BY quite another class of dark bodies than those we contemplated in the last chapter is the immediate space about us tenanted. For that, too, is anything but the void our senses give us to understand. Could we rise a hundred miles above the Earth’s surface we should be highly sorry we came, for we should incontinently be killed by flying brickbats. Instead of masses of a sunlike size we should have to do with bits of matter on the average smaller than ourselves but hardly on that account innocuous, as they would strike us with fifteen hundred times the speed of an express train. Only in one respect are the two classes of erratics alike, both remain invisible till they are upon us. Even so, the cause of their visibility is different. The one is announced by the light it reflects, the other by the glow it gives out on its destruction. These last are the meteorites or shooting-stars. They are as well known to every one for their commonness as, fortunately, the first are rare. On any starlight night one need not tarry long before one of these visitants darts across the sky, a brilliant thread of fire gone almost ere it be descried.
Usually this is all of which one is made aware. Silent, ghostlike, the apparition comes and goes, and nothing more of it is either seen or heard. But sometimes there is a good deal more. Occasionally a large ball of flame shoots through the air, a detonation like distant thunder startles the ear, and a luminous train, persisting for several seconds, floats slowly away. Finally if one be fortunate to be near,—but not too near,—one or more masses of stone are seen to fall swiftly and bury themselves in the ground. These are meteorites: far wanderers come at last to rest in graves they have dug themselves.
A great revolution has taken place lately in our ideas concerning meteorites. Indeed, it was not so very long ago, since modern man admitted their astronomic character at all. He looked as askance at them as he did at fossils. It was the fall at Aigle, in Switzerland, April 26, 1803, that first opened men’s eyes to the fact that such falls actually occurred. It is more than a nine days’ wonder at times how long men, as well as puppies, can remain blind. To admit that stones fell from heaven, however, was not to see whence they came. Their paternity was imputed to nearly every body in the sky. They were at first supposed to have been ejected from earthly volcanic vents, then from volcanoes in the Moon. That they are of domestic manufacture is, however, negatived by the paths they severally pursue. Nor can they for like reason have been ejected from the Sun.
The Earth was not their birthplace. It is alien ground in which they lie at last and from which we transfer them to glass cases in our museums. This fact about their parentage they tell by the speed with which they enter our air. They become visible 100 miles up and explode at from 20 to 10, and their speed has been found to be from 10 to 40 miles a second, which is that of cosmic bodies moving in large elliptic orbits about the Sun,—a speed greater than the Earth could ever have imparted.
Four classes of such small celestial bodies tenant space where the planets move: sporadic shooting-stars, meteorites, meteor-streams, and comets. The discovery of the relation of each of these to the solar system and then to each other forms one of the latest chapters of astronomic history. For they turn out to be generically one.
It was long, however, before this was perceived. The first step was taken simultaneously by Professor Olmstead of Yale and Twining in 1833 from reasoning on the superb November meteor-shower of that year. All the shooting-stars, “thick as snowflakes in a storm,” had a common radiant from which they seemed to come. Thus they argued that the meteors must all be travelling in parallel lines along an orbit which the previous shower, of 1799, showed to be periodic. This was the first recognition of a meteor-swarm.
The next advance was when Schiaparelli, in 1862, pointed out the remarkable connection between meteor-swarms and comets. On calculation the August meteor-stream and the comet of 1862 proved to be pursuing exactly the same path. Soon other instances of like association were discovered, and we now know mathematically that meteor-streams can be, deductively that they must be, and observationally that they are, disintegrated comets. More than one comet has even been seen to split.
Then came the recognition that comets are not visitors from space, as Sir Isaac Newton and Laplace supposed, but part and parcel of our own solar system. Without going into the history of the subject, which includes Gauss, Schiaparelli, and finally Fabry’s great Memoir, much too little known, the proof can, I think, be made comprehensible without too much technique, thanks to the fact that the Sun is speeding through space at the rate of eleven miles a second.
Orbits described by bodies under the action of a central force are always conic sections, as Sir Isaac Newton proved. There are two classes of such curves: those which return into themselves, such as the circle and ellipse, and those which do not, the hyperbolæ. If a body travel in the first or closed class about the Sun, it is clearly a member of his family; if in the second, it is a visitor who bows to him only in passing and never returns. Which orbit it shall pursue depends at a given distance solely upon the speed of the body; if that speed be one the Sun can control, the body will move in an ellipse; if greater, in an hyperbola. Obviously the Sun can control just the speed he can impart. Now a comet entering the system from without would already possess a motion of its own which, when compounded with the solar-acquired speed, would make one greater than the Sun could master. Comets, therefore, if visitors from space, should all move in hyperbolæ. None for certain do; and only six out of four hundred even hint at it. Comets, then, are all members of the solar family, excentric ones, but not to be denied recognition of kinship for such behavior.
Still, admittance to the solar family circle was denied to meteorites and shooting-stars. Thus Professor Kirkwood, in 1861, had considered “that the motions of some luminous meteors (or cometoids, as perhaps they might be called) have been decidedly indicative of an origin beyond the limits of the solar system.” Here cometoid was an apt coinage, but when comets were later shown not to be of extra-solar origin, the reasoning carried luminous meteors in its train.[1] Finally Schiaparelli, in 1871, concluded an able Memoir on the subject with the decision that “a stellar origin for meteorites was the most likely and that meteorites were identifiable with shooting-stars.”[2] A pregnant remark this, though not exactly as the author thought, for instead of proving both interstellar, as he intended, both have proved to be solar bound.