914 



ASTKONOMY. 



[KAKLY BISTOBT. 



to the diameter of this body, which he considered to be 

 T j th part of the whole dailv motion an observation not 

 Yery far from the truth. 11 in tract mi tlio RMgnttod* and 

 diitanrv* of the nun and moon ha* been preserved. In 

 addition to the tenet* of the Alexandrian school, be waa 

 of opinion that the stun were at distances immensely 

 greater than th sun, on which account this astronomer 

 must hold a high rank among those of antiquity. 



Eratosthenes of Gyrene, who liv.-d n..-. 270, is known 

 for a determination of the obliquity of tho ecliptic \<y 

 observations of the altitude of the sun at the summer ami 

 winter solstice* an observation of great value, as show- 

 ing the progressive diminution of this element when 

 compared with modern observations. He also made an 

 approximation to the magnitude of the earth, on correct 

 principle*, by the measure of the celestial arc, included 

 between Syene and Alexandria, combined with tho 

 known distance between the two places. Syene was one 

 of the most southern cities of ancient Egypt, win r.- it 

 happened that at the summer solstice the sun was exactly 

 vertical, which he determined by the fact that -a deep well 

 was wholly illuminated. At Alexandria, which waa 

 situated in the same meridian, and at the same period 

 of time, the sun's zenith distance was 7 12', or one- 

 lift ieth part of the whole circumference. The distance of 

 tin- two place* was known to be 5,000 stadia, whence he 

 concluded that the circumference of the earth was 250,000 

 stadia. The great uncertainty of the length of the stadium 

 does not permit us to make use of this determination, or 

 to compare it with the method pursued with more 

 accurate instrumental means by modern astronomers. 



Apollonius of Perga deserves also to be mentioned as 

 having invented the system of deferents and epicycles, for 

 the purpose of accounting for the direct, stationary, aud 

 retrograde appearances of the planets. 



But one of the greatest astronomers of antiquity, Hip- 

 parch us (n. c. 140), now appeared, famous alike as an 

 accurate observer and geometer. To this distinguished 

 man we are indebted for the discovery of the " precession 

 of the equinoxes." The appearance of a new star led 

 him to form a star catalogue, by a comparison of which, 

 v nh a former catalogue of the same school, this important 

 element became evident. He was also acquainted with 

 the unequal motion of the sun in its orbit, and the in- 

 equality of the solar rays. The former he endeavoured 

 to explain by a modification of tho preceding system of 

 deferents and epicycles, which will be explained in its 

 proper place. 



The revolution of the moon's nodes, and the inclination 

 of its orbit to the ecliptic, were also known to Hipparchus, 

 as well a* the eccentricity of the sun's orbit and the motion 

 of the aphides. By a comparison of his observations of 

 the sun with the former astronomers, he determined the 

 length of a tropical year. His solar tables were held in 

 high estimation by Ptolemy three centuries after their 

 construction. 



my (B.C. 130) collected the observations of the 

 Alexandrian school in his Almagest, a work which existed 

 a* a text-book among astronomers for centuries. He is 

 famous for having discovered the erection of the moon, 

 imperfectly known to Hipparchus ; aud by a comparison 

 of observation* of eclipses, aud at the other parts of tin; 

 moon's orbit, he empirically determined its amount with a 

 great degree of accuracy. He confirmed the value of the 

 precession of the equinoxes, previously discovered by Hip- 

 puvhu*. He also proportioned the magnitude of the 

 deferents and epicycles, for the purpose of explainingthe 

 planetary motions, which luul been previously invented 

 i>y Apollouius and Hiuparchus, whence the name of the 

 " Ptolemaic System.'' In his Geography, he collected 

 the longitude* and latitudes of all known places. II,- 

 also left behind him many other works, testifying to hi* 

 tkill and genius a* a philosopher. 



After the death of Ptolemy, the Alexandrian school 

 xistod only in name. During five centuries, the dis- 

 overie* of Hippanhns ami Ptolemy were not extended; 

 and the Uboun of it* followers consisted principally in 

 commenting on their works, and in noting extraordinary 

 phenomena. 



