Constellations: Eridanus 'the River' Content from the guide to life, the universe and everything

Constellations: Eridanus 'the River'

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So still runs the river
While I wait upon the shore

Amore di ritorno
For my love goes on forever more
Like the river, forever more.

The River (Le Colline Sono In Fiore), a number three hit in 1965 for Ken Dodd.

The Constellation Eridanus

Name:Eridanus (Latin: 'River')
Short form:Eri
Area:1,138 sq deg
Co-ordinates1:Right Ascension 03h, Declination −20°

The southern constellation Eridanus is one of the largest of the modern 88, ranking below Hydra, Virgo, Ursa Major, Cetus and Hercules. At 1,138 sq deg area, Eridanus is the third-largest southern constellation, and the sixth-largest overall. Eridanus starts below Taurus and Orion, then snakes through the southern sky between Lepus and Cetus, Caelum and Fornax, through Horologium and Phoenix, before finishing with its brightest star (alpha Eridani - Achernar) next to the border with Hydrus.

Eridanus is one of Greek astronomer Ptolemy's original 48 constellations. Homer called it the 'Ocean Stream', but the river it is thought to represent on Earth has been lost in the mists of time. It may have been the Sumerian Strong River, also known as Ariadan. The rivers Tigris, Euphrates, Po, Ganges and Nile have each been identified as the Eridanus by various cultures.

Objects of interest include: the 'Eridanus Supervoid', which is the largest chunk of barren space yet discovered; epsilon Eridani (Sadira) is a popular science-fiction topic due to its similarity with our own Sun; and four extrasolar planetary systems.


The constellation Eridanus represents the erratic passage taken by the Sun chariot one day when Helios (the Sun god) allowed his son Phaëton to take the reins. Phaëton convinced his father that he was strong enough to control the horses, but he was not. The result was that the Sun swung so high the Earth almost froze, then so low that parts of Africa, which had been lush, were frazzled and the skins of the populace scorched. This is how the ancient Greeks explained the ebony skin of Ethiopians.

Obviously this calamity caused mass panic and, to prevent the destruction of the Earth, the great god Zeus hurled a thunderbolt at the Sun chariot. Phaëton was killed by the blow and his body tipped out, falling into the river Eridanus below. Phaëton's grieving sisters, Aegiale, Aegle and Aetheria2, who had encouraged their brother in his misadventure, were morphed into poplar trees. Still weeping tears turned to amber by the Sun, the trees are said to grow along its banks.


The scientific star names are simple to understand (if you know your Greek alphabet). For example: 'alpha Eridani' means it is the brightest star in the constellation Eridanus. The next brightest is designated 'beta' etc. Combined with the genitive name, this is known as the 'Bayer designation'. Some stars have proper names as well, for example, alpha Eridani is Achernar. Others are known by their catalogue number.

Achernar (alpha Eridani) is a first magnitude star, the ninth brightest star in the sky. It is a blue main sequence dwarf with an unusual spectrum. This star rotates so fast that it bulges in the middle; images of it look like a blue jellybean. Achernar lies at the southernmost end of the constellation, with barely a degree separating it from Eridanus' neighbour Hydrus, the water snake.

Epsilon Eridani, Sadira, has a dust disc and an extrasolar planet system. This star, beta Pictoris, Vega (alpha Lyrae) and Fomalhaut (alpha Piscis Austrini) are dubbed the 'Fabulous Four' debris stars; the debris was discovered by the IRAS (Infrared Astronomical Satellite). Sadira is situated just ten light years3 away and, thanks to similarities to our own Sun, is often a topic of sci-fi novels.

The orange dwarf star Keid (40 Eridani) is worthy of mention because it is one of the rare stars to have a common name but no Greek letter designation. Gene Roddenberry chose this stellar area to be the origin of one of the most famous sci-fi characters, Mr Spock (played by actor Leonard Nimoy) of Star Trek fame. If it really existed, Spock's home planet, Vulcan, would boast a spectacular view because the star it orbits has two companions. The white star and a red dwarf star would shine silver-white and brilliant red, much like how we view Venus and Mars.

