Ill 



NEWTON'S THEOEY OF PLANETARY 

 MOTIONS 



PERHAPS the first system of celestial motions 

 meriting the name scientific was that devised by 

 Claudius Ptolemy, who lived about the middle of 

 the second century of our era. According to him, the earth 

 was the center of the universe around which revolved 

 daily, in the order of their supposed distances from it, the 

 Moon, Mercury, Venus, the Sun, Mars, Jupiter, Saturn, 

 and the stars. Beyond the stars, again, was the Primum 

 Mobile, or Prime Mover. To prevent the planets and 

 stars from falling down upon the earth, Ptolemy con- 

 ceived of them as set in separate crystalline spheres ; and 

 these latter, he supposed, by the noises of their rotation, 

 produced the " music of the spheres". Strange to say, 

 this notion of celestial harmonies survived the discovery 

 of Copernicus, for Kepler speaks of it, and assigns to 

 Jupiter and Saturn the bass, to Mars the tenor, to Venus 

 the contralto, and to Mercury the soprano parts in this 

 heavenly chorus. 



Having assumed that all the celestial bodies rotated 

 around our earth, it was quite natural to surmise, in the 

 first instance, that they did so in exact circles. In our 

 practical age we should be apt to do the same, were we 

 approaching the problem for the first time; but if we 

 should then speedily find that observation materially con- 

 tradicted theory, we would doubtless promptly try out 

 some other sort of curve in the effort to find a short cut. 



