4 o8 THE POPULAR SCIENCE MONTHLY. 



In observations of this sort upon Mars or the asteroids, the position 

 and displacement of the planet, as seen from different stations, are de- 

 termined by comparing it with neighboring stars. When Venus, how- 

 ever, is nearest us, she can be observed only by day, so that in her case 

 star comparisons are as a general thing out of the question. But 

 occasionally at her inferior conjunction she passes directly across the 

 disk of the sun, and her parallactic displacement from different sta- 

 tions can then be determined by making any such observations as will 

 enable the computer to ascertain accurately her apparent distance and 

 direction from the sun's centre at some given moment. Gregory in 

 1663 first pointed out the utility of such observations for ascertaining 

 the parallax, but it was not until some fifteen years later that the at- 

 tention of astronomers was secured to the subject by Halley, who dis- 

 cussed the matter thoroughly, and showed how the problem might be 

 solved with accuracy by observations such as were entirely practicable 

 even by the instruments and with the knowledge then at command. 

 In 1761 and 1769 two transits occurred which were observed in all 

 accessible quarters of the globe by expeditions sent out by the differ- 

 ent governments. From different sets of these observations variously 

 combined by different computers, values of the solar parallax were 

 obtained ranging all the way from 7.5" to 9.2". A general discussion 

 of all the materials afforded by the two transits was first made by 

 Encke in 1822, and he obtained, as the most probable result, the value 

 8."5776, which from that time for more than thirty years was accepted 

 by all astronomers as the best attainable approximation to the truth. 

 In order to harmonize the results, however, he thought himself obliged 

 to reject the observations of several stations. In 185-1 Hansen, in 

 publishing some of his results respecting the motion of the moon, 

 announced that Encke's value of the solar parallax could not be rec- 

 onciled with his investigations ; within the next six or seven years 

 several independent researches by other astronomers confirmed his 

 conclusions, and the most recent recomputations show that the ob- 

 servations rejected by Encke are as trustworthy as any, and that the 

 errors of observation were so considerable in 1769 that nothing more 

 can be fairly deduced from that transit than that the solar parallax is 

 probably somewhere between 8.7" and 8.9". 



The method of observation then used consisted simply in noting 

 the moment when the limb of the planet came in contact with that of 

 the sun an observation which is attended with much more difficulty 

 and uncertainty than would at first be supposed. The difficulties 

 depend in part upon the imperfections of optical instruments and the 

 human eye, partly upon the essential nature of light, leading to what 

 is known as diffraction, and partly upon the action of the planet's 

 atmosphere. The two first named causes produce what is called irra- 

 diation, and operate to make the apparent diameter of the planet, a& 

 seen on the solar disk, smaller than it really is smaller, too, by an 



