Constellations: Libra 'the Scales'
Created | Updated Jan 5, 2018
Camelopardalis | Cancer | Canes Venatici | Canis Major | Canis Minor | Capricornus | Carina | Cassiopeia | Centaurus
Cepheus | Cetus | Chamæleon | Circinus | Columba | Coma Berenices | Corona Australis | Corona Borealis | Corvus
Crater | Crux | Cygnus | Delphinus | Dorado | Draco | Equuleus | Eridanus | Fornax | Gemini | Grus | Hercules | Horologium
Hydra | Hydrus | Indus | Lacerta | Leo | Leo Minor | Lepus | Libra | Lupus | Lynx | Lyra | Mensa | Microscopium | Monoceros
Musca | Norma | Octans | Ophiuchus | Orion | Pavo | Pegasus | Perseus | Phoenix | Pictor | Pisces | Piscis Austrinus
Puppis | Pyxis | Reticulum | Sagitta | Sagittarius | Scorpius | Sculptor | Scutum | Serpens | Sextans | Taurus
Telescopium | Triangulum | Triangulum Australe | Tucana | Ursa Major | Ursa Minor | Vela | Virgo | Volans | Vulpecula
In all perfectly beautiful objects there is found the opposition of one part to another and a reciprocal balance.
- Victorian author, artist and poet Professor John Ruskin MA (1819 - 1900).
|Name:||Libra (Latin: 'balance')|
|Area:||538 sq deg|
|Co-ordinates1:||Right Ascension 15h, Declination −15°|
The constellations Libra and Scorpius were possibly two scorpions originally, or a scorpion whose claws were a separate asterism. During the time of the Romans, the autumn equinox took place in Libra and the days and nights were roughly the same length, hence Libra gained the perception of 'balance'. Over 2,000 years' worth of precession, the wobble of the Earth upon its axis, has shifted the autumnal equinox into neighbouring Virgo. Since being granted its own official constellation status by the IAU in 1922, Libra today symbolises a set of scales.
Libra is bordered by (clockwise, from the north) Serpens Caput, Virgo, Hydra, Lupus, Scorpius and Ophiuchus. The four main stars of Libra create a goalpost shape, straddling the ecliptic2. Gamma is the first 'weighing pan', then up to beta, which forms the first apex of the beam. Follow on down to alpha (which is directly on the ecliptic) for the second apex, then down to sigma, the other 'weighing pan', which borders Hydra.
Libra can be observed from the UK, but it doesn't rise above the horizon by very much. You have to view it from a southerly position, and the best time is May, or thereabouts. The Sun appears to pass through Libra from our vantage point, making the constellation a member of the zodiac (see 'The Zodiac' section below).
The scientific star names are simple to understand (if you know your Greek alphabet). Combined with the genitive name, this is known as the 'Bayer designation'. Some stars have proper names as well; for example, alpha Librae is Zubenelgenubi. Others are known by their catalogue number.
The stars which form Libra once represented the claws of the scorpion, hence the Arabic star name meanings. There are no stars in Libra above third magnitude. The alpha-designated star is called Zubenelgenubi, with the Arabic meaning 'the southern claw'. Zubenelgenubi is a binary star system, which isn't the brightest of the constellation despite its alpha status.
On the night of 4 May, 2004, there was a total eclipse of the Moon. While the Moon was 'switched off', astronomers were able to glimpse wonderful images that wouldn't otherwise be viewable. The Astronomy Picture of the Day website published an image showing Zubenelgenubi just before it was occulted (obscured completely) by the Moon.
The honour of being the brightest star of the constellation belongs to beta Librae, Zubeneschamali (the northern claw), which measures +2.6m. It's a blue-white dwarf, although, oddly, to the naked eye it appears to shine green3. Ancient astronomers compared its brightness to that of neighbouring first magnitude alpha Scorpii (Antares), but there's no comparison now.
