Читать книгу The Circle of Knowledge: A Classified, Simplified, Visualized Book of Answers - Various - Страница 37

In addition to the planets and their satellites, the sun is attended by numerous other bodies, moving with far less regularity, and generally much less conspicuous in the heavens. These are known as comets and meteorites or shooting stars. One of the most interesting of recent astronomical discoveries is that an intimate physical connection exists between these two classes of bodies.

Comets. Comets have been known from the earliest times, because every now and then a very large and conspicuous one hastens up to the sun from the remote regions of space, and perplexes monarchs with the fear of change. They are called comets, from the Latin coma, meaning hair, because when they are bright enough to be seen with the naked eye they look like stars attended by a long stream of hazy light, which was thought to resemble a woman’s hair flowing down her back. This train of light is known as the comet’s tail. Such bright comets are sometimes as brilliant as Venus; their tails have been known to stretch halfway across the visible sky.

These comets are very beautiful and conspicuous objects, which usually appear in the sky without any warning from astronomers, and invariably create a great popular sensation. By far the greater number of comets, however, are only visible through a telescope, and it is rare that a year passes without at least half a dozen of these being reported. Up to the present time nearly a thousand comets of all sizes have been recorded. Not more than one in five of these visitors is visible to the naked eye.

Cometary Orbits. In all cases in which a comet has been observed sufficiently often for its orbit to be calculated, it is found that it moves in one of the curves which are known to the geometer as conic sections. Less than a hundred of the known comets move like the planets in elliptical orbits, and consequently their periodical return to visibility can be predicted. As a rule the eccentricity of these cometary orbits is very much greater than that of any planetary orbit, which means that the comet approaches fairly close to the sun at one end of its orbit, but at the other flies away far beyond the outermost planet, and for a long period disappears from the view of our most powerful telescopes.

The great majority of comets have only been seen once, and their orbits appear to be either parabolic or hyperbolic. Neither of these is a closed curve, and what seems to happen in such cases is that a comet travelling in such an orbit dashes up to the sun from the remote parts of space, swings round it, often at very close quarters, and flies away again forever. Only those comets which have elliptical orbits can be said to belong to the solar system. The others are visitors from space, which in the course of their motion come near the sun and are deflected by it, but then fly away until after a lapse of ages they perhaps come within the sphere of another star’s attraction. Of the comets which move in elliptical orbits, about twenty have been observed at more than one return to the sun. Some of these complete their orbits in quite a short period, like Encke’s comet, which has the shortest period of all, less than three and a half years; the longest periodical comet is known as Halley’s, which returns to the sun after seventy-six years, and last appeared in 1910; it is a bright and conspicuous object.

The Constitution of Comets. The nature of comets was long in doubt, and even today their physical characteristics are not fully understood. They are certainly formed of gravitational matter, because they move in orbits which are subject to the same laws as those of the planets. But they also appear to be acted upon by powerful repulsive forces emanating from the sun, to which is due the remarkable phenomenon of cometary tails. Perhaps there is not much exaggeration in the statement once made by a well-known astronomer that the whole material of a comet stretching halfway across the visible heavens, if properly compressed, could be placed in a hatbox. The old fear that the earth might suddenly be annihilated by a comet striking it is thoroughly dispelled by modern investigation, which leads us to believe that the worst results of such an encounter would be an extremely beautiful display of shooting stars.

Meteors, or Fireballs, are bodies which do not belong to the earth, but come from other parts of space into our atmosphere, and are seen as bright balls of fire crossing the sky, with a train of light behind. Suddenly they are seen to go out, and very often a fall of stones occurs. Sometimes they are observed to break in two, and loud explosions like thunder are heard. They move very fast—ten or twelve miles per second, and are visible when between forty and eighty miles above the earth.

Other meteors dart across the sky and disappear, all in a very short time. These are known as shooting stars, and are sometimes big and bright, like planets. It is estimated that about six or eight meteors which drop stones come into our atmosphere every year; but some 20,000,000 of small bodies pass through the air every day—these would all appear as shooting stars if they occurred at night.

At some periods of the year there are so many shooting stars that they appear like a shower of fire. On November 14th this happens, the shower being greatest every thirty-three years. A stream of meteors is travelling round the sun, and every thirty-three years the earth just comes through them. Meteoric showers also occur about August 9th to 11th, and smaller ones in April.

The luminosity of meteors is due to the intense heat caused by the resistance of the air to their passage, and in support of this theory it is found that meteoric stones are always covered, either wholly or in part, with a crust of cement that has recently been melted.