The Heavens Above: A Popular Handbook of Astronomy

by J. A. Gillet; W. J. Rolfe

308. Meteoroids.—Astronomers now universally hold that shooting-stars, meteors, and aerolites are all minute bodies, revolving, like the comets, about the sun. They are moving in every possible direction through the celestial spaces. They may not average more than one in a million of cubic miles, and yet their total number exceeds all calculation. Of the nature of the minuter bodies of this class nothing is certainly known. The earth is continually encountering them in its journey around the sun. They are burned by passing through the upper regions of our atmosphere, and the shooting-star is simply the light of that burning. These bodies, which are invisible till they plunge into the earth's atmosphere, are called meteoroids.

309. Origin of the Light of Meteors.—When one of these meteoroids enters our atmosphere, the resistance of the air arrests its motion to some extent, and so converts a portion of its energy of motion into that of heat. The heat thus developed is sufficient to raise the meteoroid and the air around it to incandescence, and in most cases either to cause the meteoroid to burn up, or to dissipate it as vapor. The luminous vapor thus formed constitutes the luminous train which occasionally accompanies a meteor, and often disappears as a puff of smoke. When a meteoroid is large enough and refractory enough to resist the heat to which it is exposed, its motion is sufficiently arrested, on entering the lower layers of our atmosphere, to cause it to fall to the earth. We then have an aerolite. A brilliant meteor differs from a shooting-star simply in magnitude.

310. The Intensity of the Heat to which a Meteoroid is Exposed.—It has been ascertained by experiment that a body moving through the atmosphere at the rate of a hundred and twenty-five feet a second raises the temperature of the air immediately in front of it one degree, and that the temperature increases as the square of the velocity of the moving body; that is to say, that, with a velocity of two hundred and fifty feet, the temperature in front of the body would be raised four degrees; with a velocity of five hundred feet, sixteen degrees; and so on. To find, therefore, the temperature to which a meteoroid would be exposed in passing through our atmosphere, we have merely to divide its velocity in feet per second by a hundred and twenty-five, and square the quotient. With a velocity of forty-four miles a second in our atmosphere, a meteoroid would therefore be exposed to a temperature of between three and four million degrees. The air acts upon the body as if it were raised to this intense heat. At such a temperature small masses of the most refractory or incombustible substances known to us would flash into vapor with the evolution of intense light and heat.

If one of these meteoric bodies is large enough to pass through the atmosphere and reach the earth, without being volatilized by the heat, we have an aerolite. As it is only a few seconds in making the passage, the heat has not time to penetrate far into its interior, but is expended in melting and vaporizing the outer portions. The resistance of the denser strata of the atmosphere to the motion of the aerolite sometimes becomes so enormous that the body is suddenly rent to pieces with a loud detonation. It seems like an explosion produced by some disruptive action within the mass; but there can be little doubt that it is due to the velocity—perhaps ten, twenty, or thirty miles a second—with which the body strikes the air.

If, on the other hand, the meteoroid is so small as to be burned up or volatilized in the upper regions of the atmosphere, we have a common shooting-star, or a meteor of greater or less brilliancy.

Fig. 352.

311. Meteoric Showers.—On ordinary nights only four or five shooting-stars are seen in an hour, and these move in every direction. Their orbits lie in all possible positions, and are seemingly scattered at random. Such meteors are called sporadic meteors. On occasional nights, shooting-stars are more numerous, and all move in a common direction. Such a display is called a meteoric shower. These showers differ greatly in brilliancy; but during any one shower the meteors all appear to radiate from some one point in the heavens. If we mark on a celestial globe the apparent paths of the meteors which fall during a shower, or if we trace them back on the celestial sphere, we shall find that they all meet in the same point, as shown in Fig. 352. This point is called the radiant point. It always appears in the same position, wherever the observer is situated, and does not partake of the diurnal motion of the earth. As the stars move towards the west, the radiant point moves with them. The point in question is purely an effect of perspective, being the "vanishing point" of the parallel lines in which the meteors are actually moving. These lines are seen, not in their real direction in space, but as projected on the celestial sphere. If we look upwards, and watch snow falling through a calm atmosphere, the flakes which fall directly towards us do not seem to move at all, while the surrounding flakes seem to diverge from them on all sides. So, in a meteoric shower, a meteor coming directly towards the observer does not seem to move at all, and marks the point from which all the others seem to radiate.

312. The August Meteors.—A meteoric shower of no great brilliancy occurs annually about the 10th of August. The radiant point of this shower is in the constellation Perseus, and hence these meteors are often called the Perseids. The orbit of these meteoroids has been pretty accurately determined, and is shown in Fig. 353.