An Emerald in the Sky

Jupiter rises in the east. Venus still shines in the west. These two, and Mars (when it's close to the Earth) are much brighter than any star. We should never have to ask, "is that a star or a planet?" - any starlike object gleaming much brighter than the stars is a planet. Saturn and Mercury are not so bright, and we'll have to look more carefully for them, but of the five planets visible to the naked eye, three are quite special.

To the east of Jupiter is the bright star Spica. Halfway between Spica and Jupiter is the small constellation of Libra. This constellation used to represent the claws of the scorpion, and the two brighter stars have the catchy names of Zuben-el-genubi (the southern claw) and Zuben-eschamali (the northern claw.) (If you think those names strange, think how boring the modern names are: just a (alpha) and b (beta) Librae.)

It's beta (the northern, or upper one - the brightest star in Libra) thatıs most interesting. It's green! Or, at least, greenish. Though all stars have a dominant color -- yellow for our sun, red for Antares (just to the east of beta) -- green stars are very hard to find. Our eyes are not very sensitive to colors -- and most stars are pretty dim. Well, Zuben-eschamali (beta) is an exception. It's fairly bright. Part of the reason we can see it as green may its closeness to red Antares -- it's the contrast that helps us see its color.

No one can explain its color. Try to see its green hue on the next clear evening. It is unique among the brighter stars. Itıs a hot star, twice as hot as the sun, and this would make us expect a bluish-white star, like Vega. The other mystery is its brightness: in the fifth century B.C. it was brighter than Antares. A few centuries later, it was equal in brightness. Now it's dimmer. A mystery. Is Antares getting brighter or is beta getting dimmer? No one seems to know.

This is where the Sun will be in November. That's why it will be so cold. The seasons are caused by the height of the Sun, not its distance. And how high it is now!

But in 2007 B.C., the Sun was in Libra in October, so the astrological sign for October is this "house."

(06/13/07)