Star Table

StarDesignationName or
catalogue number
Brightness (m)Distance
(light years)
Spectral classification
and/or comments
α Erialpha EridaniAchernar
(end of the river)
+0.46144Blue dwarf
β Eribeta EridaniKursa+2.7990Blue-white dwarf
γ Erigamma EridaniZaurak+2.92220Red dwarf
δ Eridelta EridaniRana+3.529Orange subgiant
ε Eriepsilon EridaniSadira+3.5410Dusk disc & eso planet system
ζ Erizeta EridaniZibal+4.8120White giant
η Erieta EridaniAzha+3.9133Orange giant
θ Eritheta EridaniAcamar+2.9160Binary star system
ι Eriiota EridaniHD 16815+4.1145Orange giant
κ Erikappa EridaniHD 15371+4.24530Blue-white dwarf
λ Erilambda Eridani69 Eridani+4.2520Blue-white dwarf
μ Erimu Eridani57 Eridani+4.01500Blue-white dwarf
ν Erinu Eridani48 Eridani+3.9580Blue-white subgiant
ξ Erixi Eridani42 Eridani+5.2200White giant
ο Eriomicron Eridani38 Eridani+4.04120Triple star system
π Eripi Eridani26 Eridani+4.42500Red subgiant
ρ Erirho Eridani8 Eridani+5.7300Orange giant
τ Eritau EridaniLiberflux+3.750-4009-star group
υ Eriupsilon EridaniTheemin+4.5120Orange giant
φ Eriphi EridaniHD 14228+3.5150Blue-white dwarf
χ Erichi EridaniHD 11937+3.757Yellow dwarf
ψ Eripsi Eridani65 Eridani+4.8900Blue-white dwarf
ω Eriomega Eridani61 Eridani+4.36227White giant
40 Eri40 EridaniKeid+4.4216Orange dwarf

New General Catalogue (NGC)

The NGC was compiled by John Louis Emil Dreyer (director of the Armagh Observatory from 1882 - 1916).

NGC Objects of Eridanus

NGC Table

CatalogueTypeBrightness (m)Distance
(light years)
NGC 1187Barred ACW spiral galaxy+11.458mSupernova 1982R
NGC 1232CW spiral galaxy+10.969mHas unusual arms
NGC 1232ADwarf galaxy+10.969mInteracting with NGC 1232
NGC 1300Barred CW spiral+10.469mNo central black hole
NGC 1309CW spiral galaxy+11.6100mRecent supernova 2002fk
NGC 1637Spiral galaxy+11.525mSupernova 1999em
NGC 1535Planetary nebula+11.61,500Glowing green gaseous nebula

Hickson 21 is a group of five galaxies that are all registered separately on the NGC table. They are in such close proximity that they are interacting, and may eventually collide, providing a cosmic train-wreck of intergalactic proportions:

GalaxyNGC numberGalaxy typeMagnitude
Hickson 21ANGC 1099ACW spiral+13.1
Hickson 21BNGC 1100ACW spiral+13.0
Hickson 21CNGC 1098Elliptical+12.6
Hickson 21DNGC 1092Elliptical+13.4
Hickson 21ENGC 1091CW spiral+14.1

The Eridanus Supervoid

The Eridanus Supervoid was discovered by a combination of NASA's Microwave Anisotropy Probe and data from the Very Large Array. It is at least 6 billion light years away and the largest absence of galaxies to be discovered so far. At a billion light years in diameter4, the scale of this super-cool gap has so far escaped explanation by the scientific community. However, several ideas have been put forward, some a little wackier than others:

  • Quantum entanglement between our universe and a parallel universe.
  • Dark energy: scientists know so little about it, but it is thought to be accelerating the expansion of the universe.
  • Big Bang: this is where it happened, and the entire universe accelerated from here.
  • Heaven/the Afterlife: More than 90% of the Earth’s population believes in some form of religion including the possibility of life after death. Is this where all the souls go after Earthly life has ceased? Or maybe it is the storage facility of all the souls yet to be born?
  • It is bad news. After all, nothing wants to hang around bad news.
  • A cloaked military base camp run by hostile aliens.
  • It is the last known frontier for Star Trek - to 'boldly go where no-one has gone before'.
  • It is a space that has been cleared for an intergalactic bypass still awaiting planning permission (with apologies for the inconvenience).
  • It is the graveyard of the Great Grey Ghost Elephant, from the legend behind the song 'Ging Gang Goolie'5.
  • As it is the coolest spot in the known universe, it has to be where Elvis Presley is - that is the 'real' Elvis, of course, no impersonators, and those fans who have since followed him.
  • It is where all the unrealised dreams, lapsed friendships, regrets and unrequited love feelings go.
  • It is a rather small (solar system-sized) reflector array, pointed away from us, that was put in place 'mumblety' billion years ago during the early times of a treaty/conglomerate of beings who were running out of conventional renewable energy sources. Basically, they put together an array of small(er) mirrors, surrounded by steerable reflectors, that catch all of the light from that sector and send it back to themselves - including the light reflected back from themselves. Imagine, if you will, the complex technologies involved. Unfortunately, due to being designed under the auspices of their government, a small error occurred. We are left with the dead remains. We (well, just a few of us) know this because a probe has been recovered and its contents translated (using the 'Itsarose' tablet, recently found in Mariana Trench). It gives a brief account of the artefact but then, part way through, fades out, saying: 'Don't...' That probe and its message is held secret by certain authorities here... until now, that is.
The study of this 'cosmic nothingness' has caused extra work for people who specialise in the development of structure in the universe, particularly the University of Minnesota team which discovered it.

