XX 



THE PROGRESS OF 



KUtionary. It then acquired a motion in a con- 

 trary direction, and, after proceeding for a cer- 

 tain time westwards it became stationary, and then 

 moved eastwards as at first. These motions were 

 not easily reconciled with a uniform circular 

 motion. The explanation, however, was attempt- 

 ed by Apollonius Perseus. He conceived that, 

 in the circumference of a circle having the earth 

 for its centre, there moved the centre of another 

 circle, in the circumference of which the planet 

 actually revolved. The first of these circles was 

 called the deferent, and the second, the epicycle ; 

 the motion in the circumference of each was 

 supposed uniform. Lastly, it was conceived that 

 the motion of the centre of the epicycle in the 

 circumference of the deferent, and of the planet 

 in the epicycle, were in opposite directions ; the 

 first being towards the east, and the second 

 towards the west In this way, the alterations 

 from progressive to retrograde, with the inter- 

 mediate stationary points, were readily explain- 

 ed ; and Apollonius carried his investigation so 

 far as to determine the ratio between the radius 

 of the deferent and that of the epicycle, from 

 knowing the stations and retrogradations of any 

 particular planet. 



Hipparchus, the greatest astronomer among 

 the ancients, discovered the inequality of the 

 sun's apparent motion round the earth. To ex- 

 press this irregularity, he imagined an epicycle 

 of a small radius, with its centre moving uni- 

 formly in the circumference of a large circle 

 having the earth for its centre, while the sun 

 revolved uniformly in the circumference of the 

 small circle, but in a contrary direction. 



As other irregularities in the motions of the 

 moon and planets were observed, other epicycles 

 were introduced ; and Ptolemy, in his Almagest, 

 enumerated all which then appeared necessary, 

 and assigned to them such dimensions as enabled 

 them to express the phenomena with accuracy. 



By this contrivance the system of the heavens 

 became extremely complicated. But it had the 

 advantage of subjecting all the motions of the 

 sun, moon, and planets very readily to a geome- 

 trical construction, or an arithmetical calculation 

 of no great difficulty. Hence the predictions of 

 astronomical phenomena, the calculation of 

 tables, and the comparison of these tables with 

 observations, became easy ; and upon this the 

 progress of the science depends. We have no 

 evidence that the ancient astronomers ever con- 

 sidered the epicycles and deferents, which they 

 employed in their systems, as having a physical 

 existence, or as serving to represent these motions, 

 they merely employed them to enable them to 

 calculate the apparent motions of the heavenly 

 bodies. 



When Europe began to awake from the 



lethargy of the dark ages, astronomy was the 

 first of the sciences which drew the attention of 

 men of science. Purbach and Rugiomontanus 

 contributed most towards its advancement during 

 the 15th century. Purbach resided at Vienna, 

 under the patronage of the emperor Frederick 

 the Third, and devoted himself to astronomical 

 observations. He published an edition of the 

 Almagest ; and though he neither understood 

 Greek nor Arabic, yet his knowledge of the 

 subject enabled him to make it much more per- 

 feet than any former translation. Regiomontanus 

 was a pupil of Purbach, and became much more 

 celebrated than his master. 



Copernicus, who had the merit of first divining 

 the true system of the universe, was born at 

 Thorn, in the year 1473, studied at Cracow, and 

 ultimately went into the church. A decided tnste 

 for astronomy led him, when very young, to the 

 study of that science. It occurred to him, at a 

 very early period, to consider what effect the 

 motions of the heavenly bodies would have upon 

 a spectator, transferring that motion to the 

 objects observed, but ascribing to it an opposite 

 direction. It became immediately obvious, that 

 the rotation of the earth, on its axis from west to 

 east, would produce the apparent motion of the 

 heavens from east to west 



In considering the objections which might be 

 made to the system of the earth's motion he 

 reasons soundly, though he was not aware of the 

 full force of his own argument. Ptolemy had 

 alleged, that if the earth were to revolve on its 

 axis, the violence of the motion would be sufficient 

 to tear it in pieces, and dissipate the parts. 

 Why, says Copernicus, was he not more alarmed 

 for the safety of the heavens, if the diurnal 

 motion be ascribed to them, as their motion must 

 be more rapid in proportion as their magnitude 

 is greater ? 



We need not mention that Copernicus placed 

 the sun in the centre, and taught that all the 

 planets moved round that luminary in orbits 

 nearly circular. The moon revolved round the 

 earth, and the apparent diurnal revolution of the 

 heavens from east to west was owing to the real 

 diurnal revolution of the earth from west to east. 

 The fi^st edition of the Astronomia Instaurata, 

 in which these doctrines appeared, was dedicated 

 to the pope, and was published in 1543, a few 

 days before the death of the author. 



After Copernicus, Tycho Brahe was the most 

 distinguished astronomer of the sixteenth cen- 

 tury. An eclipse of the sun, which he witnessed 

 in 1560, when he was a young man, in conse- 

 quence of the exactness with which it answered 

 the prediction, inspired him with a veneration 

 for the science, and an anxious desire to become 

 acquainted with it Unfortunately for his pro- 



