NATURE 



[September 27, 1S94 



OS THE MACXITCDE OF THE SOLAR 

 SiSTE.UA 



'V'ATL'RE may be studied ia iwo wiiely differtm ways. On 

 * the one hand, we may employ a powerful microscope 

 which will render visible the minutest forms and limit our field 

 of view to an infinitesimal fraction of an inch situated within a 

 foot of our[own noses ; or, on the other hand, we may occupy some 

 commanding position, and from thence, aided perhaps by a 

 telescope, we may obtain a comprehensive view of an extensive 

 region. The first method is that of the specialist, the second i- 

 that of the philosopher, but both are necessary for an adequate 

 understanding of nature. The one has brought us knowledge 

 wherewith to defend ourselves against bacteria and microbes 

 which are among the mostly deadly enemies of mankind, and 

 the other has made us acquainted with the great laws of matter 

 and force upon which rests the whole fabric of science. .-VU 

 nature is one, but for convenience of classification we have 

 divided our knowledge into a number of sciences which we 

 usually regard as quite distinct from each other. Along certain 

 lines, or, mote properly, in cert.iin regions, these sciences 

 necessarily abut on each other, and just there lies the we.ik- 

 ness of the specialist. He is like a wayfarer who always finds 

 obstacles in crossing the boundaries between two countries, 

 while to the traveller who gazes over them from a commanding 

 eminence the case is quite different. If the boundary is an 

 ocean shore, there is no mistaking it ; if a broad river or a chain 

 of mountains, it is still distinct ; but if only a line of posts traced 

 over hill and dale, then it becomes lost in the natural features 

 of the landscape, and the essential unity of the whole region is 

 apparent. In that case the border-land is wholly a human 

 conception of which nature takes no cognisance, and so it is with 

 the scientific border-land to which 1 propose to invite your 

 attention this evening. 



To the popular mind there are no two sciences further apart 

 than astronomy and geology. The one treats of the structure 

 and mineral constitution of our earth, the causes of its physical 

 features and its history ; while the other treats of the celestial 

 bodies, their magnitudes, motions, distances, periods of revolu- 

 tion, eclipses, order, and of the causes of their various phenomena. 

 And yet many, perhaps 1 may even say most, of the apparent 

 motions of the heavenly bodies are merely reflections of the 

 motions of the earth, and in s.udying them we are really study- 

 ing it. Furthermore, precision, mutation, and the phenomena 

 of the tides depend largely upon the internal structure of the 

 earth, and there astronomy and geology merge into each other. 

 Nevertheless the methods of the two sciences are widely 

 diflerent, most astronomical problems being discussed quanti- 

 tatively by means of rigid mathematical formulx-, while in the 

 vast majority of cases the geological ones are discussed only 

 qualitatively, each author contenting himself with a mere state- 

 ment of what he thinks. With precise data the methods of 

 astroijomy lead to very exact results, for mathematics is a mill 

 which grinds exceeding line ; but after all, what comes out of a 

 mill depends wholly upon what is put into it, and if the data are 

 uncertain, as is the case in most co.<mological problems, theic is 

 little to choose betueen the mathematics of the astronomer and 

 the guesses of the geologist. 



If we examine the addresses delivered by former presidents 

 of this .Association, and of the sister — perhaps it would lie nearer 

 the truth to say the parent .Association on the other side of the 

 Atlaniic — we khall lind that they have generally dealt either 

 with the recent advances in some broad field of science, or else 

 with the development of some special subject. This evening I 

 propose to adopt the latter course, and 1 shall invite your at- 

 tention to the present condition of our knowledge respecting the 

 magnitude of the solar system ; but in so doing, it will be 

 necessary to introduce some considerations derived from 

 lalioratory experiments upon the luminiferous ether, others 

 derived from cxperimenis from ponderable matter, and still 

 olhcii relating both to the surface phenomena and to the internal 

 Ktructure of the caith, and thus we shall deal largely with the 

 .1 where astronomy, physics, and geology merge into 



.,,- .^.;i:ive distances of the various bodies which compose 

 the solar sy«tcm can be determined to a considerable degree of 

 approximation with very crude instrumcnis as soon as the true 

 plan of the system becomes known, and that plan was taught 



' Ai ir. . <lelivr-red b<fijrc t^c Amrncan .\*»<.»ci.iti.>n for ih-; Atlvanccmciil 

 ! ill Bnwklyn Mealing, August i6, I'jr Ihc retiring President, 



- rtiSM. 



