23-i Jr. Sarkness — Magnitude of the Solar System. 



a little before that a similar instrument had been invented by 

 Auzout in France, but observatories were fewer then than now, 

 and so far as I know J. D. Cassini was the onty person beside 

 Flamsteed who attempted to determine the solar parallax from 

 that opposition of Mars. Foreseeing the importance of the 

 opportunity, he had Richer dispatched to Cayenne some 

 months previously, and when the opposition came he effected 

 two determinations of the parallax ; one being by the diurnal 

 method, from his own observations in Paris, and the other by 

 the meridian method, from observations in France by himself, 

 Romer and Picard, combined with those of Richer at Cayenne. 

 This was the transition from the ancient instruments with open 

 sights to telescopes armed with micrometers, and the result 

 must have been little short of stunning to the seventeenth cen- 

 tury astronomers, for it caused the hoary and gigantic parallax 

 of about 180 seconds to shrink incontinently to ten seconds, 

 and thus expanded their conception of the solar system to some- 

 thing like its true dimensions. More than fifty years previously 

 Kepler had argued from his ideas of the celestial harmonies 

 that the solar parallax could not exceed 60 seconds, and a little 

 later Horrocks had shown on more scientific grounds that it 

 was probably as small as 14 seconds, but the final death blow 

 to the ancient values ranging as high as two or three minutes 

 came from these observations of Mars by Flamsteed, Cassini 

 and Richer. 



Of course the results obtained in 1672 produced a keen 

 desire on the part of astronomers for further evidence respect- 

 ing the true value of the parallax, and as Mars comes into a 

 favorable position for such investigations only at intervals of 

 about sixteen years, they had recourse to observations of Mer- 

 cury and Yenus. In 1677 Halley observed the diurnal paral- 

 lax of Mercury, and also a transit of that planet across the 

 sun's disk, at St. Helena, and in 1681 J. D. Cassini and Picard 

 observed Venus when she was on the same parallel with the 

 sun, but although the observations of Venus gave better results 

 than those of Mercury, neither of them was conclusive, and 

 we now know that such methods are inaccurate even with the 

 powerful instruments of the present day. Nevertheless Hal- 

 ley's attempt by means of the transit of Mercury ultimately 

 bore fruit in the shape of his celebrated paper of 1716, wherein 

 he showed the peculiar advantages of transits of Venus for 

 determining the solar parallax. The idea of utilizing such 

 transits for this purpose seems to have been vaguely conceived 

 by James Gregory, or perhaps even by Horrocks, but Halley 

 was the first to work it out completely, and long after his 

 death his paper was mainly instrumental in inducing the 

 governments of Europe to undertake the observations of the 



