The constellation Eridian (Eridanus-Eri)

The name, the history, the myth

The shape of the constellation Eridanus, the longest in declination of the whole sky, recalls the course of a river full of bends. The stars of this constellation have been associated to various rivers of the Earth, among which the Euphrates, the Ebro, the Rhone, the Rhine, according to some it would be the Nile, according to others the Po.

In the times of Ptolemy the constellation ended with the star Teta Eri, Acamar, but at the end of 1500, it was lengthened until the star today alpha Eri, Achernar. Both names come from the Arabic akhir al-nahar "end of the river".

Aratus (3rd century B.C.) was the first classical author to use the name Eridano, perhaps inspired by an older Mesopotamian name. When Aratus mentions "those poor remains of Eridanus, river of many tears", he alludes to the idea that the river was partly drained as a consequence of the tragic story of Phaeton (whose name means "the shining"), which would also explain the weakness of its stars. Mortal son of the Sun-god Elios (cult and history later attributed to Apollo) and the sea nymph Climene. Wishing to know the identity of his parents, he goes to the palace of Elios, who manifested himself, admits to being his real father and promises, to persuade him, to satisfy his every request. It is a fatal oath because, despite his father's grievance, Phaethon asked and obtained to drive his father's chariot, the Chariot of the Sun, built by Hephaestus. Helios and Phaethon begin their rapid ascent, but being unable to drive it, Phaethon loses control of the chariot, the horses (Eto, Piroo, Eoo and Phlegon) deviate from the road of the Sun, plunging between the constellations. The proximity to the monstrous animals of the zodiac, such as Scorpio, further frightened the four steeds and Phaethon, who plunged further into the abysses of the sky. It ended up creating panic also on Earth, in fact, passing near it the chariot inflames the top of the mountains and the fire expands in the bottom of the valleys burning the ground and draining all the rivers. At the cry of Gaea, goddess of the Earth, Zeus intervened to save the world from total destruction: throwing a lightning bolt towards the chariot that turns the crazy horses towards the sea. The burning body of Phaethon falls into the river Eridanus (or the sea into which it flows), and the waters put out the fire. The Naiads (nymphs of the rivers, lakes and springs) and the daughters of Elios run to the river weeping, shed tears that turn into amber, and they themselves turn into poplars on the banks of Eridanus.

We have arrived at a little known constellation at our latitudes since we can only see a part of it. Even less was seen for example in 4000 B.C. when its brightest star (Achernar) was near the South Celestial Pole: it was the time when from our latitudes we could also see the Southern Cross, low on the horizon.

Eridanus is one of the largest constellations in the sky and boasts some respectable records: the first is that in the 3D map that we will see shortly I have put 79 stars, all quietly visible to the naked eye, provided we go to the southern hemisphere, in a dimly lit area. From this image made with Stellarium we can't but notice the ease of interpreting this sequence of stars as a river. Having so many visible stars it's easy to imagine that this constellation has a large number of stars baptized with a name, but the most beautiful record it holds is that it has as many as 7 stars below 30 light years away from us, which rise to 12 if we set the usual threshold limit to 60al.
Another rather strange record is that of stars τ Eri there are 9, from τ1 to τ9, while of υ there are 4: in this way with the two Greek letters are indicated 13 stars, for good peace of those who over the centuries have had to give a technical name to this stream of stars!
Finally, for fans of science fiction and Star Trek in particular, one of its stars (ο2 Eri) would be the star around which the planet Vulcan revolves, home of those bizarre, pointy-eared characters, among which the most famous is undoubtedly Dr. Spock.
And now that between the serious and the facetious I have synthesized the salient characteristics of this constellation, let's discover it slowly, starting from the representation in antiquity and modern times.

The representations of Eridano

It is a river, the Nile in the tradition of the ancient Egyptians, easily represented in pictorial form: here we see it according to Bayer's Uranometry.

Here instead is the representation straightened from the one created by Hevelius

and finally we see the modern version of Stellarium

As I said, it's not very difficult to represent a river: who knows how many times we drew a river as children!

Many stars nearby

In this table I listed the 12 stars of the constellation Eridanus that are below 60al from the sun: clicking on each link will open the image of the Sun seen from the vicinity of the selected star.