After the dissolution of tho Alexandrian school, the 

 science of astronomy in Kurope had almost disappear. ,1 ; 

 but among the Arabs, it had been cultivated with some 

 success under the auspices of the more enlightened 

 caliphs. The Almagest of Ptolemy was translated into 

 tiu-ir language, and their annals transmit to us many ob- 

 servation* of the sun, moon, and planets, as well as the 

 measure of a degree of latitude. Among the Arabian 

 astronomers, Albatcgnius deserves honourable mention a* 

 an industrious an-1 an-urate observer. He confirmed and 

 corrected the rate of the precession of the equinoxes, and 

 also determined the obliquity of the ecliptic, which, from 

 several collateral circumstances, is entitled to great con- 

 fidence. He also paid considerable attention to the 

 theory of the sun, and determined the eccentricity of its 

 orbit with accuracy. He also found that the apogee of 

 the nun was subject to a small annual displacement, 

 according to the order of the signs, and determined its 

 position at that period to be in the constellation Gemini, 

 which, when compared with tho modern elements, does 

 not differ more than 40' from the truth. Tho astronomy 

 of the Arabians does not open to us any new theories, but 

 is merely an extension of the system of Ptolemy. In the 

 art of observation they had, however, improved ; larger 

 instruments and more refined calculations had been in- 

 troduced by them, on which account their investigations 

 are entitled to hold a high rank in the history of 

 astronomy. 



After the Persians had thrown off the yoke of the 

 Arabians, Holeg Ilecotikhau, one of tho Persian sove- 

 reigns, founded an academy of astronomers, where they 

 formed new tables based on the Ptolemaic system, with 

 trilling changes in their elements. But the greatest 

 praise is due to Ulugh Beigh, one of their princes, who 

 was a great patron of this science, as well as an observer, 

 and who formed a catalogue of stars, and improved tho 

 solar aud planetary tables. He also determined tho 

 obliquity of the ecliptic, and the precession of the 

 equinoxes. 



The next epoch of the history of astronomy commences 

 with its revival in Europe, when, after the overthrow of 

 the Arabian empire, it again became cultivated in Spain. 

 Alphonso, the tenth king of Castile, collected a body of 

 astronomers at Toledo ; and having made use of the ob- 

 servations of the Arabians for the purpose of correcting 

 the planetary motions, published the Alphonsitie tables, 

 which are more correct than any which preceded them in 

 some respects ; but their accuracy was not commensurate 

 with the great trouble aud expense they occasioned. The 

 great confusion of the Ptolemaic system, caused Alphonso 

 to say, "that if ho had been consulted at tho Creation, 

 he could have devised a better arrangement." In 

 Austria, the study of astronomy was cultivated under 

 the auspices of Albert, tho third duko, and Frederick 

 the second emperor. In the 15th century, i'urb.-uh, 

 Kegiouioutanus, and \Valtho lived, known as assiduous 

 cultivators of this science. Kpheinerides of the planets 

 and eclipses were published for the meridian of Vienna 

 from 1475, first by Kegiomontanus, who, being also 

 an observer, was well acquainted with the errors of tho 

 Alphonsino tables, the place of Mars l>ein_; ;il>ove 2 3 in 

 error. The art of measuring time received an important 

 improvement by the substitution of clojk-work motion 

 for the clepsydra; of the ancients, which also increased 

 the accuracy of observations ; aud about this period the 

 true system of the universe forced itself on the attention 

 of Copernicus. 



This distinguished astronomer who saw clearly the 

 errors of the I'tolemaic system, and who, after thirty- 

 six years of study, published, in a clear and conn, 

 form, the present system of tho world was born at 

 Thorn in Prussia, in 1473, four years before the death 

 'lornoiitanus. Tho ancient systems of astronomy, 

 as relati I l.y I'lntarch and Cicero, had, by their extvemo 

 simplicity, captivated his mind ; aud to this circumstance 

 wo owe his explanation of the celestial motions; an ex- 

 planation which, however imperfect in several respects, 

 shows his originality. In his system, he adopted the 

 hypothesis of tho earth's rotation on its axis, its annual 



