridanus was the river into which Phaeton fell after trying to drive the chariot of his father, the Sun God. It is also said to represent a real river such as the Euphrates, or the Nile, the unknown source of which was symbolised with the disappearance of the winding train of stars below the southern horizon (as seen from the northern hemisphere) to its termination in Achernar, (pronounced AY-kuh-nar) now α Eri.

Originally Eridanus included the stars of what is now Fornax, and it stretched only as far as θ Eridani, which was then known as Achernar, from the Arabic meaning End of the River. The present name of θ Eri, Acamar, comes from Achernar. In recent times Eridanus has been extended southwards below the horizon visible to the ancient Greeks, and another star has been assigned to Achernar.

To find Eridanus, look to Rigel in Orion, and follow the meandering line of stars to Achernar. The Southern Cross points towards Achernar, and about halfway between Achernar and the Cross is due south.

Chart showing Eridanus as seen to the north at about 11 pm NZDT on January 1.


Constellation Sculptor Constellation Phoenix Constellation Hydrus Constellation Reticulum Constellation Horologium Constellation Caelum Constellation Fornax Constellation Dorado Constellation Cetus Constellation Puppis Constellation Pictor Constellation Mensa Constellation Volans Constellation Carina Constellation Columba Constellation Lepus Constellation Canis Major Constellation Orion Constellation Taurus

Details of some of the objects shown in the chart.

α Eridani (Achernar),(the End of the River) is a magnitude 0.5 blue-white star 85 light years away.

β Eri (Cursa), (the footstool, referring to its position under the foot of Orion) is a magnitude 2.8 blue-white star 91 light years away.

ε Eri is a magnitude 3.7 star and the most like the Sun of the nearby stars. This yellow dwarf star was one of the first stars to be sent radio signals in case there is life there. It is only 10.5 light years away, so radio signals would take this long to reach it.

θ Eri (Acamar), is a striking pair of blue-white stars of magnitudes 3.2 and 4.4, 55 light years away. The two stars are only separated by just over 8 arc-seconds, so would need a small telescope to be seen separately.

32 Eri is a beautiful double star for small telescopes, consisting of a magnitude 5 yellow star and a blue-green magnitude 6.3 companion 220 light years away. Their separation is just under 7 arc-seconds, slightly closer than the two stars forming Acamar.

ο2 Eri (also known as 40 Eridani) is a remarkable triple star 15.9 light years away. A small telescope shows that the magnitude 4.4 yellow primary, a star similar to our Sun, has a widely separated magnitude 9.6 white dwarf companion, being the most easily seen white dwarf star in the sky. The separation is about 83 arc-seconds. Bigger telescopes reveal that the white dwarf has a 11th magnitude red dwarf companion, at present about 9 arc-seconds from the white dwarf.
In addition the whole system is relatively quite close to us and has a large proper motion of just over 4 arc-seconds per year. This means the star would move a distance equal to the diameter of the Moon in "only" 450 years compared to more distan background stars.

NGC 1535 is a small 9th magnitude planetary nebula 2150 light years away, appearing in a nice field of scattered stars. While the central star cannot be seen, an OIII filter helps to find and see this dying star remnant.


Eridanus ranges over almost 60° of declination from Archenar in the south to the equator near Cursa. Archenar is sufficiently far south for it to be always visible from New Zealand, although rather low to the south in winter evenings. In mid December, Archenar is at its highest at about 10 pm NZDT when it has an altitude of about 75°, to the south.

As is fitting for a river flowing north, Eridanus then winds its way as a narrow constellation northwards before broadening out as it approaches the equator. The equatorial portion of Eridanus precedes Rigel in Orion into the sky and is to the north later in the evening in December and soon after the sky becomes dark in January.