Delta Librae Zubenelakribi shines with a magnitude of +4.8m. It has a smaller companion which is so close that they complete an orbit in less than 2.5 days. The pair is aligned in such a way that from our vantage point the dimmer star passes in front of the brighter one each time around. The pair is called an 'eclipsing binary system'. This explains the main component's dip in brightness to +6m in the space of six hours from the commencement of each eclipse.
Gamma Librae Zubenelakrab (the scorpion's claw) and sigma Librae are the 'weighing pans' of the scales. Also known historically as Zubenalgubi (the south claw), sigma Librae was originally catalogued gamma Scorpii until its Bayer designation was changed in the 19th Century.
Because of the positions of kappa and lambda Librae, they can be occulted by the Moon and (more rarely) the planets of our solar system. Lambda Librae is located exactly where the new constellation borders were reorganised and consolidated by the IAU in 1930, when it was listed so close to Scorpius as to be within pinching distance.
|β Lib||beta Librae||Zubeneschamali||+2.6 var||160||The only 'green' star|
|α Lib||alpha Librae||Zubenelgenubi||+2.8||77||Binary star system|
|σ Lib||sigma Librae||20 Librae||+3.25 var||292||Red dwarf; was gamma Scorpii|
|υ Lib||upsilon Librae||39 Librae||+3.6 var||195||Multiple star system|
|τ Lib||tau Librae||40 Librae||+3.66||445||Blue-white giant|
|γ Lib||gamma Librae||Zubenelakrab||+3.9 var||150||Orange giant, possible multiple system|
|θ Lib||theta Librae||46 Librae||+4.1||160||Orange giant|
|κ Lib||kappa Librae||43 Librae||+4.75||400||Ecliptic star|
|δ Lib||delta Librae||Zubenelakribi||+4.8 var||300||Eclipsing binary stars|
|48 Lib||48 Librae||HD 142983||+4.8 var||500||Unusual emission lines|
|λ Lib||lambda Librae||45 Librae||+5.05||350||Ecliptic star|
|η Lib||eta Librae||44 Librae||+5.4||150||White giant|
|23 Lib||23 Librae||HD 134987||+6.4||84||Yellow dwarf: has a planet|
|HO Lib||HO Librae||Gl 581||+10.5 var||20||Has 6 planets|
|Wolf 1481||HN Librae||Gl 555||+11 var||19.95||Neighbouring star system|
New General Catalogue (NGC)
The NGC was compiled by John Louis Emil Dreyer (director of the Armagh Observatory from 1882 - 1916). Globular cluster NGC 5897 was discovered by William Herschel in April, 1784. It measures 135 light years across and is approximately 42,000 light years distant. Herschel described the cluster:
A very close, compressed cluster of stars 8 - 9 feet in diameter, extremely rich, of an irregular round figure, a little extended. The stars are so small as hardly to be visible, and so accumulated in the middle as to look nebulous.
All of the other NGC objects in Libra are spiral galaxies with a magnitude of around +12.
|NGC 5897||Globular cluster||+8.5||100+ stars|
|NGC 5792||Galaxy||+12.1||Almost edge-on|
|NGC 5595||Galaxy||+12.6||Sc: wide arms + small core|
|NGC 5728||Galaxy||+12.3||SBb (barred)|
Sb = Spiral barred galaxy, tightly wound arms
Sc = Multiple spiral arm galaxy (no bar)
SBb = Spiral galaxy, well-defined bar structure with arms starting from bar at right angles
Merrill 2-1 doesn't have an NGC designation but is worthy of note. Although small, its central star registers a +15 magnitude, and the surrounding nebula +13. This is a target for large telescopes, just to the south of the ecliptic.
The space debris which creates a meteor shower normally comes from the tail of a comet as the Earth crosses where the comet passed previously on its own orbit. Imagine a trail of breadcrumbs, or sawdust like that used in hashing. The meteor shower connected with this constellation is called the May Librids, which means the meteors come from the direction of Libra in May.
