Eridanus the River

Go with the flow

Photo by Hu Chen on Unsplash

What the constellation represents: a river

History: ancient, Greek and possibly older

Region of sky where it is located: equatorial, but mostly southern hemisphere

When visible: October-March

How visible: Intermediate (easier in southern hemisphere)

Objects of interest: brightest star Achernar, Witch Head Nebula (shared with Orion), Eridanus Supervoid

How to see it: Orion points out the celestial river by getting his foot wet…the star Cursa, marking one end of the river, is close to Rigel, Orion’s brilliant foot or left leg star. From there, the meandering stars of Eridanus are faint (seems this is a muddy sky river!) To see the constellation in full, travel further south.

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A quiet, dark, moonless autumn night. You stand by a riverbank, the only sound being the murmur of the water passing by, currents and eddies swirling. The stars shine gently above, flickering in a curve that matches the river flowing past you. Trees drop their leaves like tears being shed, letting go and surrendering to the moving water. You cannot see the river’s end, beyond the horizon, and you think of the journey taken by that water, that was once rain, and ocean, passing through countless animals and plants in cycles upon cycles. Water that may have reached our planet from icy comets, and was ultimately formed from the hydrogen of the Big Bang and the oxygen from the embers of stars long gone.

According to the system of constellations passed down from the ancient Greeks and compiled by Ptolemy in the 2nd century, the majority of our constellations are animals (both real and mythical), followed by human figures (both divine and mortal), and then inanimate objects. But there’s only one ancient star pattern that represents a natural feature¹, and that’s Eridanus, the river in the sky, a constellation so long that it straddles both hemispheres and cannot be seen fully in the north.

Eridanus as viewed from the UK (Stellarium)

Eridanus flows from Orion’s foot, moving westward, going on a meander past Cetus, then straight down and below the horizon. Out of sight, as though cascading into the underworld. But go closer to the equator, or the southern hemisphere, and that’s where the whole river presents itself. There, the star marking the river’s mouth shines bright, like the splash pool at the end of a starry waterfall. This star, Achernar, was not known to the Greeks, who ended their constellation a bit further upstream at a star called Acamar. In the 16^th century, when Dutch and English astronomers on colonial voyages mapped the stars of the southern hemisphere, Eridanus got extended. A few more stars were tacked on to the existing constellation to stretch it out a bit, so it could terminate at Achernar.

Eridanus is part of a segment of sky designated as the ‘Waters’, where other aquatic constellations like Pisces and Delphinus (the Dolphin) are found.

The Fall of Phaethon by Rubens (from Wikipedia)

River of Myths

‘High in heaven, Orion and his dogs hunt at the mouth of the “deep eddying” River Eridanus. The sad River-now a stream of stars-recalls the tragic fall of Phaethon, who, like Orion, was punished for his pride.’

David Weston Marshall, Ancient Skies: Constellation Mythology of the Greeks

This constellation, also called Potamos (river) by the Greeks, was first named as Eridanus by the writer Aratus in the 3^rd century BCE. In his poem the Phaenomena, he called it the River of Many Tears, and associated it with the story of Phaethon. This is one of many starry myths that caution against hubris and impetuousness.²

Only Helios, the great sun god, had the ability to control the fiery horses that pulled his solar chariot, and his son Phaethon found this out the hard way when he decided to go on a jolly on it without his father’s permission. The ride started out smoothly, but the horses sensed that their driver was inexperienced, and started panicking. As Phaethon clung on for dear life, the steeds bucking and galloping wildly, the flaming vehicle plunged over the land of Libya, scorching its vegetation such that it became a desert, then charged across the sky, startling various constellations along the way. Zeus put a stop to this mayhem by striking the chariot with a flash of lightning, causing Phaethon to plummet into the river Eridanus. The nymphs living by the river’s banks, who also happened to be Phaethon’s sisters, watched in horror as the blazing chariot hit the water. Upon hearing of the death of their brother, the nymphs cried tears of pure, shining amber, and transformed into poplar trees, weeping like willows.

The chariot’s descent was described ‘as sometimes a star from the clear heavens…seems to fall’, which suggests a meteorite³ strike, though there are no impact craters in the Mediterranean area dating to the time the myth was first told. Another thought that came to mind regarding the ‘scorching of Libya’ is that perhaps it is a folk memory of a time long ago when the Sahara was green, before natural climate changes caused it to become arid, a centuries-long process rather than something fast and fiery like the myth implies.

Eridanus as river god, some say inspired by the Sumerian god Enki (image source)

In star maps, Eridanus is usually pictured as a river proper, sometimes with islands and reeds lining its banks, but in some older maps, the constellation is shown as a reclining river god, with green pondweed hair, holding reeds and a jar of spilling water, similar to Aquarius.

I have wondered about the position of Orion close to Eridanus. One of the hunter’s superpowers was walking on water, and maybe this was based on the placement of the two constellations (see the star map below). There’s also the Egyptian interpretation, where Orion is Osiris, and in some tomb paintings he is shown riding his boat on the celestial Nile. In Arabic star lore, the star Cursa, aka Beta Eridani, gets its name from a phrase meaning ‘footstool of Orion’. And in India, Eridanus is known as Srotaswini (meaning ‘the course of a river’) a starry stream that runs close to Nataraja, one of the names given to Orion.

