Groovin' with the Moon and Zubenelgenubi

The half-moon will shine just above the bright double star Zubenelgenubi in Libra tonight (August 14), making for a stellar sight in binoculars. Stellarium

It's fortunate that stars have such cool names: Nunki, Aldebaran, Rigil Kentaurus, Betelgeuse just to name a few. One of the most fun to say out loud is Zubenelgenubi (zoo-BEN-el-je-NEW-bee), which also goes by the more prosaic moniker Alpha Librae. It forms part of the outline of the diamond-shaped Libra the Scales, a zodiac constellation which represents weighing scales. 

In this 16th century globe created by Gerard Mercator Libra is shown both as a balance and the scorpion's claws. Public domain

In ancient times, the Babylonians and Greeks alternatively pictured Libra as the Claws of the Scorpion, and included it as part of the neighboring constellation Scorpius. Its brightest stars still bear Arabic names that refer to the arachnid's pincer-like claws. Zubenelgenubi means "northern claw"; Zubeneschamali (zoo-BEN-ess-sha-MAH-lee) southern claw and Zubenelhakrabi (zoo-BEN-el-hak-RAH-bee) simply means "claws." 

Watch the moon pass just north of ancient Libra's "northern claw" tonight. Stellarium

On Saturday night August 14, the first quarter moon will blaze less than 1/2° above Zubenelgenubi. A half-degree is equal to the apparent diameter of a full moon. They'll be closest around 9:30 p.m. Central Time (10:30 p.m. Eastern, 8:30 p.m. Mountain) but near one another until moonset no matter where you live. Although the star is just 3rd magnitude — one brightness level fainter than the Big Dipper stars — you might still be able to see it with the naked eye even with the moon nearby.  

Why is the conjunction of a dim star and the moon of interest you ask? It so happens that Zubenelgenubi is a double star that you can "split" in any pair of binoculars. In fact, if you have really good vision you can see both stars without optical aid. The companion, called Alpha-1, shines just to the upper right (northwest) of the brighter star, dubbed Alpha-2. The two suns are physically connected in space and revolve about their common center of gravity. 

Zubenelgenubi is a naked-eye double star that's a snap to see in binoculars. AAO/STScI, WikiSky

Both stars are 77 light-years distant from the Earth and move through space together separated by about 420 billion miles, equal to about 140 times Pluto's distance from the sun. Alpha-2 is double again, but its two suns are so tightly bound that ordinary telescopes can't separate them.

Ordinarily you'd have to star-hop from Scorpius to arrive at Zubenelgenubi. Tonight you can let the moon do the walking for you. What a pretty sight this trio will be!