by Pythagoras more than live hundred years before Christ. I' 

 must have been known to the Egyptians and Chaldeans stil' 

 earlier, if Pythagoras really acquired his knowledge of astronomy 

 from them, as is affirmed by some of the ancient writers, but on 

 that point there is no certainty. In public Pythagoras seem- 

 ingly accepted the current belief of his time, which made the 

 earth the centre of the universe, but to his own chosen disciples 

 he communicated the true doctrine that the sun occupies the 

 centre of the solar system, and that the earth is only one of the 

 planets revolving around it. Like all the world's greatest sages, he 

 seems to have taught only orally. A century elapsed before his 

 doctrines were reduced to writing by Philol.ius of Crotona, and 

 it was still later before they were taught in public for the first 

 lime by Ilicetas, or, as he is sometmies called, Nicelas, of 

 Syracuse. Then the familiar cry of impiety was raised, and the 

 Pythagorean system was eventually suppressed by that now 

 called the Ptolemaic, which held the field until it was over- 

 thrown by Copernicus almost two thousand years later. Pliny 

 tells us that Pythagoras believed the distances to the sun and 

 moon to be respectively 252,000 and 12,600 stadia, or taking 

 the stadium at 623 feet, 29,837 and 1492 English miles; but 

 there is no record of the method by which these numbers were 

 ascertained. 



After the relative distances of the various planets are known, 

 it only remains to determine the scale of the system, for which 

 purpose the distance between any two planets suffices. We 

 know little about the early history of the subject, but it is clear 

 that the primitive astronomers must have found the quantities 

 to be measured too small for detection with their instruments, 

 and even in modern times the problem has proved to be an ex- 

 tremely difficult one. .Aristarcus of Samos, who liourished 

 about 270 li.c, seems to have been the first to attack it in a 

 scientific manner. 



Slated in modern language, his reasoning was that when the 

 moon is exactly half full, the earth and sun as seen from its 

 centre must make a right angle with each other, and by iiieasur- 

 ir.g the angle between the sun and moon, as seen from the earth 

 at that instant, all the angles of the triangle joining the 

 earth, sun, and moon would become known, and thus 

 the ratio of the distance cf the sun to the distance 

 of the moon would be determined. Although per- 

 fectly correct in theory, the difficulty of deciding visually upon 

 the exact instant when the moon is half full is so great that it 

 cannot be accurately done even with the most powerful tele- 

 scopes. Of course, .Aristarcus hail no telescope, and he does 

 not explain how he effected the observation, but his conclusion 

 was that at the instant in question the distance between the 

 centres of the sun and moon, as seen from the earth, is less than 

 a light angle by 1/30 part of the same. We should now express 

 this by saying that the angle is 87 ; but Aristarcus knew 

 nothing of trigonometry, and in order to solve his triangle he 

 had recourse to an ingenious but long and cumber.some geomet- 

 rical process which has come down to us, and afl'ords conclusive 

 proof of the condition of Greek mathematics at that time. 

 His concli:sion was that the sun i« nineteen times further from 

 the earth than the moon, and if we combine that result with 

 the modern value of the moon's parallax, viz. 342238 seconds, 

 we obtain for the solar parallax iSo seconds, which is more than 

 twenty limes too great. 



The only other method of determining the solar parallax 

 known to the ancients w.as that devised by Hipparchus about 

 150 B.C. It was based on m;asuring the rate of decrease of the 

 <liaineter of the earth's shadow cone by noting the duration of 

 lunar eclipses, and as the result deduced from it Inppened to be 

 nearly the same as that found by .Aristarcus, sulistanlially his 

 value of the parallax remained in vogue for nearly two thousand 

 years, and the discovery of the telescope was required to reveal 

 its erroneous character. Doubtless this persistency was due to 

 the extreme minuteness of the true parallax, which we now 

 know is far too sm.ill to have been visible upon ihc ancient 

 instruments, and thus the supposed measures of it were really 

 nothing but measures of their inaccuracy. 



The telescope was first pointed to the heavens by Galileo in 

 1609, but it needed a micrometer to convert it into iin accurate 

 measuring instrument, and that did not come into being until 

 '<'39, when it was invented by William G.ascoignc. After his 

 deathin 1644, his original instrument passed to Richard 'I'ownley, 

 who attached it to a fourteen-foot telescope at his residence in 

 Townley, Lancashire, l-^ngland, where it was used by Elam- 

 steed in observing the diurnal parallax of Mars during its oppo- 



NO. 1300, VOL. 50] 



