Libra

Authors: Susanne M Hoffmann, Youla Azkarrula

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star chart

Lib star chart (IAU and Sky & Telescope magazine, Roger Sinnott & Rick Fienberg).

One of the 88 IAU constellations. The constellation Libra is Babylonian—like all constellations of the zodiac. What is special about this constellation is that it was initially lost in Greek times and returned in Roman times. The fact that Libra is the only instrument in the zodiac today is rather coincidental: we could just as well count the sextant in the zodiac in addition to Orion's club, because stars in this constellation are sometimes covered by the moon, and then we would have another inanimate object. So it is rather coincidental that Libra ended up in the zodiac and other instruments did not.

Etymology and History

Origin of Constellation

The constellation name is attested at least since MUL.APIN (around 1000 BCE). However the shape of the image and the exact identification with stars was occasionally varied over time.

If you draw the Babylonian equivalent of the constellation Virgo, Spica is the easternmost star. Between this and the stars at the tips of the Scorpion's claws (α and β Lib), there is about 20° of space where you can draw a set of scales. The scales then lie towards Virgo and not towards Scorpius as in today's/Greek image. Perhaps this was the original image, which first had to give way to the enlarged Greek Virgo before the Babylonian roots were revisited in Roman times. In the 1st century, Libra appears in astrological literature, for example in Geminos and Manilius, and is thus drawn again – but then in the scissors of Scorpio: on the Farnese Globe, Scorpius “holds” Libra with the tips of its scissors, so that the scales dangle in its face.

Greek Absence

Aratos, Eratosthenes, Hipparchus, and Ptolemy did not know the constellation Libra: for them, this area was named the scorpion's claws. The reason for this is probably a change in the constellation Virgo: in Mesopotamia, there were two variants of the constellation Virgo (the Virgin), both of which occupy a small space of about 30° along the ecliptic. However, today's constellation Virgo is approximately 43° long along the ecliptic, and today's constellation is based on the Greek one.

If you draw the Greek Virgo on a star map and next to it a giant scorpion, for which the tips of the claws are marked by the stars α and β Librae, then there is no room left in between for another figure, e.g., a scale. On the globe of the Farnese Atlas, the scorpion holds the scales in its claws, i.e., anyone who wanted to imagine a set of scales in Greek superimposed them on the claws of the scorpion.

Roman Renaissance

Manilius passed down the mnemonic rule that day and night “keep the scales balanced” in the constellation. He cannot have meant that the sun is in Libra at the equinox, because that was no longer the case in his era. The equinox was located next to α Lib around 1250 BC, which fits the date of MUL.APIN. Heliocentric risings of the Libra stars α and β Lib in the morning sky thus took place in the 2nd millennium at the equinox and no longer in Roman times. Manilius can therefore only have referred to heliocentric settings in the evening, i.e., observations at a time when people could comfortably observe the sky and could use this as a farmer's rule. In his time, the stars of Libra were just visible in the evening sky around the equinox and set quickly after the sun.

Manilius was best known for his astrology and was not read in mathematical astronomy. Astronomy in the Arabic and Latin Middle Ages refers primarily to Aratus and Ptolemy, neither of whom knew the constellation Libra. That is why the two brightest stars in Libra are still called by their Arabic names “northern scissors” and “southern scissors” today; they are direct translations from the Almagest. In modern star charts, Libra is consistently depicted again – at least since it was drawn in the Uranometria in 1603, it has become an integral part of uraniumography.

Modern authors sometimes wonder why Libra is the only inanimate object in the zodiac. This was by no means always the case: the Mesopotamian precursor to the zodiac was the moon's path. The “path of the moon” is the area in which the moon can cover stars. Today, as in MUL.APIN, it consists of 17 constellations. Among them are several “devices”; for example, the stars of Auriga once formed a throwing stick, i.e., a weapon—a kind of Babylonian boomerang. Orion holds up a club (or, in modern terms, sometimes a sword) or, in Babylonian terms, a shepherd's crook. Whatever he holds in his northward-facing hand is an object whose tip lies in the zodiac. This is also already mentioned in MUL.APIN.

Greco-Roman

Hipparchus

Rising (Lib II Cap V §10)

The Rising of ...

	east		south
	lam1	lam2	lam1	lam2
star
duration	2 1/8 hours = 127.5 min = 31 7/8 degree

Setting (Lib II Cap VI §10)

	west		south
	lam1	lam2	lam1	lam2
star
duration	2 2/3 hours = 160 min = 40°

Stars Mentioned

	Greek	German	English	ident.	src		lam_culm
					Lib II Cap V §10	rising, east, first
					Lib II Cap V §10	rising, east, last
					Lib II Cap VI §2	setting CrB, south, last	Psc 13.5
					Lib II Cap VI §13	setting Aql, south, first	Ari 2
					Lib III Cap I §9	rising Ori, south, last	Psc 13

Almagest Χηλαί.

