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The river of the night: going with the flow on January evenings means a trip down Eridanus, the River.

THE CONSTELLATIONS VARY not only in their size (giant Hydra versus tiny Crux) and prominence (bright Scorpius versus dim Octans), but also in their lore and meaning. Two of the sky's 88 constellations stand out from their retinue because they are the only ones that represent natural but non-living features. I touched on one of them briefly in last November's column--Mensa (the Table), named after Table Mountain in South Africa. Nicolas-Louis de Lacaille observed from Cape Town in the mid-18th century, and the spectacularly picturesque landform was very much in his sights as he explored the southern sky.

The second constellation is Eridanus, the River. Its origins lay in antiquity; some legends link it to actual rivers such as the Euphrates or the Nile, others to rivers found in mythology. Among all the constellations, Eridanus is the sixth largest, but it has by far the greatest north-south extent, flowing from just north of the celestial equator all the way to nearly 60[degrees] south declination.

Achernar, Alpha ([alpha]) Eridani, is at the southernmost tip of Eridanus. At magnitude 0.4, Achernar (140 light-years from us) is the ninth-brightest star in the night sky. It's also one of five stars of 1st magnitude or brighter that are circumpolar for observers at latitude 33[degrees] south. The other four are Alpha and Beta ([beta]) Centauri and Alpha and Beta Crucis.

Achernar's name originates from the Arabic for "the end of the river," but it wasn't always so. Only in modern times was Eridanus extended southward to allow the river to end at Achernar. Before Europeans ventured regularly to the Southern Hemisphere, the river's flow dried up at Acamar, Theta ([theta]) Eridani, some 17[degrees] farther north.

Acamar is a 2.9-magnitude white star, the brightest that lies between Achernar and Beta Eridani. However, with just a small telescope, Acamar transforms into two white stars, of magnitude 3.2 and 4.4, separated by just 8.3 arcseconds. Noted Australian observer and author E. J. Hartung described Acamar as "a brilliant white pair, one of the gems of the southern sky." Some 160 light-years distant, the star was marked as 1st magnitude by the ancient Greek astronomer Ptolemy. Was this just a mistake, a gremlin of the star's low altitude as seen by Ptolemy, or some unusual chapter in this binary's history?

As we make our way upriver, 3.0-magnitude Gamma ([gamma]) Eridani is the next bright stellar buoy we'll anchor at. We're not stopping here for the star itself, also known by the name Zaurak (Arabic for "boat"), but rather because it draws our attention to two nearby sights. The first is Epsilon ([epsilon]) Eridani, the westernmost of a small triangle of stars to the northwest of Gamma. Despite being only magnitude 3.7, Epsilon Eridani is the 13th-closest star to our solar system, lying 10.5 light-years away, and the closest star known to host a planet.

The other interesting object near Gamma is [Omicron.sup.2] ([o.sup.2]) Eridani to the north-northeast. At first glance, [Omicron.sup.2] is one of the fainter Bayer-labeled stars in Eridanus (magnitude 4.4). However, it too is nearby (16 light-years), making it the eighth-closest naked-eye star. The 4th-magnitude yellow star has two companions--a 9th-magnitude white dwarf and an 11th-magnitude red dwarf. What a combination!

As we continue to navigate eastward to Beta Eridani, we reach the location where astrophotographers and advanced observers with large telescopes seek out IC 2118, the Witch Head Nebula, which lies about 2 1/2 [degrees] west of Rigel in Orion's western foot. The nebula's glow is due to reflected light from Rigel. It seems appropriate to have a reflection nebula in a river constellation, and for the head of the river to have a namesake nebula.

We've now reached the river's headwaters. Whereas Achernar is circumpolar and thus visible year-round for observers south of latitude 33[degrees], we're now at the foot of equatorial Orion, just north of the zenith--a sign that midsummer in the Southern Hemisphere has finally arrived.

Greg Bryant is the editor of Australian Sky & Telescope. He can be reached in Sydney at greg@austskyandtel.com.au.
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Author:Bryant, Greg
Publication:Sky & Telescope
Geographic Code:1USA
Date:Jan 1, 2007
Words:693
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