Libra

The Scales between the Maiden and the Scorpion

Libra, the Scales, fills the quiet stretch of sky between Virgo and Scorpius. It is a modest pattern: no star in it is brighter than second magnitude, so from a bright city you may pick out only two or three of its stars. Under a dark sky it becomes a small, tilted box of stars.

Libra lies on the ecliptic, the line the Sun appears to follow against the background stars as Earth orbits it each year. The Sun passes in front of Libra from about October 30 to November 22, so in late autumn the constellation is hidden in daylight. Around November 7 the Sun slides almost directly past Zubenelgenubi, which sits very close to the ecliptic.

Why scales? Between roughly 2300 and 700 BC, the September equinox point, where the Sun crosses the celestial equator heading south, lay inside Libra's modern borders. On that date day and night are about equal in length, a natural match for a balance. Earth's axis slowly wobbles like a spinning top, an effect called precession, and the equinox point has since drifted into Virgo. The name 'First Point of Libra' has stayed, and astrological sign dates still follow that old equinox convention rather than the Sun's actual position among the stars today.

How to find it

Use two bright neighbors as signposts: blue-white Spica in Virgo to the west and red Antares in Scorpius to the southeast. Libra's main stars lie between them, closer to Antares. Zubeneschamali is about two fist-widths, measured at arm's length, northwest of Antares.

From about 40°N, Libra is best placed on June and July evenings, fairly low in the southern sky. It is highest at midnight around mid-May, so late spring nights work too if you stay up. Southern observers have a much better view: from southern Australia or New Zealand, Zubeneschamali can climb to about 55–60 degrees above the horizon.

The Moon passes through Libra every month and planets cross it too, often slipping between the two claw stars like travelers through a gateway.

The main stars and their names

Zubeneschamali (Beta Librae) is the brightest star, at magnitude about 2.6 and about 185 light-years away. Its Arabic name means 'the northern claw'. It is a hot B-type star with a surface temperature of about 21,700 °F (12,300 K), roughly 130 times as luminous as the Sun. Some ancient writers, including Eratosthenes and Ptolemy, described it as brighter than it looks now, and there is no accepted explanation.

This star is famous for its color debate. The stargazing writer William Tyler Olcott called it the only green star visible to the unaided eye. Astronomers class it as blue-white, and green stars are unlikely for a simple reason: green light sits in a narrow band between blue and yellow, so a star that shines strongly in green also shines in its neighbors, and the mix looks white. Look for yourself and decide.

Zubenelgenubi (Alpha Librae), 'the southern claw', is only slightly fainter, at about magnitude 2.7, and much closer, at about 75 light-years. Binoculars show a white magnitude 5.2 companion beside the blue-white main star. The two are about 5,400 astronomical units apart (one astronomical unit is the Earth–Sun distance), and they may not be bound to each other by gravity. Gamma Librae, an orange star of magnitude 3.9, completes the main figure.

Beyond the bright stars

Libra has few showpiece deep-sky objects. The best is NGC 5897, a globular cluster, a dense ball of very old stars. It shines at about magnitude 8.5, but its stars are loosely packed and it lies about 50,000 light-years away, so in a small telescope it looks like a faint, round glow.

Gliese 581, a dim star about 20 light-years away inside Libra, is far too faint for the naked eye but well known to astronomers because it hosts a system of planets.

From claws to scales

Around 2000 BC Babylonian sky watchers already linked these stars with a pair of scales, perhaps because the Sun stood here at the autumn equinox. The Greeks preferred another picture. The poet Aratus, in the third century BC, described them as the claws of the Scorpion, and the Arabic star names still remember that view.

The Romans settled on the scales again, often as the balance of justice held by Astraea, the goddess linked with neighboring Virgo. In Turkish the constellation is called Terazi, which simply means a pair of scales.

Practical observing tips

Go out on a June or July evening, about an hour after full darkness, and find Antares first. Then move northwest to the two claw stars. Compare their colors side by side; this is the easiest way to judge whether Zubeneschamali looks green to you.

Brace binoculars on a post or tripod and split Zubenelgenubi into two stars. Because Libra is low for northern observers, choose a site with an open southern horizon, away from the glow of a town. From late October to late November Libra is lost in the Sun's glare.