354 THE POPULAR SCIENCE MONTHLY. 



that the earth was the center of the universe ; that the sun, the 

 planets, and the countless stars revolved around and were acces- 

 sory to this little abode of man. 



Ptolemy, the greatest astronomer of the ancients, put this false 

 theory on a strict mathematical basis. By means of his cumber- 

 some system of epicycles he could roughly compute the positions 

 of the planets at any time, could foretell the time of rising and 

 setting of the moon, and predict eclipses. But, while the Ptole- 

 maic system is false, while it does not agree with what we now 

 know to be the true system of the heavens, yet it is mathematic- 

 ally possible. In discussing the various positions and motions of 

 the planets, it would be perfectly possible to consider the earth as 

 the fixed point around which they move ; we could thus arrive 

 at correct results, but the processes would be infinitely long and 

 complicated. And yet a modification of this antiquated method 

 was the only means of tracing further the path of this interesting 

 comet, for the pull of Jupiter became now so strong that, in our 

 backward path, we would have had to take steps, not of a few 

 days, not of a few hours, but of twenty or thirty minutes at a 

 time. The task would have been endless. 



Jupiter was now the ruler of the comet's destiny, the sun a 

 mere disturbing element, so that it became simpler to give Jupiter 

 its just position as ruler at the center of the comet's motion. 



Jupiter was made the momentary center of the universe; 

 comet, sun, earth, and planets were all considered as revolving 

 .around this monster planet. The change of the center of motion 

 ^f rom the sun to Jupiter was easily effected, and the resulting orbit 

 ,of the comet about Jupiter was found to be a hyperbola, an open 

 curve. And now, just as before, this curve is merely the path the 

 comet would have described about Jupiter if it and the planet 

 were the only two bodies in existence ; the long-suffering comet 

 is still pulled and hauled at by various bodies, notably the sun, 

 and step by step its path had to be traced out. At first, steps 

 of ten days each were found to be sufficiently accurate, but as 

 the comet approached closer and closer to Jupiter it began to 

 move faster and faster, and consequently the length of the steps 

 had to be shortened to four days each. After the comet had 

 passed Jupiter the length of the steps was gradually lengthened 

 again. 



The remarkable character of this appulse should be clearly 

 understood. The comet passed the center of Jupiter in 1886, July 

 19th, at no greater distance than two and one third radii of that 

 planet. It must then have passed the surface of Jupiter at a dis- 

 tance of only one and one third radii — that is, the center of the 

 comet was only about sixty thousand miles from the surface of 

 the planet. It is not at all improbable that parts of the diffused 



