Читать книгу Astronomy of To-day: A Popular Introduction in Non-Technical Language - Cecil Goodrich Julius Dolmage - Страница 9

It is still well under four hundred years since the modern, or Copernican, theory of the universe supplanted the Ptolemaic, which had held sway during so many centuries. In this new theory, propounded towards the middle of the sixteenth century by Nicholas Copernicus (1473–1543), a Prussian astronomer, the earth was dethroned from its central position and considered merely as one of a number of planetary bodies which revolve around the sun. As it is not a part of our purpose to follow in detail the history of the science, it seems advisable to begin by stating in a broad fashion the conception of the universe as accepted and believed in to-day.

The Sun, the most important of the celestial bodies so far as we are concerned, occupies the central position; not, however, in the whole universe, but only in that limited portion which is known as the Solar System. Around it, in the following order outwards, circle the planets Mercury, Venus, the Earth, Mars, Jupiter, Saturn, Uranus, and Neptune (see Fig. 2, p. 21). At an immense distance beyond the solar system, and scattered irregularly through the depth of space, lie the stars. The two first-mentioned members of the solar system, Mercury and Venus, are known as the Inferior Planets; and in their courses about the sun, they always keep well inside the path along which our earth moves. The remaining members (exclusive of the earth) are called Superior Planets, and their paths lie all outside that of the earth.

Fig. 2.—The Copernican theory of the Solar System.

The five planets, Mercury, Venus, Mars, Jupiter, and Saturn, have been known from all antiquity. Nothing then can bring home to us more strongly the immense advance which has taken place in astronomy during modern times than the fact that it is only 127 years since observation of the skies first added a planet to that time-honoured number. It was indeed on the 13th of March 1781, while engaged in observing the constellation of the Twins, that the justly famous Sir William Herschel caught sight of an object which he did not recognise as having met with before. He at first took it for a comet, but observations of its movements during a few days showed it to be a planet. This body, which the power of the telescope alone had thus shown to belong to the solar family, has since become known to science under the name of Uranus. By its discovery the hitherto accepted limits of the solar system were at once pushed out to twice their former extent, and the hope naturally arose that other planets would quickly reveal themselves in the immensities beyond.

For a number of years prior to Herschel's great discovery, it had been noticed that the distances at which the then known planets circulated appeared to be arranged in a somewhat orderly progression outwards from the sun. This seeming plan, known to astronomers by the name of Bode's Law, was closely confirmed by the distance of the new planet Uranus. There still lay, however, a broad gap between the planets Mars and Jupiter. Had another planet indeed circuited there, the solar system would have presented an appearance of almost perfect order. But the void between Mars and Jupiter was unfilled; the space in which one would reasonably expect to find another planet circling was unaccountably empty.

On the first day of the nineteenth century the mystery was however explained, a body being discovered[1] which revolved in the space that had hitherto been considered planetless. But it was a tiny globe hardly worthy of the name of planet. In the following year a second body was discovered revolving in the same space; but it was even smaller in size than the first. During the ensuing five years two more of these little planets were discovered. Then came a pause, no more such bodies being added to the system until half-way through the century, when suddenly the discovery of these so-called "minor planets" began anew. Since then additions to this portion of our system have rained thick and fast. The small bodies have received the name of Asteroids or Planetoids; and up to the present time some six hundred of them are known to exist, all revolving in the previously unfilled space between Mars and Jupiter.

In the year 1846 the outer boundary of the solar system was again extended by the discovery that a great planet circulated beyond Uranus. The new body, which received the name of Neptune, was brought to light as the result of calculations made at the same time, though quite independently, by the Cambridge mathematician Adams, and the French astronomer Le Verrier. The discovery of Neptune differed, however, from that of Uranus in the following respect. Uranus was found merely in the course of ordinary telescopic survey of the heavens. The position of Neptune, on the other hand, was predicted as the result of rigorous mathematical investigations undertaken with the object of fixing the position of an unseen and still more distant body, the attraction of which, in passing by, was disturbing the position of Uranus in its revolution around the sun. Adams actually completed his investigation first; but a delay at Cambridge in examining that portion of the sky, where he announced that the body ought just then to be, allowed France to snatch the honour of discovery, and the new planet was found by the observer Galle at Berlin, very near the place in the heavens which Le Verrier had mathematically predicted for it.

Nearly fifty years later, that is to say, in our own time, another important planetary discovery was made. One of the recent additions to the numerous and constantly increasing family of the asteroids, a tiny body brought to light in 1898, turned out after all not to be circulating in the customary space between Mars and Jupiter, but actually in that between our earth and Mars. This body is very small, not more than about twenty miles across. It has received the name of Eros (the Greek equivalent for Cupid), in allusion to its insignificant size as compared with the other leading members of the system.

