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Monday, June 12

Libra the Scales

Once considered by the Greeks as the claws of Scorpius the Scorpion, Libra is now a constellation unto itself. The Romans made the break back in the time of Julius Caesar, ascribing the star pattern to the goddess of justice.

Although Libra is not the most exciting constellation in terms of "deep-sky wonders" for small telescopes, it contains a curious star that has been cause for discussion for centuries. The star's name is Zubeneschamali, or easier to remember, the "Northern Claw.

The debate began with the Greek astronomer Eratosthenes. (He's the one who made the first almost-dead-on calculation of the circumference of the Earth in 240 BC or thereabouts.) Eratosthenes compiled a catalog of some 675 stars in which he measured their position, and estimated their brightness and color. Zubeneschamali, said Eratosthenes, was the brightest star in Libra and in Scorpius, even outshining Antares, which we recognize as the brightest star in that region today.

A century or so later, another Greek astronomer, Ptolemy, found the star equal to Antares in brightness. Deepening the mystery still, the 18th-century British astronomy popularizer, T. W. Webb, thought the star looked pale green in color. And in the early part of the 20th century, another astronomy writer, William T. Olcott, described Zubeneschamali as having a decidedly green tint. If true, that would make it the only naked-eye star with that hue.

Although I can't speak for other observers, many of whom claim Zubeneschamali is indeed green in color, to me it just looks white, or maybe blue-white. Perhaps this star, over the centuries has gone through both brightness and color changes. Although we can't go back in time and see for ourselves, we can plainly see Zubeneschamali tonight. What color do you think it is?

Current Moon phase

Updated every 4 hours, courtesy USNO

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Jeff Kanipe is the author of A Skywatcher's Year, an astronomy guide just published by Cambridge University Press. He is a former editor at Astronomy and StarDate magazines and a writer for the Earth & Sky radio series.

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