128 Determination of the Sun's Distance. 



There is another advantage in this method by Mars, viz., 

 that the -observations need not be confined to a short space 

 of time, as is requisite in the case of Venus, but many 

 repetitions may be made, both as the planet is approaching 

 and receding from opposition, and in combining many 

 observations a greater amount of accuracy is obtained. 

 Were it not for this, the Mars method would claim far less 

 reliability than, with the beautiful graduated instruments 

 now available, it seems entitled to. 



This method appears to have been first used for deter- 

 mining the sun's distance, in 1672, when Richer was sent to 

 Cayenne to observe the planet in conjunction with astro- 

 nomers in Europe ; the result obtained gave the sun's 

 distance eighty-six millions of miles ; although one-sixteenth 

 less than was subsequently found from its nearer approach 

 to the truth than had hitherto been reached, the value of the 

 method was clearly indicated. Cassini shortly afterwards 

 obtained nearly the same result by observing Mars parallax 

 in right ascension, or its displacement with respect to the 

 stars, due to the distance in space an observatory is moved 

 through during a large portion of the earth's revolution. 

 For, suppose the position of Mars with respect to certain fixed 

 stars, be observed when it is far east of the meridian ; when 

 it appears far west of the meridian, the earth will have 

 moved through a considerable amount of space, which can be 

 very easily calculated. Its position with regard to the same 

 stars is then observed, the change in position, combined with 

 base line described by the observatory in the course of the 

 earth's rotation with the motion of Mars in that time, then 

 gives all the data for computing its parallax. A great 

 advantage of this method is, that it can be fully carried out 

 at a single observatory with one observer, but, on the other 

 hand, depen dance has to be placed on the absolute stability 

 of the instrument used for many hours, and this is a most 

 difficult condition to obtain in instruments mounted in a way 

 suitable for such observations. Nevertheless, unusual pre- 

 cautions may be taken to secure it, as was done during the 

 late opposition by Mr. Airy, when he most successfully used 

 this method. 



The transit of Venus, which took place in 1761, was 

 observed at the Cape of Good Hope and in Lapland ; the 

 sun's parallax deduced from the observations was 8" '53, 

 equal to a distance of 95,141,830 miles. For the transit of 

 Venus, of 1769, great efforts were made by most of the 