Extrasolar Planets in Libra
The nomenclature that's been decided on for extrasolar planet discoveries is to use a lower-case letter after the parent star catalogue number (or name), commencing at 'b': eg 'Gliese 581 b'. This stays with the planet regardless of whether subsequent discoveries are made within the same solar system, and despite the position of the new planet relative to the star. Therefore, the first-discovered planet of Gliese 581 is Gliese 581 b, with Gliese 581 c, and so on, being detected later.
Gliese 581 (also known as HO Librae) is a red dwarf star that's approximately 20.5 light years distant. Some planets have been discovered in orbit around it. The Gliese 581 system has its own h2g2 Entry.
The orbital period given in the table below is the time the planet takes to orbit its parent star, which we know of as a year. The mass of the extrasolar planet is compared with that of Jupiter, our solar system's largest planet, known to astronomers as the 'Jovian scale'.
Extrasolar Planets Table
|Star name or
|Year of discovery||Comments|
|23 Librae||23 Librae b||1.7||260||1999||Superjovian; habitable zone|
|23 Librae||23 Librae c||0.8||5,000||2009||Gas giant|
|HD 141937||HD 141937 b||9.68||653||2002||Possible brown dwarf|
|WASP-109||WASP-109 b||0.9||3.32||2014||Hot gas giant|
|HD 133131A||HD 133131A b||1.43||649||2016||Superjovian|
|HD 133131A||HD 133131A c||0.48||3,407||2016||Gas giant|
|HD 133131B||HD 133131B b||2.5||6,119||2016||Superjovian|
The modern abbreviation for pound weight is lb. This comes from the word 'libra', as scales were used to measure out a Roman soldier's pay - a pound5 of salt. The British currency symbol £ is an elaborate 'L' for the same reason: it is the initial letter of Libra.
The emblem of the Virgin Mary is also a set of scales.
As well as an instrument for weighing, a set of scales is a recognised symbol for justice. There's a statue of Justice, represented by a woman holding a sword in one hand and a set of scales in the other, outside the Old Bailey in London.
Some religions depict body parts or spiritual essence being weighed as a precursor to gaining entry to Heaven. The ancient Egyptians decorated tomb walls with images of Anubis, the god of the underworld, in the process of placing the heart of the deceased into a dish to be weighed. According to St Michael, the person's soul should be light.
The constellation Libra is a member of the zodiac, one of 13 which the Sun appears to pass through over the course of a year. Yes, 13. When the IAU redrew the constellation borders it found that the Sun passed through 13. Precession (the wobble of the Earth upon its axis) has shifted the timings somewhat, moving the points at which the equinoxes occur to a different place, and shifting the Earth's axis so it points to other north/south pole stars. Rather than upset astrologers by asking them to change the dates for their horoscopes, astrological predictions have carried on as before, with no discernible difference.
They probably wouldn't want to recognise 13 zodiacal constellations anyway, as some people consider 13 to be an an unlucky number. Of the zodiacal signs (zodiac means 'circle of animals'), Libra is the only one that isn't represented by a living creature (if you can call a centaur and a sea-goat 'living'; perhaps we'd better say 'animated').
Libra is the seventh sign of the zodiac and rules over Friday. It supposedly covers those born between 23 September and 21 October. Their birthstone is sapphire (for September-born Librans) and opal (for October babies). Libra's element is air and its quality cardinal.
For those who choose to believe the writers of horoscopes, Librans' lucky numbers are five and seven. Librans are compatible with fellow air signs Aquarius and Gemini. Their qualities include a love of beauty; this is reflected in their home surroundings - they aren't averse to paying top dollar for the best furnishings. Librans are good-looking, likeable, flirty, diplomatic and intelligent - and happy to use their charms to further their ambitions, so end up in high-flying jobs with good contacts.
Libra in Literature
In Chaucer's Prologue to the Parson's Tale, the pilgrims near their destination when Libra is ascending, and it's likely that this is part of an astrological allusion to Christ's Passion.