Follow the river

For winter we are turning our attention to the long and winding constellation Eridanus (also known as the river) which extends northwards from Achernar, deep in the southern hemisphere to a point a little way to the north west of the bright star Rigel in Orion. Although portions of Eridanus are visible from anywhere in the world, this long and winding constellation can be seen in its entirety from most of Mexico and India, north Africa and latitudes to the south of these.

Eridanus chart

As its name suggests, the constellation Eridanus depicts a river, although which river it resembles is open to debate. The Greek astronomer Eratosthenes took it to represent the River Nile, although the Akkadians identified it as being the River Euphrates. The River Eridanus features in the legend attached to Phaeton. Phaeton persuaded his father, the god of the sun, to let him drive the chariot of the sun across the sky but was unable to control the steeds pulling the chariot. To avert a catastrophe, Zeus hurled a thunderbolt at Phaeton, striking him down and hurling him into the River Eridanus far below.

We will commence our journey along Eridanus by working our way southwards from the magnitude 2.78 star Cursa (click here for more about magnitudes) which marks the river’s northern extremity. The light from Cursa set off on its journey towards us around 90 years ago and from here we can identify the nearby brilliant star Rigel, located just to its south east. Rigel is one of the leading stars in the constellation Orion which, as you can see from the accompanying chart, is easily the most prominent of the constellations that grace the night sky at this time of year.

Eridanus

The brightest star in Eridanus is Achernar which, at magnitude 0.45, shines from a distance of about 140 light years. The name of this star is derived from the Arabic akhir al-nahr meaning ‘the river’s end’, which is fitting in view of its location at the southern end of the constellation (click here for more about star names). Just as aptly named is the magnitude 2.97 red giant Zaurak, located near the northern end of Eridanus – its name means ‘boat’. The light from Zaurak set off towards us around 200 years ago.

Of particular interest is the magnitude 3.72 Epsilon Eridani which, shining from a distance of just 10.5 light years, is one of our closest stellar neighbours. Immediately to the west of Epsilon is Zibal, a magnitude 4.80 white giant star whose light has taken 110 years to reach our planet. Slightly to the west again is Azha, a magnitude 3.89 orange giant star shining from a distance of 135 light years (click here for more about star colours).

Moving further south, and navigating around the northern bend of the river Eridanus, we eventually arrive at the pair of stars Upsilon1 and Upsilon2 Eridani. Shining at magnitude 3.81 from a distance of 214 light years, the yellow giant Upsilon2 is marginally brighter than its magnitude 4.49 orange giant neighbour Upsilon1, the light from which set off on its journey towards us around 125 years ago. Upsilon1 and Upsilon2 form just one of several wide pairs of stars scattered along the entire length of the constellation.

Northern Winter Southern hemisphere

Acamar (whose name is also derived from the Arabic akhir al-nahr) marks the original end of the river Eridanus, the stars south of this point being inaccessible to the astronomers of Ancient Greece who first devised this constellation. It wasn’t until explorers ventured into the southern hemisphere that additional stars were seen, this resulting in the original constellation Eridanus being extended to the south and to a new termination point marked by the bright star Achernar. Located at a distance of 160 light years, Acamar is widely regarded as being one of the finest double stars in the southern sky, its magnitude 3.4 and 4.4 components easily resolvable in a small telescope.

If you are fortunate enough to be located anywhere to the south of latitude 30°N the whole of the constellation Eridanus will be visible, and you can use your imagination to navigate northwards from Achernar along the full course of the heavenly river, eventually to arrive at Cursa, where our journey began. Happy stargazing! 

Magazine of the Ramblers