Two
spring constellations of the zodiac are Virgo the virgin and Libra
the scales. Put them together and you have Astraea (Star-Maiden)
holding the scales, the goddess of justice. Astraea was the daughter
of Zeus and Themis, the original goddess of justice. According to
Greek mythology she was the last of the immortals to have lived on
the earth, and when mankind became wicked, she retreated to the
heavens where we see her and her scales of justice in the sky as the
two constellations.
We tend to think of Virgo as another kind of goddess, that of the harvest, as Demeter or the Roman Ceres. Virgo's bright star Spica would represent the head of a stalk of wheat. It is known that certain ancient Greek temples were aligned to Spica's setting point. The earth's axis wobbles under the gravitational influence of the moon and sun. This effect, called precession, causes the entire sky to appear to shift accordingly. For the last seven to eight thousand year Spica has been heading south. And so its setting point in the west has also been shifting southward with time. Thus the building of the temples can be dates by their orientation.
Spica is a first magnitude star of magnitude +0.98. It is the hottest and bluest of the first 21 magnitude stars, having a spectral type of B on the OBAFGKM spectral type scale that goes from the hottest and bluest O type stars to the coolest red M stars. My first encounter with how blue Spica is came with the total lunar eclipse of April 12, 1968, in which the moon was located close to Spica in the sky. In photographing the partially eclipsed moon, Spica was in the field. It's image was blue in the short exposures. Much of that color is lost by burning the image in on longer exposures to also record fainter stars.
The constellation of Virgo itself, except for Spica, is made of dimmer stars. I imagine the virgin reclining, as can be seen in the constellation lines in the Stars and Planets chart later in this publication. Others, as seen in the picture below standing, with lines drawn to match. Notice the position of Jupiter for June 15th this year (2006) betweem Virgo and Libra
Incidently that picture is from the free program Stellarium, written by a team headed by Fabien Chereau. The constellation image artwork is by Johan Meuris. Version 8.0 of Stellarium has just been released. Go to the web site to download it.
The Babylonians saw the constellation as Ishtar the Queen of Heaven. To the early Christians Virgo was naturally thought to depict the Virgin Mary. To other Christians, the twelve constellations of the zodiac represented the twelve apostles, and the stars here were those of St. James the less.
Libra the scales, or balance is an oddity among the constellations of the zodiac in that it does not portray either an animal or human. The origins of the word have to so with living beings. Being so, Libra being an inanimate object, doesn't fit. Even stranger are the names of its two brighest stars. Zubebelgenubi is the star of Libra closest to Jupiter in the image on the right. Zubeneschamali is the top star of Libra. Their names mean north and south claws respectively. What were the Arabs thinking? These two stars, like most stars have Arabic names. To the Arabs and even the early Greeks, the constellation Scorpius the scorpion, just east of Libra extended into what we now call Libra. These stars just represented the tips of the scorpion's claws.
So
we have a deeply ambiguous part of the sky here: Some saw it as part
of Scorpius, while at the same time part of Virgo or Astraea with her
scales. It seems that Libra as a separate constellation was the last
sign or constellation of the zodiac to be established. Apparently
there were always twelve zodiacal constellations, and some suggest
that Libra took the place of Boötes, located north of Libra.
There seems to be a good reason to names these stars the balance back when they did just over 2,000 years ago. Back then the sun was in that part of the sky around the autumnal equiniox. So the balance would depict the equality of day and night at that time of the year. That relationship is no longer true. Precession has moved Libra southward and eastward along the ecliptic, the path of the sun in the sky. Now Virgo is the location of the autumnal equinox, as can be seen on the Stars and Planets chart at the intersection of the ecliptic that passes through the zodiacal constellations and the celestial equator that intersects the horizon at the east and west compas points. The third line that intersects the other two and passes close to Polaris in the north is the 12 hour right ascension line. It's no coincidence that that third line is there, it is defined by the intersection of the other two.
Libra and Virgo aren't the brightest of constellations, but their stars and their history make them well worth seeking out.
Questions? Send Email to me at bob@bjmoler.org
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