A RIXG OF WORLDS. 



4GU 



the questions of astronomers on such points, 

 while two others have recently been put under 

 examination, the occasion seems a suitable one 

 for giving a brief account of this ring of worlds, 

 of the manner of their discovery, and of the 

 ideas which have been suggested as to their 

 origin. 



If the solar system could be seen at a single 

 view, its appearance at any moment would give 

 no idea of regularity in its construction. The 

 pictures of the solar system in our books present 

 a certain symmetry even when the paths of the 

 planets are shown with their true eccentricity 

 of position (which is, unfortunately, but seldom 

 done). The symmetry is like that of a leaf or 

 flower, not perfect, not geometrical or rigid, but 

 still it is sufficiently striking. But if from a 

 picture of the orbits, presenting this symmetry 

 of appearance, we prick off the positions of the 

 central sun and of the planets in various parts 

 of their paths around him, we can see no sym- 

 metry at all in the resulting set of points. The 

 solar system thus shows how there may be real 

 symmetry of arrangement among bodies apparent- 

 ly scattered without law or order. And it shows 

 us also the part which time plays in educing sym- 

 metry from apparent disorder. Conceive a being 

 so constituted that the circuit even of the planet 

 Neptune around the sun, though lasting more than 

 a hundred and sixty of our years, would seem to 

 last but a single instant, so that to his vision the 

 planet would be visible during its entire circuit 

 even as a spark swiftly whirled round appears 

 as a circle of light. To such a being the solar 

 system'would present a symmetrical and doubt- 

 less a most beautiful appearance. At its centre 

 would be the glowing orb of the sun, round which 

 would appear four rings of light, representing the 

 paths of Mercury, Venus, the Earth, and Mars ; 

 far outside these again four other rings of light, 

 much brighter and with much wider spaces be- 

 tween them, showing where Jupiter and Saturn, 

 Uranus and Neptune, traverse their wide courses ; 

 and between these families will be seen the multi- 

 tudinous intertwining paths of the small planets, 

 scarcely discernible separately, but forming, as a 

 whole, a faintly-luminous ring between the well- 

 defined sets of bright rings marking the paths of 

 the eight planets. We need not here consider ! 

 how the beauty of this scene would be enhanced 

 by the rings of light which the moons of the 

 giant planets and of our earth would produce. 

 Let it suffice to note that the symmetry of the \ 

 solar system, as thus seen, would be altogether j 

 marred if the rings of asteroids were removed, j 



It is not given to man, whose span of life is less 

 than half the orbital period of the outermost 

 planet, to witness, scarcely even to conceive 

 rightly, the scene we have described. But the 

 mathematician can perceive what is necessary to 

 its completeness. Accordingly, the astronomer 

 Kepler, inquiring into the harmonies of the solar 

 system, perceived that one note was wanting ; or, 

 returning to our ideal description of the system 

 as it would be seen if centuries were fractions 

 only of seconds, he perceived that the absence of 

 a certain feature impaired the symmetry of the 

 picture. He saw that though the distance sepa- 

 rating the path of Mars from that of Jupiter is in 

 reality much less than that which separates the 

 path of Jupiter from that of Saturn, the next 

 planet beyond him, yet there is a certain regular- 

 ity in the progression of the distances which re- 

 quires that the space between Mars and Jupiter 

 should not be untenanted, as, according to the 

 astronomy of his day, it was supposed to be. In 

 his youth Kepler had noted the want, and had 

 suggested certain fanciful relations which might 

 be fulfilled by a planet occupying the gap. He 

 had written to Galileo on the subject, who had 

 advised him to base his theories on observed facts 

 only. Later, when unwearying researches for 

 nineteen years had revealed to him the laws of 

 the solar system, Kepler suggested as the relation 

 which connects the distances of the planets that 

 which is now commonly called Bode's law. It 

 may be thus simply expressed : Calling Mercury's 

 distance from the sun 4, the distances of the other 

 planets' orbits from Mercury's orbit are in order 

 as the numbers 3, 6, 12, and so on, doubling as 

 we proceed. According to this law, the distance 

 of Mars from Mercury's orbit should be 12, and 

 the distance of the next planet 24. But there 

 was no known planet at that distance. Jupiter, 

 the planet next beyond Mars, travels at a distance 

 from Mercury's orbit represented on this scale by 

 48, and Saturn — the most distant known planet — 

 at a distance of 91, the former corresponding ex- 

 actly, the latter fairly enough, with the law we 

 have indicated. But the planet which, according 

 to the law, should have traveled between Mars 

 and Jupiter at a distance of 24 from Mercury's 

 orbit, or 28 from the sun, either did not exist, or 

 was invisible. 



In Kepler's day it was thought by many a suf- 

 ficient solution of the difficulty to conclude that a 

 planet formerly traveling along this seemingly va- 

 cant track had been destroyed on account of the 

 wickedness of its inhabitants. And we are told 

 that there were not wanting preachers who used 



