A post on The JAR Blog last week included the word libration, which has the expected Spanish counterpart libración. The English-language Wikipedia article on the subject warns readers not to confuse the word with liberation, libation, or vibration, and then goes on to explain that “In astronomy, libration is an oscillating motion of orbiting bodies relative to each other, notably including the motion of the Moon relative to Earth…. Libration is manifested as a slow rocking back and forth of the Moon as viewed from Earth, permitting an observer to see slightly different halves of the surface at different times.”

Libración/libration is taken straight from Latin lībrātiō, with stem lībrātiōn-, which meant ‘oscillation.’ That noun was based on the past participle of the verb lībrāre ‘to balance,’ which in turn had come from the noun lībra, which designated ‘a balance, a pair of scales.’ English tends to say a scale or a balance scale, but the older phrase a pair of scales emphasized the way many ancient scales worked: the weigher put an object of unknown weight in a pan hanging from one end of the scale and tried to make it balance against an object or combination of objects of known weight in a similar pan hanging from the other end of the scale. In the process, the scale would typically rock back and forth until the two weights balanced.

All this talk of weighing reminds us that lībra was also the Latin word for a particular weight, namely ‘a pound.’ Spanish, of course, still uses libra that way, and so does English, even if only in the abbreviation lb. (Why didn’t my elementary school teachers ever explain where that strange abbreviation came from?) There’s also £, the stylized L that represents the type of pound that is the British monetary unit. And speaking of currency, Latin lībra evolved to Old Provençal liura, which passed into Old Italian and became the lira that until the adoption of the euro was the Italian monetary unit.

In the world of astrology, Libra is one of the twelve astrological signs of the Zodiac, represented by a balance scale. Astronomy still recognizes Libra as the corresponding Southern Hemisphere constellation but affords it no special significance. Tilting back to Libración/libration, I’ll recommend the musically accompanied video of the moon’s libration that you can find on The JAR Blog and in the English-language Wikipedia libration article. The Spanish-language Wikipedia libración article has a couple of animations but no music, nor does it warn about any confusion with the words liberación, libación, or vibración.

© 2013 Steven Schwartzman

6 Comments (+add yours?)

  1. Jim in IA
    Sep 28, 2013 @ 10:18:55

    How interesting. Thank you for the connection. I appreciate that.

    We have been entertaining our grand-daughter this morning and it delayed my getting into your blog post. It is hard to lībrāre all the parts of life sometimes. 🙂


  2. shoreacres
    Sep 28, 2013 @ 17:44:40

    I can’t believe I got to this stage of life without knowing how that strange lb. abbreviation came about. I never thought to ask. Like so much in life, it just was there to be used. So I did.

    Of course, the pound sterling symbol fell in the same category: life’s little givens.

    When I lived in Liberia, balance scales still could be seen in the markets, usually for selling things like ground peppers. I was lucky enough to come across some old Asante gold weights and one of the boxes used for gold dust or nuggets. You can see a wonderful set here. They were used with the same sort of scale – the gold weight(s) placed on one side, the nuggets or gold dust on the other, until the scale balanced.


    • Steve Schwartzman
      Sep 28, 2013 @ 21:07:07

      So your elementary school teachers were as remiss as mine. Oh well, what they didn’t know, they couldn’t teach us.

      It’s good of you to bring in your experience living in a place where the ancient type of balance scale was (and may still be) in use. I see that the weights you linked to were from the 19th century, which was perhaps their heyday.


  3. kathryningrid
    Oct 01, 2013 @ 12:45:58

    All of this is fun, but I especially appreciate *finally* knowing from whence that weird lb. abbreviation came! I’d never have guessed it. Thanks!


    • Steve Schwartzman
      Oct 01, 2013 @ 13:07:53

      You’re welcome. If I were still in my smarty-pants math teacher mode, I’d tell my students that we abbreviate pound as lb because the word pound doesn’t have an l or a b in it. (I used to give that sort of tongue-in-cheek explanation to algebra students to “explain” why we use the letter m to stand for slope.)


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