(544) The discovery of Jupiter's satellites, one of the first fruits of the invention of the telescope, and of Galileo's early and happy idea of directing it s new-found powers to the examination of the heavens, forms one of the most memorable epochs in the history of astronomy. The first astronmical solution of the great problem of "the longitude" - practically the most important for the interests of mankind which has ever been brought under the dominion of strict scientific principles, dates immediately from their discovery. The final and conclusive establishment of the Copernican system of astronomy may also be considered as referable to the discovery and study of this exquisite miniature system, in which the laws of the planetary motions, as ascertained by Kepler, and especially that which connects their periods and distances, were speedily traced, and found to be satisfactorily maintained. And (as if to accumulate historical interest on this point) it is to the observation of their eclipses that we owe the grand discovery of the successive propagation of light, and the determination of the enormous velocity of that wonderful element. This we must explain now at large.
(545) The earth's orbit being concentric with that of Jupiter and inferior to it (see fig. art. 536), their mutual distance is continually varying, the variation extending from the sum of the difference of the radii of the two orbits; and the difference of the greater and least distances being equal to the diameter of the earth's orbit. Now, it was observed by Roemer (a Danish astronomer, in 1675), on comparing together observations of eclipse of the satellites during many successive years, that the eclipses at or about the opposition of Jupiter (or its nearest point to the earth) took place too soon - sooner, that ,is, than, by calculationfrom an average, he expected them ; whereas those which happened when the earth was in the part of its orbit most remote from Jupiter were always too late. Connecting the observed error in their computed times with the variation of distance, he concluded, that, to make the calculation on an average period correspond with fact, an allowance in respect of time behoved to be made proportional to the excess or defect of Jupiter's distance from the earth above or below its average amount, and such that a difference of distance of one diameter of the earth's orbit should correspond to 16m26.6s of time allowed. Speculating on the probable physical cause, he was naturally led to think of a gradual instead of an instantaneous propagation of light. This explained every particular of the observed phaenomenon, but the velocity required (192,000 miles per second) was so great as to startle many, and, at all events, to require confirmation. This has been afforded since, and of the most unequivical kind, by Bradley's discovery of the aberration of light (art. 329). The velocity of light deduced from this last phaenomenon differs by less than one eightieth of its amount from that calculated from the eclipse, and even this difference will no doubt be destroyed by nicer and more rigorously reduced observations. The velocity has also been determined by M. Fizeau (by direct experiments with a reflecting apparatus on a most ingenious principle, suggested by Mr. Wheatstone for measuring the velocity of the electric current) at 70,000 geographical leagues, 25 to the degree = 194,600 statute miles per second.