It is a very hot topic in the cosmology world right now.
- Professor Lawrence Rudnick of the University of Minnesota.

Extrasolar Planets in Eridanus

Several extrasolar planetary systems have been found in the constellation Eridanus; the first was discovered in 2000. Figures given in the table below are the length of the planet's orbital period around its parent star, which we know of as a year, the mass of the extrasolar planet as compared to that of Jupiter, our solar system's largest planet (the 'Jovian scale'), and the year of discovery.

Extrasolar Planets Table

Star name or
catalogue number
catalogue number
Planet mass
(Jovian scale)
Orbital period
(Earth days)
Year of discoveryComments
HD 10647HD 10647 b0.91,0502003Slight eccentric orbit
HD 28185HD 28185 b63832001Habitable zone
Gliese 86 AGliese 86 A b4162000Binary system: planet orbits primary star Gliese 86 A
epsilon Eridaniepsilon Eridani b/AEgir1.72,5002000Eccentric orbit
epsilon Eridaniepsilon Eridani c0.1102,2002002Unconfirmed (2007)
HD 30562HD 30562 b1.291,1572009Eccentric orbit
HIP 12961HIP 12961 b0.3557.42009Hot Saturn
HD 28254HD 28254 b1.161,1172009Gas giant
WASP-22WASP-22 b0.563.52010Hot gas giant
WASP-35WASP-35 b0.723.172011Hot gas giant
WASP-50WASP-50 b1.471.952011Hot gas giant
82 Eridani82 Eridani b2.7 (Earth)182011Super-Earth
82 Eridani82 Eridani c2.4 (Earth)402011Super-Earth
82 Eridani82 Eridani d4.8 (Earth)902011Super-Earth
HD 18742HD 18742 b2.77722011Superjovian
HD 30856HD 30856 b1.89122011Superjovian
HD 13808HD 13808 b0.032514.22011Super-Earth
HD 13808HD 13808 c0.03653.82011Super-Earth
WASP-78WASP-78 b1.162.182012Hot gas giant
WASP-79WASP-79 b0.893.62012Hot gas giant
WASP-97WASP-97 b1.322.072013Hot gas giant
WASP-98WASP-98 b0.832.962013Hot gas giant
WASP-99WASP-99 b2.785.752013Hot superjovian
WASP-117WASP-117 b0.28102014Hot Saturn
HATS-5HATS-5 b0.244.762014Hot Saturn
51 Eridani51 Eridani b914,9652015Superjovian
HD 19467HD 19467 b5218,6152016Brown dwarf
HIP 8541HIP 8541 b5.61,5602016Superjovian
WASP-139WASP-139 b0.185.922016Hot Saturn
WASP-140WASP-140 b2.442.242016Hot superjovian
WASP-141WASP-141 b2.693.32016Hot superjovian

Gamma-ray Burst

On 13 September, 2008, a record-breaking gamma-ray burst, 12.8 billion light-years from Earth, was recorded by the Swift Space Telescope. GRB 080913 is 70 million years older than the previous oldest GRB. The light is only just reaching us now because of the immense distance involved.

It's coming to us from near the edge of the visible Universe.
– NASA scientist Dr Neil Gehrels

Down to Earth

  • Eridanus Giffin is a self-representing Californian artist who likes to paint flowers, particularly poppies, tulips and daffodils.

1Current IAU guidelines use a plus sign (+) for northern constellations and a minus sign (−) for southern ones.2Known as the Heliades or the Electrides, meaning 'children of the sun'.3A light year is the distance light travels in one year, roughly 5.88 trillion miles or 9.46 trillion km.4If you were to go to this hole and enter one side, you would have to travel at the speed of light for a billion years before you would reach the other side.5This is perhaps best known as the scout/guide campfire song.

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