Page:Popular Astronomy - Airy - 1881.djvu/222

208 and Jupiter were in such a position as E'J'. The earth went travelling on in its orbit, and came to such a position as E". Jupiter, who is very slow in his motions, travelled perhaps as far as J" in his orbit. And now came the remarkable thing: it was found that, when the earth came to such a position as E", the tables and preliminary calculations upon which had been founded the predictions of the eclipses of the satellites would not apply. The eclipses of the satellites invariably occurred later than they ought to have done. This occurred year after year, and it was a long time before people could guess at the cause. Every time the earth came to that part of its orbit in which it is nearest to Jupiter, the eclipses of the satellites, happened as predicted: every time the earth approached the part of its orbit furthest from Jupiter, the eclipses of the satellites occured later than predicted. At last a very celebrated man, a Dane, of the name of Römer, gave the explanation, that in these latter observations the earth was further off from Jupiter than at the time when those observations were made on which the tables and calculations were founded: and therefore the light from Jupiter had to travel over a path longer by very nearly the breadth of the earth's orbit. Upon this, calculations were made, and the result was this: that the time occupied by the passage of light across the semi-diameter of the earth's orbit is 8m. 18s.; and therefore the time occupied by the passage of light across the whole breadth of the earth's orbit is 16m. 36s. Upon applying corrections, proportionably to the distance, to the observations made in other positions, it was found that they all harmonized perfectly well, and no doubt was left of the truth of the result, that the time the light occupies in travelling from the sun to the earth is 8m. 18s.