A Visitor from Outer Space

Coming to us from the distant regions of our solar system, where the sun shines dimly and its heat is scarcely felt, an interplanetary traveler is visiting us. The visitor is Comet Holmes. Most keen-eyed visual observers can see it without optical aid, and it is a spectacular sight in binoculars for everyone.

High in the sky is the "W" of Cassiopeia. The left side of the "W" points to the constellation Perseus. There, next to the brightest star in Perseus and looking like a hazy patch - definitely not a star - is the comet.

Comets are just giant dirty snowballs: mixtures of rocks, ice, and frozen gasses usually only a few miles across. If that were all there were to a comet, we could never see it. It would just be too small. But as the comet approaches the sun and is warmed by it, the ice and frozen gasses evaporate and form a halo around the comet millions of miles across. It's the sun's light reflected by this ball of gasses that makes it look so spectacular.

The tail of a comet is a stream of gasses and dust millions of miles long, pointing -- not behind the comet, as we would expect, but always away from the sun, pushed by the sunıs light and heat. In the morning, the tail points to the west, away from the rising sun. In the evening it points to the east, with the setting sun below it in the west.

So insubstantial is the tail of a comet that astronomers have called it "the nearest to nothing that anything can be and still be something." We can look through thousands of miles of the tail and see the stars beyond. In 1910, the Earth actually passed through the tail of Halleyıs comet. No one was hurt, despite dire predictions to the contrary.

(11/07/07)