126 Determination of the Sun's Distance. 



third law, it will be apparent, that, if the distance of any one 

 plane b from the earth, can be accurately determined, the 

 relative distances of the others can be converted into absolute 

 ones. This became the next part of the problem. 



The distance of a planet is obtained by ascertaining the 

 angle the earth's radius will subtend at the planet : the 

 larger this angle becomes the more accurate is the result. 

 The nearest planets, of course, will give the largest angles, 

 and for this very sufficient reason Yenus and Mars have 

 always been selected for observation in connection with the 

 determination of the sun's distance. The angle above 

 referred to as subtended by the earth's radius is generally 

 known as the 'parallax, and it is by this name I shall now 

 speak of it. 



Of course the determination of distance from the parallax 

 of a planet pre-supposes that the length of the earth's radius 

 is known, but, although many geodetic enterprises have been 

 undertaken for this purpose extending so far back as some 

 centuries before the Chri stain era, Picard's measure of the 

 French meridianal arc, in 1664, was the first undertaking 

 that furnished reliable results. This measurement, performed 

 with great skill and care, and with a better class 

 of instruments than had been previously constructed, 

 is memorable from the fact that the earth's diameter deduced 

 therefrom enabled Newton to establish his laws of gravita- 

 tion, which laws he had discarded for many years previous, on 

 account of the measures which had been determined from 

 former surveys being so utterly at variance with his theory. 



Since Picard's survey of the French arc, others have 

 followed in quick succession, each more exact than the pre- 

 ceding. 



The methods adopted in modern times for the determina- 

 tion of the sun's distance are two, involving the parallaxes of 

 the two nearest planets, Mars and Venus. 



The parallax of Venus will give the largest angle, and 

 consequently the most accurate result, but she only attains a 

 position favourable for its measure at certain conjunctions, 

 when her latitude is so small that she appears to a terrestial 

 observer to transit across the sun's disc. This forms the best 

 of all conditions for determining the sun's distance from her 

 parallax, but, unfortunately, such occurrences are few and 

 far between. 



The celebrated astronomer Halley, in 1725, wsffe the first 

 to propose the observation of the transit of Venus across the 