This completes the list of the planets which, so far, have revealed themselves to us. Whether others exist time alone will show. Two or three have been suspected to revolve beyond the path of Neptune; and it has even been asserted, on more than one occasion, that a planet circulates nearer to the sun than Mercury. This supposed body, to which the name of "Vulcan" was provisionally given, is said to have been "discovered" in 1859 by a French doctor named Lescarbault, of Orgères near Orleans; but up to the present there has been no sufficient evidence of its existence. The reason why such uncertainty should exist upon this point is easy enough to understand, when we consider the overpowering glare which fills our atmosphere all around the sun's place in the sky. Mercury, the nearest known planet to the sun, is for this reason always very difficult to see; and even when, in its course, it gets sufficiently far from the sun to be left for a short time above the horizon after sunset, it is by no means an easy object to observe on account of the mists which usually hang about low down near the earth. One opportunity, however, offers itself from time to time to solve the riddle of an "intra-Mercurial" planet, that is to say, of a planet which circulates within the path followed by Mercury. The opportunity in question is furnished by a total eclipse of the sun; for when, during an eclipse of that kind, the body of the moon for a few minutes entirely hides the sun's face, and the dazzling glare is thus completely cut off, astronomers are enabled to give an unimpeded, though all too hurried, search to the region close around. A goodly number of total eclipses of the sun have, however, come and gone since the days of Lescarbault, and no planet, so far, has revealed itself in the intra-Mercurial space. It seems, therefore, quite safe to affirm that no globe of sufficient size to be seen by means of our modern telescopes circulates nearer to the sun than the planet Mercury.

Next in importance to the planets, as permanent members of the solar system, come the relatively small and secondary bodies known by the name of Satellites. The name satellite is derived from a Latin word signifying an attendant; for the bodies so-called move along always in close proximity to their respective "primaries," as the planets which they accompany are technically termed.

The satellites cannot be considered as allotted with any particular regularity among the various members of the system; several of the planets, for instance, having a goodly number of these bodies accompanying them, while others have but one or two, and some again have none at all. Taking the planets in their order of distance outward from the Sun, we find that neither Mercury nor Venus are provided with satellites; the Earth has only one, viz. our neighbour the Moon; while Mars has but two tiny ones, so small indeed that one might imagine them to be merely asteroids, which had wandered out of their proper region and attached themselves to that planet. For the rest, so far as we at present know, Jupiter possesses seven,[2] Saturn ten, Uranus four, and Neptune one. It is indeed possible, nay more, it is extremely probable, that the two last-named planets have a greater number of these secondary bodies revolving around them; but, unfortunately, the Uranian and Neptunian systems are at such immense distances from us, that even the magnificent telescopes of to-day can extract very little information concerning them.

From the distribution of the satellites, the reader will notice that the planets relatively near to the sun are provided with few or none, while the more distant planets are richly endowed. The conclusion, therefore, seems to be that nearness to the sun is in some way unfavourable either to the production, or to the continued existence, of satellites.

A planet and its satellites form a repetition of the solar system on a tiny scale. Just as the planets revolve around the sun, so do these secondary bodies revolve around their primaries. When Galileo, in 1610, turned his newly invented telescope upon Jupiter, he quickly recognised in the four circling moons which met his gaze, a miniature edition of the solar system.

Besides the planets and their satellites, there are two other classes of bodies which claim membership of the solar system. These are Comets and Meteors. Comets differ from the bodies which we have just been describing in that they appear filmy and transparent, whereas the others are solid and opaque. Again, the paths of the planets around the sun and of the satellites around their primaries are not actually circles; they are ovals, but their ovalness is not of a marked degree. The paths of comets on the other hand are usually very oval; so that in their courses many of them pass out as far as the known limits of the solar system, and even far beyond. It should be mentioned that nowadays the tendency is to consider comets as permanent members of the system, though this was formerly not by any means an article of faith with astronomers.

Meteors are very small bodies, as a rule perhaps no larger than pebbles, which move about unseen in space, and of which we do not become aware until they arrive very close to the earth. They are then made visible to us for a moment or two in consequence of being heated to a white heat by the friction of rushing through the atmosphere, and are thus usually turned into ashes and vapour long before they reach the surface of our globe. Though occasionally a meteoric body survives the fiery ordeal, and reaches the earth more or less in a solid state to bury itself deep in the soil, the majority of these celestial visitants constitute no source of danger whatever for us. Any one who will take the trouble to gaze at the sky for a short time on a clear night, is fairly certain to be rewarded with the view of a meteor. The impression received is as if one of the stars had suddenly left its accustomed place, and dashed across the heavens, leaving in its course a trail of light. It is for this reason that meteors are popularly known under the name of "shooting stars."