Friday, May 20, 2011

Spica in the constellation Virgo

As the earth makes its way around the sun, the constellation Scorpio is becoming more prominent in the night sky as Orion recedes from the scene (because the sun is now between the earth and Orion, meaning he is up primarily during the day, when he cannot be seen). This process is described in greater detail here.

The constellation Virgo is fairly easy to find from the Scorpion -- just continue along the ecliptic from his head and claws to the right as you face south, past the faint stars of Libra, and on to brilliant Spica, an important star with a historic place in the story of precession (see the above-mentioned post on Orion for further explanation of precession).

Another way to find Spica and Virgo is to follow the sweep of the handle of the Big Dipper (see the discussion in "The Undying Stars") to reddish Arcturus and then continue along the same arc to Spica (an old saying advises "follow the arc to Arcturus and then drive a spike to Spica").

Spica is also designated alpha virginis -- meaning "the alpha of the constellation Virgo" or "the Virgin's alpha." Under a system of star designation introduced by Johann Bayer (1572 - 1625) in 1603, every constellation has a first star (often the brightest star) designated as its alpha, a second star (usually its second-brightest star) designated as its beta, a third star designated as its gamma, and so forth. The genitive (possessive) form of the Latin name for the constellation precedes the designation, so that the brightest star in the constellation Leo would be alpha leonis ("the lion's alpha"), which is the star Regulus.

Spica itself is a Latin word meaning an ear of wheat, perhaps because the constellation Virgo was formerly associated with harvest time (all the constellations have now "been delayed" by the long march of precession). In any event, Spica played an important role in the re-discovery of precession by Hipparchus of Nicaea (c. 190 BC - c. 126 BC or 120 BC).

The great modern scholar of the history of astronomy, Otto Neugebauer (1899 - 1990) recounts that one of the most important observations that led Hipparchus to the re-discovery of precession was his calculation that the star Spica had moved relative to the autumnal equinox, compared to the notations of Spica's position recorded by the earlier astronomers Timocharis of Alexandria (320 BC - 260 BC) and his contemporary and colleague Aristyllus.

Hipparchus can be said to have "re-discovered" precession because there is abundant evidence that far more ancient civilizations had understood it in even greater detail and precision literally thousands of years earlier. This is not to take anything away from the achievement of Hipparchus, as perceiving precession is by no means easy -- the phenomenon causes only one degree of change relative to the equinoxes in 71.6 years.

Spica is indicated in the above chart of Virgo by a red arrow (note that in the rather fanciful drawing from the 1825 star book Urania's Mirror shows a sheaf of wheat at Spica). The heavy black lines connect the stars of Virgo using the alternative (and much more useful) method created by H.A. Rey, who was also the author of the beloved Curious George stories. His book The Stars: A New Way to See Them is one of the best books for star-gazing ever written.

He argued that "allegorical drawings" such as the one above were decorative but unhelpful -- that in fact they were confusing. He also decried the geometrical but shapeless connections that are often used in which lines connect stars in a constellation but with no relation to the shape that the constellation's name suggests. His alternate outlines are both simple enough to actually use and suggestive enough of the object named to be the best of both worlds.

Spica is currently reaching its highest point around 10:30 pm each night (and getting earlier each evening) and setting at about 4:00 in the morning (at latitude 35o north). Spica is actually a binary star and one of the twenty brightest in the entire sky.

Also in the constellation Virgo is the star Porrima, which is also a binary, also designated gamma virginis. It is indicated by a green arrow in the above diagram. The planet Saturn is currently near Porrima as it arcs across the night sky along the ecliptic path (see discussion in this previous post).