Rays of aurora punctuate a green arc near the northern horizon last night. They were spawned by particles streaming from a coronal hole in the sun’s outer atmosphere aimed at Earth. Chances for minor activity continues through tonight and tomorrw. Details: 35mm lens at f/2.8, ISO 800 and 25-second exposure. Photo: Bob King
During a twilight walk yesterday with the dog I thought I saw a patch of those night-loving noctilucent clouds in the direction of sunset. Upon returning home, I drove out to an wide-open field hoping to catch a glimpse of them but encountered a modest show of northern lights instead. The time was about 10:45 p.m. Lots of stars were out but twilight still lingered along the northern horizon. Several auroral rays, faint as distant smoke, materialized in the northeast before melting away five minutes later. Hoping for the display to strengthen, I drove further north to even darker skies, but sadly, the show was already over by 11:15. Curious how fickle the aurora is — such is the nature of subatomic particles and our moody magnetosphere.
As Earth blazes at 18 1/2 miles per second in its orbit around the sun, constellations slowly drift westward over the weeks and months. You’ve probably noticed Orion’s gone, and he’ll soon by followed by Gemini. Remember how Leo the Lion once dominated the southern sky at nightfall? Even he’s padding off in search of new prey. Virgo is now due south during late evening twilight and accompanied by the brilliant pink-orange Arcturus. Once darkness takes hold, another constellation pipes up in the south-southeast hoping to catch our attention, Libra the Scales or Balance.
To find Libra, go out between about 10:15 and midnight and face south. First get oriented with the huge triangle composed of the bright stars Arcturus and Spica and the planet Saturn. Alpha Librae is two outstretched fists to the left of Spica. Maps created with Stellarium
Back in 2000+B.C. the Sumerians saw this unassuming group of stars as a balance (right) because the sun crossed into Libra during the autumnal equinox when the hours of day "balanced" those of night. The Greeks didn’t see a separate constellation here at all. To them Libra’s stars comprised the claws of the nearby Scorpius the Scorpion. We owe it to the Romans who restored the balance to the night sky giving us today’s Libra, the newest constellation of the zodiac. Libra is also the only zodiac constellation that doesn’t represent an animal.
Don’t expect to be wowed by this diamond-shaped pattern. It has neither 1st nor 2nd magnitude stars. The brightest are Alpha and Beta Librae which outline the right side (western half) of the constellation. They’re 3rd magnitude or one level below the Big Dipper stars’ brightness. You’ll find them easily enough two outstretched fists to the left of Spica in Virgo. The other stars in the diamond are fainter yet and require some persistence and fairly dark skies.
The westernmost star in Libra is a double star called Alpha-1 and Alpha-2. Oddly, Alpha-2 is the brighter star, the one you see with your naked eye. The 5th magnitude companion sits just to its upper right (northwest). If you’re vision is extremely good, you may be able to split this pair with nothing more than your eyes. Give it a try, and if that doesn’t work, binoculars will easily cleave the two Alphas. Alpha Librae’s proper name is Zubenelgenubi (zu-BEN-el-ge-NEW-bee), Arabic for "southern claw" and named back in the day when Libra’s stars belonged to Scorpius. Beta Libra has an equally mellifluous moniker: Zubeneschamali (zu-BEN-ess-sha-MAH-lee) or "northern claw". Say them outloud when you’re out tonight in search of Libra and you’ll feel like a stargazer of old.