Use binoculars to peer at Zubenelgenubi – otherwise known as Alpha Librae – 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 orbital period of perhaps 200,000 years. That suggests these two stars may not be bound by gravity, after all. Zubenelgenubi is more intrinsically luminous than our sun. It resides some 77 light-years away.
How to see Zubenelgenubi. Shortly after Halloween each year, Zubenelgenubi rises and sets with the sun, and can’t be seen at all. Annually, the sun and this star are in conjunction on or near November 7.
Half a year later, by early May of each year when this star stands opposite the sun in Earth’s sky, the best time to view Zubenelgenubi has arrived. 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 for all observers around the globe (1 a.m. local daylight saving time). Because this star (and all stars) returns to the same spot in the sky 4 minutes earlier daily (or 2 hours earlier monthly), Zubenelgenubi transits due south around 10 p.m. (11 p.m. local daylight saving time) in early June, and earlier still in July and August.
That’s why Northern Hemisphere summer (or Southern Hemisphere winter) evenings present a good time for viewing this star. During these months, it’s high up at a convenient time of night. Zubenelgenubi, though a rather faint star, is easily visible in a dark country sky. It is fairly easy to locate near its fellow star in Libra, Zubeneschamali.
Zubenelgenubi is a touch fainter than Zubeneschamali. Nonetheless, Zubenelgenubi enjoys the alpha designation in the constellation Libra the Scales, probably because of its proximity to the ecliptic – the path of the sun, moon and planets in our sky.
Zubenelgenubi sits midway between two brilliant stars in other constellations. It’s between Antares in 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 mythology of Zubenelgenubi. 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 to 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.
Zubenelgenubi’s position is at RA: 14h 51.4m, dec: -16° 5′
Bottom line: Learn to recognize Zubenelgenubi, the constellation Libra’s alpha star.
Bruce McClure has served as lead writer for EarthSky's popular Tonight pages since 2004. He's a sundial aficionado, whose love for the heavens has taken him to Lake Titicaca in Bolivia and sailing in the North Atlantic, where he earned his celestial navigation certificate through the School of Ocean Sailing and Navigation. He also writes and hosts public astronomy programs and planetarium programs in and around his home in upstate New York.