How to see it
Zubenelgenubi, the alpha star of the constellation Libra the Scales, is a touch fainter than its fellow Libra star, Zubeneschamali. Nonetheless, Zubenelgenubi enjoys the alpha designation, probably because of its proximity to the ecliptic – the sun’s annual path in front of the backdrop stars. Annually, the sun and Zubenelgenubi are in conjunction on or near November 7. Shortly after Halloween, the star rises and sets with the sun, and can’t be seen at all. Zubenelgenubi maintains a low profile throughout October and November, because it’s too close to the sun’s glare to be visible.
One half year later – in early May – this star stands opposite the sun in Earth’s sky. Shortly after May Day, Zubenelgenubi rises around sunset, stays up all night, then sets around sunrise. In early May, this star transits – soars to its highest spot in the southern sky – around midnight (1:00 a.m. daylight saving time). Because Zubenelgenubi returns to the same spot in the sky 4 minutes earlier daily (or 2 hours earlier monthly) this star transits due south around 10:00 p.m. (11:00 p.m. daylight saving time) in early June, and around 8:00 p.m. (9:00 p.m. daylight saving time) in early July.
Summer evenings present a good time for viewing this star, because it’s high up at a convenient time of night. Zubenelgenubi, though a rather faint star, is easily visible in a dark country sky. Moreover, it is fairly easy to locate, because it sits midway between two brilliant stars: Antares of the constellation Scorpius, and Spica of the constellation Virgo. Zubenelgenubi shines to the west (right) of ruddy Antares, and to the east (left) of blue-white Spica.
History and Myth
The names of Libra’s two brightest stars are derived from Arabic. Zubenelgenubi means “the Southern Claw (of the Scorpion)” and Zubeneschamali means the “the Northern Claw.” These names hark back to the times of the ancient Babylonians, who saw these Libra stars as part of the constellation Scorpius the Scorpion.
Apparently, the Greeks and Romans separated this part of Scorpius into the constellation Libra the Scales, because the sun shone in front of this constellation on the autumn equinox. The balance symbolizes the equal lengths of the day and night that come with the equinox. Libra marked the position of the autumn equinox well over 2,000 years ago. At present, the sun shines in front of the constellation Virgo on the autumn equinox.
According the Greek mythology, Virgo represents Astrea, the goddess of justice, holding Libra the Scales. Richard Hinkley Allen, in his classic work Star Names, says Libra in Roman eyes may have been the deification of Augustus as the arbiter of justice.
Look at Zubenelgenubi through binoculars and you’ll see that it’s a double star. Astronomers have studied the motions of Zubenelgenubi’s two stars, thinking that it’s probably a binary – two physically related stars orbiting a common center of mass. However, the rather wide separation between these two stars must mean a long oribtial period of perhaps 200,000 years. Zubenelgenubi, a star more intrinsically luminous than our sun, resides some 77 light-years away.
Zubenelgenubi’s position is at RA: 14h 51.4m, dec: -16° 5′