THE SUN.
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92,000,000 miles which Mr. Stone obtained from that of 1769,than to the 95,000,000 which Encke published in 1824, andwhich was for more than thirty years generally accepted. Itis also a remarkable tact that successive endeavours to solve theproblem by other methods, subsequent to Encke’s investiga-tion, all indicated a much smaller value than his.
We must not stop to describe these various attempts at anylength. Two of them depend upon certain small irregula-rities respectively occurring in the motions of the Moon , andin the apparent position of the Sun during each lunar month,which involve, in a special manner, the distance of the latter.Others are connected with minute perturbations in the orbitsof Mars and Venus . From these LeVkrrier, in the year 1872,very confidently announced that the Sun ’s distance must inall probability be between 92,200,000 miles and 92,300,000miles.
Another very interesting method is founded upon observa-tions of Jupiter ’s satellites, combined with experimental deter-minations of the velocity of Light . If, by such appliances asthose invented by Fizeau, * or Foucault,! we can measure thevelocity of Light, we may estimate our distance from the Sun by noting how much sooner, or later, than would otherwisebe the case, the eclipses of these satellites are seen to takeplace, according to the distance of the Earth at any given timefrom Jupiter .
It is true that if the Earth is so placed that the Sun isalmost exactly between it and Jupiter , or, in other words, if theEarth is at its greatest possible distance from Jupiter , thatplanet and its satellites will be invisible, owing to their ap-parent proximity to the intense brightness of the Sun ’s lightf.But if the Earth be not very far from such a position, as at Ej,the satellites and their eclipses may be detected, and (as isshown in Fig . VI.) the light coming from them must, in orderto reach the Earth at e,, traverse a distance not much less thanthe diameter of the Earth ’s orbit, in addition to that which itwould traverse to reach the Earth , when, as at e 2 , it is exactly
* See Arago’s “Popular Astronomy,” vol. ii. •