In this table I listed the 12 stars of the constellation Eridanus that are below 60al from the sun: clicking on each link will open the image of the Sun seen from the vicinity of the selected star.

Some notes on the photos: the Sun starts from a magnitude of 2.4 for the nearest star, to reach the limit of visibility with the naked eye for the farthest one. Note that in almost all the photos the Sun is in a desolately empty area of the sky: only in the case of 58 Eri it is in the company of Sirius, Vega, Arthur and Raccoon, but they have lost some of their brilliance. Instead, going to my friends Quunoerini, the Sun is in a zone of the sky where stars of the Bear, Benetnasch, Mizar, Alioth and Dubhe coexist with Arturo, Vega and Sirius: the same thing I found going to see my colleagues Chierini. Very disappointing is the Sun seen by the dearest Ranisti: did you know that they live swimming on a planet entirely covered with water, like the one in the movie Solaris ? Discounted, right ? Just as it is obvious that their traditional food, exported all over the galaxy, even to Bologna, is tortellini.

A few big stars

It's time to get serious again and compare the stars of Eridano with other monsters met during the various episodes: in this constellation there are few really big stars, as we can see in the diagram, made by the undersigned to compare the width of new stars with others already met during our celestial wanderings. In Eridanus the biggest stars are all of spectral class M and therefore red-orange. Finally another disappointment, going to visit the Oduerini: hoping to meet some true Vulcan, I realized instead that my friends do not like to be called by the other nickname, Keidioti.

We have the star 47 Eri 153 times the Sun, a couple of stars just above 100 times (one of the family "tau", τ4 and 54 Eri), a couple above 70 times and 7 Eri) and one from 57x (γ Eri)

To see how big is a star "153x", I approached with Celestia to the remarkable distance of 10UA (that of Saturn from the Sun): from that distance the star reddens imposingly big 7°. My friends Quarantasettieri know something about it, who always have to protect themselves from sunburn with their quarantasettieri cream.

Nebulae and galaxies in this constellation

Enough nonsense and now let's dive into deep space... In Eridanus we find two barred spiral galaxies, respectively NGC1232 ...

...and the fantastic NGC1300, which I invite you to see in high resolution by clicking on the picture with the mouse

here is a ring galaxy, NGC1291.

lastly I show you a strange nebula (the Witch Head Nebula ), to the century IC2118, in Italian translated (and well depicted!) as $Nebula$ of the Witch Head: the brightest star we see is Rigel and also of other stars I added the name

I'll never get tired of saying that all these fantastic photos were made by that marvel of technology that is the Hubble Space Telescope.

The names of the stars of Eridano

In this constellation there are many names, the most famous of which are Achernar and Acamar.

  • AchernarEri): from Arabic, the mouth of the river
  • CursaEri): from Arabic, the chair
  • ZaurakEri): from Arabic, the light of the boat
  • RanaEri): from Latin, with obvious meaning
  • SadirahEri): from Arabic
  • ZibalEri): from Arabic, young ostrich
  • AzhaEri): from Arabic, the ostrich's nest
  • Acamar (θ1 Eri):from Arabic, the end of the river
  • Kursi al jauzah I and II and λ Eri) :from Arabic, the central chair
  • Beid (ο1 Eri): from Arabic, the egg
  • Keid (ο2 Eri): from Arabic, the eggshell
  • Aludhi I, II and III (ρ1, ρ2 and ρ3 Eri): from Arabic, still the ostrich's nest
  • Angetenar (τ2 Eri):from Arabic, the crease of the river
  • Liberflux (τ4 Eri): from Latin
  • Theemin (υ2 Eri): from Arabic, in the water
  • Beemin I, II and III (υ4, υ3 and υ1 Eri): from Arabic, always in the water
  • Sceptrum (l Eri): from the Latin, the scepter

They are rejoicing in the sky

I said that this constellation cannot be seen entirely from our latitudes: towards the end of October, at 9 p.m., we find it low on the horizon in the South-East, while towards mid-April we find it very low in the South-West. The culmination, in the South, is about Befana, when the constellation goes from the horizon up to 40° of , always followed on the left by the imposing Orion. 

Audio Video The constellation Eridian (Eridanus-Eri)
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