124? Determination of the Sun's Distance. 



Art. XXXI. — On the Determination of the Sun's Distance. 

 By Robert L. J. Ellery, Esq., F.R.A.S., &c, Government 

 Astronomer. 



[Read 27th June, 1863.] 



The sun's distance from the earth forms the great base line 

 of astronomical measurements, and outside the moon all 

 dimensions and distances depend upon it ; so that its deter- 

 mination may well be considered the noblest problem in 

 astronomy. 



It is a problem, too, in which the conditions to render it 

 determinate with any probability of success, are only presented 

 to us at long intervals ; even then it demands all the skill 

 and ingenuity of both the observer and mathematician to 

 obtain the necessary measures, and to deduce from them 

 results unaffected by the complications introduced by the 

 movements of the bodies involved in the determination. At 

 the same time, the object to be attained is of such surpassing 

 interest and importance, that no care or cost can be considered 

 too much to expend in its attainment. 



It should be of special interest to Australians, for it was 

 when Captain Cook was sent in command of the expedition 

 to the South Seas, for the purpose of observing the transit of 

 Venus, in order to determine the sun's distance, in 1769, that 

 he re- discovered Australia and New Zealand, and it is more 

 than probable that the colonization of Australia by the 

 British, owes its origin to that very expedition of Captain 

 Cook. 



The fact of our Victorian Observatory having taken so 

 prominent and successful a part in the late determination of 

 the sun's distance, will possibly render a brief sketch of the 

 history of this problem, and of the methods hitherto adopted 

 in its solution, not altogether devoid of interest. 



Among the Ancients the distance of our great luminary 

 was scarcely anything more than conjecture. The first 

 attempt at its determination appears to have been by 

 Aristarchus of Samos, about 280 B.C., who used a method 

 which only aimed at obtaining the relation of the distances 

 between the earth and the moon, the moon and the sun ; it 

 consisted of measuring the angle between the moon and the 

 sun when the former was dichotomized, in other words, 

 exactly half illuminated — literally, cut in two. It is evident 

 that at this time the earth, moon, and sun will form a 



