Page:The Music of the Spheres.djvu/252

 has no tail, but a large comet usually possesses a long and beautiful one, and has been known to possess two, and even half a dozen. This tail, or tails, are at their best when close to the sun, although at all times swung carefully away from the terrific flames of that luminary. Scientists almost hold their breath sometimes at the close call of comets, for the Great Comet of 1843 almost grazed the surface of the sun and many another has passed right through the outstretched banners of the corona. After darting swiftly around this brilliant center, the comet again heads into fathomless space, its long, bright tail gradually disappearing and its fierce speed relaxing as it escapes farther and farther from the sun's control.

The word comet means a "long-haired" star. The star-like nucleus of the comet is called the head and the long nebulous streamer which trails behind, the tail. The head may be colossal, or it may be a mere shred; the head of Donati's comet would have more than filled the space between the earth and moon, while that of the comet of 1811 was considerably larger than the sun itself. The length of a comet's tail is equally amazing for it extends often for millions of miles. The comet of 1882 had 100,000,000 miles of tail while that of 1483 had one twice as long. The comet of 1680 extended below the horizon when its head touched the zenith.

The mean density of a comet is very small. Young states that it is nothing more than a sandbank—a swarm of solid particles of unknown size and widely separated. This light and airy object darts through space at a most hectic speed, often at the rate of a number of hundred miles per second, when close to the sun. It is of such flimsy construction that its light does not even obscure the stars although there have been comets so bright that they were visible in the daytime.

It is now believed that a comet derives its light, except that