IIISTOKY OF PHYSICAL ASTBONOMY.

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an important contribution to science; but in the beginning of the seven-teenth century, when men’s minds were wavering between the ancientand modern ideas of the system of the world, it exercised an influence ofwhich it is impossible to form an adequate conception in the present day.The existence of four bodies revolving round one of the principal planetsof the solar system, exhibited a beautiful illustration of the moon’s motionround the earth, and furnished an argument of overwhelming force infavour of the Coperuiean theory. The announcement of this fact pointedout also the long vista of similar discoveries which have continued fromtime to time down to the present day to enrich the solar system, and toshed a lustre on the science of astronomy. In more recent times thephysical theory of Jupiter and his attendants has supplied evidence of themost varied and satisfactory character in favour of the principle of Uni­ versal Gravitation . All the irregularities which arise from the mutualaction of the larger bodies of the system are here exhibited in miniature.Their study also offers peculiar advantages to the mathematician, for, asthey generally pass through all their values in short periods, their realcharacter is readily appreciable, and on this account they are eminentlyfavourable for testing the conclusions of his theory. Nor is it merely inits relation to speculative science that the discovery of Jupiter ’s satellitesis to be regarded as of capital importance. The eclipses of these bodiessoon suggested a new solution of the great problem of the longitude.Their theory thus came to be associated with one of those questionswhich most deeply affect the progress of civilization—the promotion ofmutual intercourse between the various nations of mankind,—and a moreearnest and more generally diffused interest was naturally felt in the re-searches connected with its improvement.

When Galileo first turned his telescope to the planets, he was delightedto perceive that they exhibited a round appearance like the sun or moon.Jupiter presented a disc of considerable magnitude, but in no other re-spect was he distinguishable from the rest of the superior planets.Having, however, examined him with a new' telescope of superior power onthe 7th January, 1610, his attention was soon drawn to three small butvery bright stars that appeared in his vicinity, two on the east side andone on the west side of him. He imagined them to he three fixed stars,and still there w'as something in their appearance which excited his ad-miration. They were all disposed in a right line parallel to the plane ofthe ecliptic, and were brighter than other stars of the same magnitude.

This did not, however, induce him to alter his opinion that they werefixed stars, and therefore he paid no attention to their distances fromeach other, or from the planet. Happening, by mere accident, to examineJupiter again on the 8th January *, he was surprised to find that thestars were now arranged quite differently from wliat they w'ere when he

* “ Cum autem die octava, nescio quo fato ductus, ad inspectionem eandem reversusessem.”— Sidereus Nuncius , p. 20.