Libra
Libra is the only one of the 12 zodiac constellations that represents an object, a set of scales, rather than a person or animal. Look for it in the southern sky on early summer evenings, between Virgo and Scorpius.
| Latin name | Libra |
|---|---|
| Abbreviation | Lib |
| Brightest star | Zubeneschamali (mag 2.6) |
| Best seen | Early summer evenings (Northern Hemisphere) |
| Astrological dates | September 23 – October 22 |
| Sun actually passes | October 31 – November 23 |
| Stars used by Planetica | 1,228 |
How to find it
Libra is visible in the evening sky from May to July and climbs highest in the south in the early evening in June. Even then it doesn't get very high, so it's best viewed from a spot with an open southern horizon.
With no bright stars, it's easier to find using its neighbors than on its own. Locate Spica in Virgo and the red star Antares in Scorpius, then look roughly halfway between them. Four stars of 2nd–3rd magnitude form a lopsided quadrilateral, or something like a triangle pointing upward. The top corner is Zubeneschamali, and the one to its lower right is Zubenelgenubi. Picture these stars as the beam and two pans of a balance.
If you have binoculars, aim at Zubenelgenubi. You'll see a fainter star sitting close to the bright one. It's a wide double star that people with sharp eyes can even split with the naked eye.
The myth: the scales of justice
Libra is usually seen as the scales held by the goddess of justice. In the story that identifies neighboring Virgo as Astraea, goddess of justice, these scales are the tool she uses to weigh right and wrong fairly. It's the same idea behind the scales that still appear in images symbolizing courts and the law.
But in very early Greece, these stars weren't a separate constellation at all. They were thought of as the great claws of the scorpion next door. Traces of this survive in the star names: Zubenelgenubi means "the southern claw" in Arabic, and Zubeneschamali means "the northern claw." Libra became established as its own constellation, the Scales, around Roman times.
The scales also have a seasonal connection. About 2,000 years ago, the Sun was near this constellation at the September equinox, when day and night are nearly equal in length, a natural fit for a symbol of balance. In the East Asian 28 lunar mansions, the stars of Libra belong to the Root mansion (氐宿).
Main stars
| Name | Apparent magnitude | Distance (approx., ly) | Notes |
|---|---|---|---|
| Zubeneschamali (Beta) | 2.6 | ~185 | The brightest star in Libra, blue-white. Its name means "the northern claw." |
| Zubenelgenubi (Alpha²) | 2.75 | ~77 | A wide double star; binoculars show it together with its 5th-magnitude companion (Alpha¹). |
| Sigma Librae | 3.3 | ~290 | A red giant that was once classified as a star of Scorpius. |
| Zubenelhakrabi (Gamma) | 3.9 | ~150 | An orange giant on the eastern pan of the scales. |
Libra has almost no bright clusters or nebulae worth viewing with the naked eye or binoculars. On the other hand, several of its stars are known to have planets, so it comes up often in exoplanet research.
Birthday sign dates: two different answers
In astrology, Libra runs from September 23 to October 22. But during that period, the real Sun is passing through Virgo. The Sun is actually within the Libra constellation from about October 31 to November 23, about 24 days.
The first reason is precession. Earth's axis slowly traces a circle, about once every 26,000 years, so the equinox points move along the ecliptic by about 1 degree every 72 years. The astrological signs divide the ecliptic into 30-degree slices from the vernal equinox point, based on the reference of about 2,000 years ago. So even though the sign is still called Libra, today the Sun is in Virgo at the September equinox.
The second reason is the constellation boundaries set by the International Astronomical Union (IAU) in 1930. Real constellations vary in size, so the Sun doesn't spend equal time in each. The dates can vary by about a day from year to year.
In Planetica
If your birthday's astrological sign is Libra, Planetica places your planetary system next to a real star inside the Libra constellation. The candidates are the 1,228 stars in the HYG catalog that lie within about 3,000 light-years of Earth and inside the IAU boundaries of Libra.
One of them is chosen at random, and your system is placed about 1–6 light-years from it. "Twin systems" from people with the same name and birthday all sit in Libra, but, like the two pans of a balance, each sits next to a different star.
Planetica also shows which constellation the Sun was really in on your birthday. For example, someone born on October 10 is a Libra in astrology, but on that day the Sun was actually in Virgo.
Fun facts
- There's a long-standing claim that Zubeneschamali looks greenish, but it's actually a blue-white star.
- Gliese 581, a red dwarf about 20 light-years away known to have several planets, is in Libra.
Other constellations
Create your own planetary system
Enter a name and a birthday to get a one-of-a-kind planetary system, placed next to a real star in your birthday constellation.