% INTRODUCTION. 



must move round the earth. Now as astronomers, by calculations 

 founded on the principles of geometry, are able to ascertain very 

 nearly the distances of the heavenly bodies from the earth and 

 from each other, it appeared that if we conceived the heavenly 

 bodies to move round the earth, we must suppose them endowed 

 with a motion or velocity so immense as to exceed all conception j 

 whereas all the appearances in nature may be as well explained by 

 imagining the earth to move round the sun in the space of a year, 

 and to turn on its own axis once in twenty-four hours. 



The earth, therefore, in the space of twenty-four hours, moves 

 from west to east, while the inhabitants on the surface of it, like 

 men on the deck of a ship, who are insensible of their own motion^ 

 and think that the banks move from them in a contrary direction, 

 will conceive that the sun and stars move from east to west in the 

 same time of twenty-four hours in which they, along with the earth, 

 move from west to east. This daily or diurnal motion of the earth, 

 being once clearly conceived, will enable us easily to form an idea 

 of its annual or yearly motion round the sun. For, as that luminary 

 seems to have a diurnal motion round our earth, which is really 

 occasioned by the daily motion of the eaith round its own axis, so, 

 in the course of a year, he seems to have an annual motion in the 

 heavens, and to rise and set in different points of them ; which is 

 really occasioned by the annual motion of the earth in its orbit or 

 path round the sun, which it completes in the space of a year. Now, 

 as to the first of these motions we owe the difference of day and 

 night, so to the second we are indebted for the difference in the 

 length of the days and nights, and in the seasons of the year. 



Different systems of the universe... .Thalesj the Milesian, 

 who, about 600 years before Christ, first taught astronomy in 

 Greece, had made a sufficient progress in this science to calculate 

 eclipses, or interpositions of the moon between the earth and the 

 sun, or of the earth between the sun and the moon. Pythagoras, a 

 native cf Samos, flourished about fifty years after Thales, and was, 

 no doubt, equally well acquainted with the motion of the heavenly 

 bodies. He conceived an idea, which there is no reason to believe 

 had ever been suggested before, namely, that the earth itself was in 

 motion, and that the sun was at rest. He found that it was impos- 

 sible, in any other way, to explain consistently the heavenly motions. 

 His system, however, was so extremely opposite to all the prejudi- 

 ces of sense and opinion, that it never made great progress, nor 

 was ever widely diffused in the ancient world. The philosophers 

 of antiquity, despairing of being able to overcome ignorance by 

 reason, endeavoured to adapt the one to the other, and in some 

 measure to reconcile them. Ptolemy, an Egyptian philosopher, 

 Who flourished 138 years before Christ, supposed with the vulgar, 

 that the earth was fixed immovably in the centre of the universe, 

 and that the seven planets, considering the moon as one of the 

 primaries, were placed near to it. Above them he placed the fir- 

 mament of fixed stars, then the crystalline orbs, then the primum 

 mobile, and, last of all, the caelum empyreum,or heaven of heavens. 

 All these vast orbs he imagined to move round the earth once in 

 twenty-four hours, and also to perform other revolutions round it, in 

 certain stated and periodical times. To account for these motions., 

 he was obliged to conceive a number of circles, called eccentrics, 

 and epicycles, crossing and interfering with each other. This sys- 



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