id	Greek (Heiberg 1898)	English (Toomer 1984)	ident.
	Χηλῶν ἀστερισμός.	Constellation of The Claws
1	τῶν ἐπ’ ἄκρας τῆς νοτίου χηλῆς ὁ λαμπρός	Stars on the tip of the southern claw: the bright one	alf Lib
2	ὁ βορειότερος αὐτοῦ καὶ ἀμαυρότερος;	Stars on the tip of the southern claw: the star to the north of this and fainter than it	mu Lib
3	τῶν ἐπ’ ἄκρας τῆς βορείου χηλῆς ὁ λαμπρός	Stars an the tip of the northern claw: the bright one	bet Lib
4	ὁ προηγούμενος αὐτοῦ καὶ ἀμαυρός.	Stars an the tip of the northern claw: the faint star in a,dvance of this	del Lib
5	ὁ ἐν μέσῃ τῇ νοτόῳ χηλῇ	The star in the middle of the southern claw	iot Lib
6	ὁ τούτου προηγούμενος ἐπὶ τῆς αὐτῆς χηλῆς	The one in advance of this an the same claw	nu Lib
7	ὁ ἐν μέσῃ τῇ βορεύῳ χηλῇ.	The star in the middle of the northern claw	gam Lib
9	ὁ ἑπόμενος αὐτῷ ἐπὶ τῆς αὐτῆς χηλῆς	The one to the rear of this on the same daw	tet Lib
	ἀστέρερ ἡ, ὥν β’ μεγέθους β, δ’ ὅ, ε’ β.	{8 stars, 2 of the second magnitude, 4 of the fourth, 2 of tl;le fifth}
	Οἱ περὶ τὰς χηλὰς ἀμόρφωτοι	Stars araund Libra outside the constellation
10	τῶν βορειοτέρων τῆς βορείου χηλῆς β' ὁ προηγούμενος	The most advanced of the 3 stars north of the northern claw	37 Lib
11	τῶν ἐπομένων β ὁ νοτιώτερος	The southernmost of the rearmost 2 [ of these]	48 Lib
12	ὁ βόρειος αὐτῶν	The northernmost of them	xi Lib
13	τῶν μεταξὺ τῶν χηλῶν » ὁ ἐπόμευος	The rearmost of the 3 stars between the claws	lam Lib
14	τῶν λοιπῶν β καὶ προηγουμένων ὁ βόρειος.	The northernmost of the other 2 in advance f of the lauer 1	41 Lib
15	ὁ νότιος αὐτῶν	The southernmost of them	HR 5810
16	τῶν νοτιωτέρου τῆς νοτίου χηλῆς 7 ὁ προηγούμενος.	The most advanced of the 3 stars south of the southern claw	20 Lib
17	τῶν λοιπῶν καὶ ἐπομένωυ β ὁ βορειότερος.	The northernmost of the other, rear 2	39 Lib
18	ὁ νοτιώτερος αὐτῶν	The southernmost of them	40 Lib
	ἀστέρες 9, ὧν γ’ μεγέθουρ ἄ, δ’ ἓ, ε β, ς ἂ.	9 stars, I of the third magnitude, 5 of the fourth, 2 of the fifth, 1 of the sixth

Convex Hull for the stars inside Chilai (CC BY Youla Azkarrula).

Stars within the Constellation Area

id	Label	IAU design.	description	Vmag
1	Zubeneschamali	HIP 74785	Constellation lines (Vertex)	2.62
2	Zubenelgenubi	HIP 72622	Constellation lines (Vertex)	2.75
3	Zubenelhakrabi	HIP 76333	Constellation lines	3.91
4	θ Librae	HIP 77853	Constellation lines (Vertex)	4.16
5	ι 1 Librae	HIP 74392	Constellation lines (Vertex)	4.54
6	δ Librae	HIP 73473	Constellation lines (Vertex)	4.93
7	ν Librae	HIP 73945	Constellation lines	5.2
8	μ Librae	HIP 72489	Constellation lines (Vertex)	5.31
9	η Librae	HIP 77060	Inside the hull	5.412
10	ξ 2 Librae	HIP 73133	Inside the hull	5.45
11	ζ 4 Librae	HIP 76126	Inside the hull	5.499
12	ζ 1 Librae	HIP 75730	Inside the hull	5.626
13	-	HIP 75352	Inside the hull	5.716
14	ξ 1 Librae	HIP 72934	Inside the hull	5.788
15	ζ 3 Librae	HIP 75944	Inside the hull	5.806
16	18 Lib	HIP 73310	Inside the hull	5.857
17	ι 2 Librae	HIP 74493	Inside the hull	6.066
18	ο Librae	HIP 75118	Inside the hull	6.143
19	28 Lib	HIP 75110	Inside the hull	6.17
20	26 Lib	HIP 74600	Inside the hull	6.172
21	-	HIP 77007	Inside the hull	6.295
22	22 Lib	HIP 73953	Inside the hull	6.404
23	30 Lib	HIP 75294	Inside the hull	6.48
24	-	HIP 74391	Inside the hull	6.49

Transfer and Transformation of the Constellation

• 
  Libra (two versions) in as-Sufi, Kitāb Ṣuwar al-kawākib al-thābitah, Mosul 1170, Bodleian Libraries, University of Oxford,
• 
  Libra in Bayer (1603).
• 
  Libra in Flamsteed (1776)
• 
  Libra in Bode (1782,1805)
• Error creating thumbnail:
  Libra in Jamieson (1822)
• Error creating thumbnail:
  Libra zodiac sign, Jantar Mantar, Jaipur, India

Mythology

For the Babylonians, Libra was located next to Spica, the ear of corn, and was therefore probably an instrument for weighing the harvest. The Babylonian equivalent of the constellation Virgo was associated with the goddess Shala. Although there was no constellation called Shala, this goddess is sometimes depicted holding the ear of corn in her hand instead of lying in a furrow. This pragmatic proximity of the ear of corn and the scales is thus documented from the 2nd millennium BCE to Greek times. There is no story behind it; rather, it seems to be a mnemonic device for the farmers' calendar.

When the Romans reintroduced the constellation Libra, however, it was an instrument or symbol of justice for them. It is said to be the scales of the goddess Justicia (Greek Dike), which is one of the possible interpretations of the Greek Virgin.

There is therefore no specific myth associated with Libra, only various ideas in ancient religions or as a farmer's rule for determining the calendar: day and night are in balance, the world experiences justice, or the harvest is weighed.

References

• References (general)
• References (Babylonian)
• References (ancient Greco-Roman)
• References (medieval)