Moving away from rivers but staying with a watery theme, the Chinese called some of the stars in the north east part of Eridanus the Jade Well. Other Chinese constellations made from stars in Eridanus represented orchards and vegetable gardens, and a chain of banners from a larger hunting scene also associated with Orion’s stars.

And the lore of Eridanus doesn’t stop at old myths. Science fiction is, to me, an ongoing continuation of the stories we tell about the stars. In Project Hail Mary by Andy Weir, 40 Eridani is home star of the Eridian alien Rocky. And it’s not the first time this star has featured in sci-fi. If you’re a Trekkie, you might be pleased to know that Vulcan (the planet where Spock comes from) also orbits 40 Eridani. Live long and prosper! 🖖

Eridanus with Orion and Lepus (source)

Whose river is it anyway?

There is debate regarding exactly which river the constellation Eridanus was inspired by. Was it the Jordan, the Danube, the Rhine or the Rhone, or a river in Greece? Or maybe a mythical stream, like the Styx or the Acheron, two of the rivers that flow through the realm of Hades. In India, it was linked to the Ganges. But the most popular candidates for Eridanus’s earthly counterpart are the Nile, the Euphrates, and the Po in northern Italy.

Eratosthenes, who may have been inspired by ancient Egyptian cosmology in this case, suggested a connection to the Nile and Eridanus. In this interpretation, the starry river flows south-north, from an unseen point below the horizon, mirroring how in antiquity, the exact location of the Nile’s source was not known.

The cosmology of the Sumerians and Babylonians mentioned a starry river mirroring the Euphrates, though it’s uncertain if this is Eridanus or the Milky Way. Those who advocate for a Mesopotamian origin for the constellation point to its name, which could hint at the Sumerian city of Eridu, and an association with Enki, the god of freshwater and wisdom. Other sources say the name Eridanus comes from a Babylonian term meaning ‘strong river’, but it’s more likely that it comes from Greek words meaning ‘early’ and ‘burnt’, a reference to the parched river in the myth of Phaethon.

Talking of which, the association of Eridanus and the Po came about later as a result of that myth, where it was the river where the unfortunate youth splashed down after his ill-fated chariot ride, a story popularised by the Romans. The Po valley was full of poplars, a tree that produces a sweet scented amber-like sap. Perhaps this is where the story of the amber tears of the nymphs comes from. The Po Valley was also an important area for the trade of amber across the Mediterranean.

The River Po in Turin, Italy. (A fascinating, highly underrated and untouristy city, I must add!)

A Whirling Star and a Cosmic Void

Now for some of the astronomy of this long and winding constellation. Eridanus’s alpha star Achernar ranks 10th on the list of brightest stars in the sky. But you need to travel to the southern hemisphere, or a latitude below 32 degrees north, to see it. I’ve seen it from the Canaries, flickering so low down in the sky that I first thought it was a plane.

The name Achernar initially makes me think of the chthonic river of Greek mythology, the Acheron. But it’s actually derived from the Arabic phrase achir-al-nahr, meaning ‘end of the river’. As a star so far south in the sky, it doesn’t feature in Greek myth or Babylonian cosmology, but it is a part of the celestial stories of peoples in the southern hemisphere. In southern Africa, the Tswana and Sotho people call Achernar Senakane, meaning Little Horn. In their calendar, the rising of this star and the setting of the Pleiades mark the cold season.

Achernar also features in the star lore of First Nations people in Australia. For the Boorong people, it is Yerredetkurrk, the owlet nightjar, and the mother in law of Totyarguil, the Lorikeet star (Altair). To the Wati of Australia’s Western Desert, the stars Canopus (the second brightest star in the sky after Sirius) and Achernar represent the fires of the Magellanic Clouds⁴, which were seen as two heroic characters.

Artistic rendition of Achernar’s unusual egg-like shape (source)

A hot blue-white star six to eight times more massive than the Sun, Achernar is one of the fastest spinning stars in the sky, rotating on its axis once every two days. This gives the star a squashed shape, more like an egg or a lozenge than a sphere. As a blue star, it is hotter than the Sun, but its speedy spin and subsequent flattening means that the star’s temperature varies, with its poles being hotter than the equator. It is 139 light years away. As of 2025, its light reaches us from 1886, the year in which Karl Benz invented the first automobile.

There are also intriguing objects to be found within the long stretch of this constellation, including many galaxies and a few exoplanets (so maybe Star Trek got something right there). The galaxy NGC 1300 is one of the most striking examples of a barred spiral galaxy, in which a straight, thick ‘bar’ of stars intersects the spiral shape. And in orbit around the star Epsilon Eridani, which is similar to the Sun and only 10.5 light years away (making it one of the nearest stars to the solar system), there is a ring of dust and a Jupiter sized planet.

The Witch Head Nebula is considered to be part of Orion, because it is illuminated by Rigel and is part of the larger Orion molecular cloud, but it is technically within Eridanus if you consider the official constellation boundaries. The Witch Head glows an ethereal blue (though some filters-see image below-show this nebula in a fabulous shade of Elphaba green!) and you can see her long nose, cackling mouth and jutting chin shining in the stardust.

