Eridanus was born from Oceanus and Tethys, the eldest divine couple whose union produced the rivers, springs and waters of the world. Oceanus was the great river imagined to flow around the edges of the earth, while Tethys was the nourishing mother of its streams. Their children were the Potamoi, the river gods who took the forms of powerful men with bull-like horns, flowing hair and water pouring from their jars. Eridanus was one of this vast family, but his waters lay far beyond the familiar rivers of Greece. Ancient writers placed him among the Hyperboreans, a blessed people who lived beyond the north wind and received Apollo's visit during the winter months. Beyond their land lay the northern ocean, imagined as the source of wonders denied to the Mediterranean world: tin, gold, fertile abundance and amber.

The earliest descriptions made Eridanus less a fixed river than a gateway to the unknown. Travellers' tales said that its current reached a sea surrounding Europe and led towards islands rich in amber and tin. Herodotus heard the story and rejected it as a Greek poetic invention, doubting that any real barbarian river carried those names. Yet he accepted the larger truth that amber and tin came from the remote ends of the inhabited world. The river existed on the border between geography and myth. It could be beyond the north wind in one telling, among the Celts in another, or near the setting sun where Heracles would later search for the Garden of the Hesperides.

Eridanus entered his most famous myth through the ambition of Phaethon. The boy was the son of Helios, god of the sun, and the Oceanid nymph Clymene. Taunted over his parentage, Phaethon travelled to the palace of his father and begged for proof. Helios swore an oath by the Styx, the river of unbreakable promises, and granted any request. Phaethon asked to drive the sun chariot for one day. Helios warned him that even Zeus could not guide the fiery horses along the narrow path between heaven and earth, but the oath was already spoken. At dawn Phaethon mounted the blazing chariot and took the reins.

The horses sensed the weakness of their driver. They bolted from the accustomed road, climbing too high and scorching the constellations, then plunging too low and burning the earth. Ovid's account makes the whole world cry out: cities and forests blaze, mountains split, rivers shrink in their beds, Libya is scorched into desert and the darkened skin of the Ethiopians is explained as the mark of that fire. Earth herself calls to Zeus for help. To save the cosmos, the king of the gods hurls a thunderbolt. Phaethon falls like a shooting star, hair and breast still smouldering, and lands in the remote waters of Eridanus.

The river god receives the body. Far from home, Phaethon is buried by the Hesperian Nymphs, the western water maidens who cared for Eridanus, and an epitaph is carved upon his stone. In another telling, the body lies unburied in a deep lake at the river's outfall, still smouldering beneath the water. Birds that attempt to fly across the vapour fall from the air. The lake breathes steam day and night, and the Argonauts, sailing the Argo through the far northern waters during the voyage of Jason, hear the hissing wound and smell the stench of the divine fire. The river becomes both tomb and memorial.

Phosphorescent grief soon surrounds the water. The Heliades, daughters of Helios and sisters of Phaethon, come to the banks and weep without end. Their feet take root in the earth, bark climbs their bodies, and their arms stretch into branches until they become poplar trees. Their mother Clymene tears at the bark and finds living flesh beneath, but the transformation cannot be reversed. The sisters continue to weep through their leaves, and their tears harden in the sun into drops of amber. Eridanus catches the golden drops and carries them downstream. The river god's grief thus becomes a source of wealth: amber, called electron in Greek, a stone of light born beside the water that extinguished the son of the sun.

Other mourners gather at the river. Cycnus, king of Liguria and a devoted companion or kinsman of Phaethon, cries until the gods take pity and transform him into a swan. The bird's song becomes the voice of lament. The people living near the river are said to wear black in mourning for Phaethon. Even Eridanus himself is personified in tears, a river god bowed over his water as he receives the body and gathers the amber tears of the poplars. In Nonnus' late epic, he brings shining gifts of amber to the wedding of Poseidon and Beroe. The god who swallowed the fire now delivers beauty from it.

The location of Eridanus never stopped moving. Early tradition placed him beside the Hyperboreans in the north, close to the sources of amber. Aeschylus transferred the fall of Phaethon to Iberia and was understood to identify the Eridanus with the Rhone. Other writers set the river near the Po, the broad northern Italian waterway whose mouth opens into the Adriatic and whose valley stood near the end of amber trade routes reaching from the Baltic world. Still others identified Eridanus with the Danube, called the Ister by the Greeks, or left him in an imagined ocean beyond the known lands. The myth followed the amber. Wherever the precious golden tears of the Heliades might have travelled, the river could be found.

Geographers attacked the uncertainty and the poets defended it. Strabo dismissed the tales of Phaethon, the amber islands and the poplar sisters near the Po as inventions, noting that no such features existed along that real river. Yet Pliny and later mythographers still called the Po by the name Eridanus, preserving the mythical identity inside geography. The real river gave the myth a body, while the myth gave the real river a tragic history. Every attempt to pin Eridanus to one map revealed how fully he belonged to all of them: a northern stream, a western boundary, a trade road and a watery road to the Underworld.

Roman poets widened his journey still further. Virgil calls Eridanus the king of rivers and places him among the rivers flowing through the Underworld. There he joins Styx, Acheron and Cocytus, the mournful waters of the dead, as if the river that received Phaethon had always possessed one foot in the world below. His transformation into a chthonic stream was natural. He was a boundary between fire and water, life and death, the earth that burned and the darkness beneath it. The river god who received the sun god's son had become a passage from the upper world into the hidden one.

The sky offered a final home. Aratus, in his Hellenistic poem on the constellations, calls the starry river the River of Many Tears and links its winding course to the tragedy of Phaethon. The constellation begins near Orion, close to the hunter's foot, then curls southward past Cetus and disappears below the horizon before re-emerging in southern skies. Some early astronomers simply called it Potamos, the River. Eratosthenes and Hyginus connected it with the Nile, whose unknown southern source made it another river flowing from mystery. Ptolemy included it among the forty-eight constellations of the ancient sky. Renaissance star charts later extended it to Achernar, a brilliant southern star whose name means the end of the river.

The celestial Eridanus preserves the shape of the catastrophe. It bends and loops rather than following a direct path, like the track Phaethon cut when he lost control of the sun chariot. It starts beside Orion and then descends into the dark, echoing the flaming fall from heaven into water. Its stars are cold where the myth is hot, remote where the grief is intimate, yet the river remains recognisable. Every age remapped its earthly course but kept the same image above: a stream of stars that begins in tragedy and ends in darkness, carrying amber tears through heaven as its waters once carried them through the world.

Eridanus endured because he held so many forms at once. He was a god, a river, a tomb, an amber road, an entrance to the Underworld and a constellation. His waters cooled the fire of Phaethon but could not cool the grief of the Heliades. They turned human sorrow into a precious substance and carried it from the edges of myth into the markets, jewellery boxes and temples of Greece and Rome. His uncertainty became his power. No single river could contain him, so the entire cosmos did.