12 



HISTORY OP ASTRONOMICAL DISCOVERY. 



parently described by the moon and stars ; and on a calm autumnal or winter night, 

 when the winds are lulled, and scarce a twig stirs, or a ripple is seen upon the waters, 

 but the " river wanders at its own sweet will," how perfect the illusion in which the 

 scene involves the senses the quiescence of our terrestrial globe, and the ceaseless revo- 

 lution of the firmament round it as a central point ! These were dogmas firmly held by the 

 Platonists and Aristotelians, who stoutly clung also to the idea of circular orbits and uniform 

 velocities in relation to the celestial luminaries ; because the circle was deemed the most 



perfect of all geometrical figures, 

 and on that account alone proper 

 to represent their motions ; and be- 

 cause of the fancy, that in such 

 divine and eternal bodies no irregu- 

 larity can exist. 



We have seen, however, that the 

 Greeks had a cultivated acquaint- 

 ance with other movements besides 

 the apparent diurnal revolution of 

 the sphere, movements in an opposite 

 direction to it. They had carefully 

 traced the sun's path in the eclip- 

 tic, that of the moon in her orbit, 

 and they had marked likewise the 

 striking peculiarities of planetary 

 motion. To an observer on the 

 earth, the planets appear to pursue 

 a course of the most irregular and 

 unsystematic kind, which was a 

 perfect puzzle to the ancient theorists. Sometimes they seem to go along with the 

 sphere, but with greater celerity ; then, to remain stationary ; then, to retrograde, or 

 make an angular movement, or describe a circuit like a loop in a knot of ribbons. The 

 paths of Mars and Jupiter, as observed from the earth, described the courses roughly 

 represented in the diagram, at the intervals stated. It seems surprising therefore, in 

 the face of these appearances, that the centrical station of the 

 earth, and all the orbs of heaven moving round it in perfect 

 circles, should have become so much the grand leading idea 

 of the ancient mind. Hipparchus, indeed, was too well aware 

 of the imperfection of observation, and of the importance of 

 supplying data upon which to found a theory, to be given to 

 theorising himself. But when he did systematise, he departed 

 from one of the cardinal maxims of his compeers and succes- 

 sors, by supposing the sun to revolve round the earth in a 

 circle, the earth not being at the centre. He had found the in- 

 terval between the vernal equinox and the summer solstice 94^ days, and that from the 

 summer solstice to the autumnal equinox only 92^ days, thus making the length of the 

 summer half year 187 days, and that of the winter half year 178 days. To account for 

 this unequal division of the year, and to reconcile it with the idea of the sun's orbit being 

 a circle and his velocity uniform, Hipparchus dismissed the hypothesis of the centrical 

 position of the earth. He supposed an eccentric circle, according to which, A B c D 

 may represent the solar orbit, E the place of the earth, not coincident with F the centre 

 of the orbit. E F will be the eccentricity, G H the line of the apsides, I the position of 



Mercury. $ Venus. Sun. 



Jupiter. ]? Saturn. sjc Fixed Stars. 



