JAXIARY II, 1S95.] 



SCIENCE. 



31 



tlif Pytliajjorean system wasi-vi'iitually sup- 

 pressed by that now ealled the PtoU'iiiaie. 

 whieli held the field until it was overtlirown 

 by Copernieus. almost two tlumsand years 

 later. Pliny tells us that Pythagoras be- 

 lieved the distances to the sun and moon to 

 be respectively -JoL'.oOO and 12.(i00 stadia, 

 or takiufi the stadium at (JL'S feet, 25t.s;j7 

 and 1.4!)'_* English miles; but there is no 

 record of the method by whicli these num- 

 bers were ascertaint'd. 



After the relative distances of the various 

 planets are known, it only remains to de- 

 termine the scale of the system, for which 

 l)urpose the distance between any two 

 planets suffices. We know little about the 

 i-arly history of the subject, but it is clear 

 that the primitive astronomers mu.st 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 extremely difficult one. 

 Aristarchus of Samos. who flourished about 

 '270 B. C, seems to have been the first to at- 

 tack it in a scientific manner. Stated in 

 modern language, his reasoning was that 

 when the moon is exactly half full, the earth 

 aiul sun as seen from its center must make 

 a right angle with each other, and by meas- 

 uring 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 of the sun to 

 the distance of the moon would be deter- 

 mined. Although perfectly correct in theory, 

 the difficulty of deciding visually ui>on the 

 exact instant when the moon is half full is 

 so great that it cannot be accurately done 

 even with the most powerful telescopes. Of 

 course Aristarchus had no telescope, and he 

 dws not explain how he effected the obser- 

 vation, but his conclusion was that at the 

 instant in question the di.stance between the 

 centers of the sun and moon, as seen from 

 the earth, is li'ss than a right angle by ^'^ 



part of the saim'. We sliould now exjyress 

 this by saying that the angle is (S7 degrees, 

 but Aristarchus knew nothing of trig(mom- 

 etry, and in order to solve his triangle, he 

 had recourse to an ingenious, but long and 

 cumbersome geometrical process which has 

 con\e down to us, and aft"ords conclusive 

 proof of the condition of Greek mathematics 

 at that time. His conclusion was that the 

 sun is nineteen times further from the earth 

 than the moon, and if we combine that re- 

 sult with the modern value of the nuum's 

 parallax, viz. : 3,4'22.38 seconds, we obtain 

 for tlu' solar parallax ISO seconds, which is 

 more than twenty times too great. 



The only other method of determining 

 the solar parallax known to the ancients 

 was that devised l>y Hipparchus about 150 

 B. C. It was based on measuring the rat'.' 

 of decrease of the diameter of the earth's 

 shadow cone l)y noting the duration of lunai- 

 eclipses, and as the result deduced from it 

 happened to be nearly the same as that 

 found by Aristai-chus, substantially his valu' 

 of the parallax remained in vogue for nearly 

 two thousand yeai"s, and the discovery of 

 the telescope was required to reveal its er- 

 roneous character. Doubtless this pei"sist- 

 ency was due to the extreme minuteness 

 of the true parallax, which we now know is 

 far too small to have been visible upon the 

 ancient instruments, and thus the sui)posed 

 measures of it were really nothing but 

 measures of their inuc uracy. 



The telescope was first pointed to the 

 heavens by Galileo in 1()09, but it needed 

 a micrometer to convert it into an accut-at'.' 

 measuring instrument, and that did not 

 come into being until 1(539, when it was in- 

 vented by Wm. fiascoighe. After his death 

 in 1644, his original instrument passed to 

 Richard Townley who attached it to a four- 

 teen foot teh'scope at his residt'nce in Towti- 

 ley. Ijancashire. England, where it was usc<l 

 by Flamsteed in observing the diurnal j)aral- 

 lax of ^lars during its oj»positif)ii in 1(>72. 



