River constellation takes meandering route

In 150 CE, the Alexandrian astronomer Ptolemy wrote the Almagest, a work that was to have an ongoing influence on astronomy for 1,400 years.

Among its many lasting accomplishments, the Almagest lists 48 constellations still recognized as canonical nearly two millennia later. Of Ptolemy’s constellations, Eridanus, the River, has the strangest and most circuitous history.

The classical scholars of the 19th and early 20th centuries trace, without much evidence, the constellation’s origin to the fertile-crescent region centered on the Tigris and Euphrates rivers. The early civilizations in the region certainly had river gods, but the connection to the constellation is tenuous at best.

The first mention of Eridanus is by the Greek poet Hesiod in his Theogony, composed sometime between 730 and 700 BCE. Hesiod’s Eridanus is a river god, the son of the Titans Oceanus and his wife — and sister — Tethys.

Later Greek and Roman constellational commentators believed the constellation Eridanus represented an actual river on Earth. Chief among those writers is the poet Aratus from the third century BCE.

However, they couldn’t agree on what river it was or where it was located. According to classicist Theony Condos, some commentators position it in Southern Europe and some in Western Europe. Some doubt the existence of the earthbound version of the river entirely.

Hyginus, a Roman writer in the first century BCE, identifies Eridanus as the Nile River. First-century CE writer Pseudo-Eratosthenes echoes that identification in his Catasterisms.

Most Roman commentators think Eridanus represents the Po River, Italy’s longest and most storied river.

Like many earthly rivers, Ptolemy’s Eridanus takes a somewhat meandering route.

Even Ptolemy couldn’t decide where it began. In his description of the constellation Orion, he states that Eridanus and Orion share the star Rigel, Orion’s left foot.

However, Ptolemy’s description of Eridanus claims that the river begins with “the star after the one in the foot of Orion.” We now call that faint star Lambda Eridani, just up and to the right of much brighter Rigel.

From there, the constellation moves east and then south. It then moves to the WSW and abruptly to the southeast. The Ptolemaic constellation ends at the star Theta Eridani, which Arabic astronomers called Achernar, the “river’s end.”

The ancient constellation looked roughly like a backward “S.” In modern times, the reversed “S” developed a tail. Astronomical cartographers added a stream of stars that plunged south toward the southern celestial pole.

Eridanus now terminates at the constellation’s brightest star Alpha Eridani, which stargazers today call Achernar since it is the actual end of the modern river.

But another star already had that name. So the name of the original Achernar became Acamar, which has the same Arabic root as Achernar. Thus, one extraordinary aspect of Eridanus is its two termini, one ancient and one modern.

Ptolemy could not have seen the modern constellation, of course. Alexandria, where the ancient astronomer observed the stars, is at 31 degrees north latitude. The current constellation stretches down to Achernar at 57 degrees south latitude. Ptolemy was too far north to see several of the constellation’s stars far below the celestial equator.

The modern constellation was first identified on a celestial globe created by Dutch cartographer Petrus Plancius in 1598.

No one knows for sure from whom Plancius got his information. By the end of the 16th century, many explorers were venturing below the equator by ship. Ian Ridpath’s masterwork on the constellations suggests two possible sources.

One is English explorer Robert Hues, who circumnavigated the globe from 1591-1592. While doing so, he spent long hours studying the southern sky.

According to Ridpath, “Hues wrote of seeing three first-magnitude southern stars that are never visible from England, one of them ‘in the end of Eridanus’; this can only have been the present-day Achernar.”

Most probably, Plancius got the idea from the Dutch navigator Pieter Dirkszoon Keyser, who made his first voyage to the Dutch East Indies from 1595 to 1597. According to encyclopedia.com, Plancius instructed Keyser in the art of celestial navigation. He also asked him to measure the position of any new stars he saw during his trip south.

In any case, the five stars of the southern extension, including the new “end of the river,” were copied by Johannes Bayer in his influential star catalog and map Uranometria. The extended constellation has been canonical ever since.

The heavenly river plays a part in one of the most catastrophic tales in Greco-Roman mythology, the ill-fated chariot ride of Phaethon.

In the myth, the universal predominance of the gods began with an apocalyptic war between the gods and the Titans, the race of supernatural creatures that preceded them.

The gods were triumphant. They cast many of their Titanic enemies into the outer darkness.

They gave other Titans onerous tasks. They burdened Helios with the sweaty task of carrying the sun across the sky in a horse-drawn chariot.

Carrying the hot, flaming rock was the least of his worries. The powerful horses took all his Titanic strength to keep under control, and he had to repeat the task every day for all eternity.

Presumably, he took a long break at night. To him and the sea nymph Cymene was born Phaethon, a son.

Phaethon’s playmates teased him relentlessly. They did not believe that his father was the solar charioteer.

He begged his father for some proof, and his father foolishly promised him anything he wanted. The boy insisted on driving the chariot for a day, but he could not control the solar steeds.

His calamitous ride is described artfully in Book II of the Roman poet Ovid’s first-century CE work, the Metamorphoses.

As he careened around the sky, he passed so close to the Earth that parts of the planet caught fire. Verdant northern Africa became the Sahara Desert. Rising up, Phaethon dislodged some of the stars. The Milky Way is the burn mark left by one of the dislodged stars or even the sun itself.

Eventually, Zeus zapped him with a thunderbolt, and his smoldering corpse, still hot from his proximity to the sun, fell into the river Eridanus.

As Ovid puts it,

“At once from life and from the chariot driven,

The ambitious boy fell thunderstruck from heaven.”

In Ovid’s version of the story, the body, still aflame, was found by his sisters, the Heliades, and they buried him.

The Heliades mourned him with pitiful wailing so great that Jupiter, the king of the gods, took note. Out of pity, he turned them into a stand of poplar trees beside the river.

Their conversion to trees was painfully slow. Their human tears transform into drops of amber. Their precious amber tears fall from the trees into the river. Perhaps we see them as the stars of the constellation Eridanus:

“As bark closed over lips, their tears still ran,

Tears that were drops of amber in the sun

Fallen from green sides and branches of young trees,

To flow in the clearest waters of the river

And later worn as jewels by Roman brides.

And to this day, many poplars stand in sad silence in the Po River Valley

All the long nights their mournful watch they keep,

And all the day stand round the tomb and weep.”

The tale has many variants. In the earliest Greek account of Eridanus, the Argonauts sail up the much-diminished river during their quest for the Golden Fleece. They discover poor Phaethon’s still-smoking corpse and bury him.

Most of the River’s stars are faint, a fact that Aratus sought to justify mythologically. Phaethon’s fiery, Titanic frame caused the river to boil and evaporate. He describes the “poor remains” of the river encountered by the Argonauts.

My favorite version comes to us again from the ancient Greeks.

As Phaethon reached manhood, he finally discovered his father’s identity. With his friend Cycnus, he sought out Helios and demanded to drive the chariot the god used to carry the sun across the sky.

When Helios finally relented, the horses that pulled the chariot immediately sensed Phaethon’s incompetent hand on the reins. Up and down they plunged, creating death and devastation wherever he went.

To end the destruction, Zeus hurled his deadly thunderbolt, and Phaethon plummeted into Eridanus. Cycnus dove into the River but could not find his friend. The gods changed Cycnus into a swan so that he might continue his search.

Phaethon died because he tried to take the place of a god. Cycnus looks for his friend to this very day. He sails as the constellation Cygnus over a shimmering river of stars we call the Milky Way.


By Tom Burns


Tom Burns is the former director of the Perkins Observatory in Delaware.