Defying gravity in outer space: the Witch Head Nebula (source)

But this sorcerous cloud is nowhere near as ominous as…the Eridanus Supervoid! (dun dun duuuuun!)

It is in Eridanus, a region of the sky,

Southwest of Orion, but nobody knows why

A billion light years wide and a googleplex away

There’s a hole in the universe!

-The Destroyers, ‘Hole in the Universe’

There’s something strange in this part of space. To understand why it is so weird, I need to introduce the Cosmic Microwave Background. This is the leftover radiation from the Big Bang. When our universe inflated and expanded into existence 13 billion years ago, it was filled with heat and light, like a fireball. This hot, bright radiation cooled over millions of years as the universe grew, changing from light and heat to microwaves that fill the entire universe almost uniformly. I say almost, because space telescopes like WMAP and Planck have since found that there are slight inconsistencies in this otherwise smooth microwave background, with some spots appearing ‘warmer’ with a tiny bit more radiation, and some ‘cold spots’, where there’s less radiation. One of the largest cold spots happens to be in Eridanus. And it coincides with a region of space a billion light years across where there are hardly any galaxies, and is largely devoid of stars, gas or dust. That this empty region of our universe also happens to be in a cold part of the microwave background can’t be down to chance, but why it exists at all is a mystery. There are even theories suggesting that this void could be caused by the collision of our universe with another parallel universe!

The CMB from the Planck survey (source)

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Bringing it down to Earth

I create with the river because I am water too. My blood vessels and neurons dance and flow like rivers dance and flow through the land - the life force of our beautiful planet.

Agnes Becker

That the constellations have a river among their ranks reflects their importance to society and the natural world. But people have also attempted to tame rivers by changing their courses, straightening their meanders, building dams. And their precious water is often spoiled with sewage and agricultural runoff, microplastics and forever chemicals. Not to mention the impact of climate change on rivers worldwide, which is either drying them up, or causing them to flood more violently. There is hope though. From wild swimmers who interact with rivers most intimately by immersing themselves in their waters leading the way for change, to personhood rights being granted upon rivers-thus treating them as living beings, to looking towards the relationships Indigenous people have with their bodies of water for inspiration, there are glimmers, like starry glints on ripples of water, that our kinship with rivers can be healed.

Art is one way to reconnect. Artist and science communicator Agnes Becker (someone who I have worked with for years and has inspired me greatly with my writing and projects, and the way I approach nature as a whole) has made artwork in collaboration with her local river, the Great Ouse in Bedfordshire. By ‘being in collaboration’, it means paying loving attention to the river, the land on which it flows, the plants that grow on its banks, how the weather affects it, and how humans have impacted it. She collects mud and water from the river and uses it in paintings, and she also makes ink-based art based on the river’s movements.

More directly linked to this constellation is the work of US artist Melissa McGill. Her art is deeply inspired by water and the night sky, and her project Eridanus explores the constellation and the River Po, though a cosmic and environmental lens, blending myth and ecology, hydrology and astronomy. As her website explains: ‘Through these interventions, the technical and scientific representation of the Po River represented in archival maps, are combined with a unique combination of organic materials, connecting us once again with the environment these maps delineate…carrying both the water’s creative expression and the reflections of the stars of the Eridanus Constellation.’

If you can’t see Eridanus (you really need a dark sky for this one) I suggest you get to know your local river. It can be a celebrated and storied river, or an unassuming small stream. Preferably a natural water channel, but a canal will also do at a stretch. Visit your river, and observe. What colour is the water, is it clear or muddy, how deep is it, how fast is it flowing? What sounds does it make, or smells? How developed or altered by humans is it? What plants and animals live there? What stories or history are associated with it? Where does it start and end? Is there anything you can do to tend to and help care for the river, such as maybe writing to your council about keeping it clean and protecting its wildlife?

And Eridanus teaches us that sometimes it is best to surrender to life’s currents and let them guide the way. As Mark Westmoquette says regarding this constellation: ‘Life is like the flow of a river. It moves ceaselessly onwards, following twists and turns, meeting boulders, falling into chasms, and gliding across sunny pastures. If we try and swim against the current, the harder life will be and the more frustrating…all we can really do is let ourselves be swept along with the flow.’

Photo by Jack Anstey on Unsplash

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Further exploration

Agnes Becker: Rivers Run Through Us (this is a shop site but this is not an ad or commercial plug for her shop, but here you can see what her art looks like)

Melissa McGill: Eridanus

1. When astronomers during the 16th century onward added newer constellations to the list, another natural feature was added to the stars: the southern constellation Mensa, which depicts South Africa’s Table Mountain.

2. I have discussed this myth before, most recently in my Cygnus article.

3. There is an asteroid called Phaethon, responsible for December’s Geminid meteor shower, but it’s unconnected to this myth.

4. Two dwarf galaxies orbiting the Milky Way that can only be seen in the Southern Hemisphere. In areas without light pollution, they look like smudges in the sky.