Constellations: Libra 'the Scales' Content from the guide to life, the universe and everything

Constellations: Libra 'the Scales'

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The shield of the Science, Mathematics and Engineering faculty of the h2g2 University.Constellations: Overview | Andromeda | Antlia | Apus | Aquarius | Aquila | Ara | Aries | Auriga | Boötes | Caelum
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Scales weighing up right and wrong
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).

Constellation Data

Name:Libra (Latin: 'balance')
Genitive:Librae
Short form:Lib
Area:538 sq deg
Co-ordinates1:Right Ascension 15h, Declination −15°
Zodiac:Yes
Origin:Ancient

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).

Stars

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 green. 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.

Star Table

StarDesignationName or
catalogue number
Brightness (m)Distance
(light years3)
Spectral classification
and/or comments
β Libbeta LibraeZubeneschamali+2.6 var160The only 'green' star
α Libalpha LibraeZubenelgenubi+2.877Binary star system
σ Libsigma Librae20 Librae+3.25 var292Red dwarf; was gamma Scorpii
υ Libupsilon Librae39 Librae+3.6 var195Multiple star system
τ Libtau Librae40 Librae+3.66445Blue-white giant
γ Libgamma LibraeZubenelakrab+3.9 var150Orange giant, possible multiple system
θ Libtheta Librae46 Librae+4.1160Orange giant
κ Libkappa Librae43 Librae+4.75400Ecliptic star
δ Libdelta LibraeZubenelakribi+4.8 var300Eclipsing binary stars
48 Lib48 LibraeHD 142983+4.8 var500Unusual emission lines
λ Liblambda Librae45 Librae+5.05350Ecliptic star
η Libeta Librae44 Librae+5.4150White giant
23 Lib23 LibraeHD 134987+6.484Yellow dwarf: has a planet
HO LibHO LibraeGl 581+10.5 var20Has 6 planets
Wolf 1481HN LibraeGl 555+11 var19.95Neighbouring 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 Table

CatalogueTypeBrightness (m)Remarks
NGC 5897Globular cluster+8.5100+ stars
NGC 5792Galaxy+12.1Almost edge-on
NGC 5595Galaxy+12.6Sc: wide arms + small core
NGC 5728Galaxy+12.3SBb (barred)
NGC 5861Galaxy+12.3Peculiar
NGC 5878Galaxy+12.4Sb
NGC 5885Galaxy+12.3Face-on

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

Planetary Nebula

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.

Meteor Showers

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. Five planets have been discovered orbiting it since 2005.

Gliese 581 c is a terrestrial (rocky) planet, these are classed as 'Super Earth' worlds as they are much bigger than the Earth. Even though it orbits quite close to its star, the planet is considered to be in the system's habitable zone, because the star is a red dwarf and therefore cooler than our Sun. It's hard to convey how exciting this discovery is to anyone who isn't involved in the search for evidence of life beyond our planet.

On the treasure map of the Universe, one would be tempted to mark this planet with an X.
- Study team member Xavier Delfosse, of Grenoble University, France.

Gliese 581 d was discovered by the HARPS instrument (a high-precision spectrograph) on the 3.6-metre telescope at the European Southern Observatory in La Silla, Chile. It's also a suspected terrestrial planet, seven times the mass of Earth. It orbits just inside the system's habitable zone.

Yet another planet, Gliese 581 e, was discovered in April 2009. It is a rocky world unfortunately orbiting far too close to the star for life to be considered.

Two more planets, Gliese 581 f and Gliese 581 g, were detected in 2010. Gliese 581 g is a super-Earth orbiting within the habitable zone. Its co-discoverer dubbed it 'Zarmina's World' after his wife.

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
catalogue number
Planet
catalogue number
Planet mass
(Jovian scale)
Orbital period
(Earth days)
Year of discoveryComments
Gliese 581Gliese 581 b0.055.362005Hot gas giant
Gliese 581Gliese 581 c0.016132007Super Earth
Gliese 581Gliese 581 d0.02566.82007Super-Earth
Gliese 581Gliese 581 e0.0063.152009Super-Earth
Gliese 581Gliese 581 f0.0234332010Super-Earth
Gliese 581Gliese 581 g0.0136.652010Super-Earth
23 Librae23 Librae b1.72601999Superjovian; habitable zone
23 Librae23 Librae c0.85,0002009Gas giant
HD 141937HD 141937 b9.686532002Possible brown dwarf

Scales

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 pound4 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 Zodiac

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.

1Current International Astronomical Union (IAU) guidelines use a plus sign (+) for northern constellations and a minus sign (−) for southern ones.2The apparent path of the Sun through the sky.3A light year is the distance light travels in one year, roughly 5.88 trillion miles or 9.46 trillion km.4A Roman pound was about three quarters of a modern pound.

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