312 



HISTORY OK ASTRONOMY. 



centre of gravity by twos Hirers, Sec. Compare 

 your observations with those of yoiir predecessors, 

 and you will find Umt new stars have appeared at 

 different times, and afterwards disappeared, and that 

 others have experienced a change in the intensity of 

 their light. Of the actual magnitude ami distance of 

 the .stars we. know nothing. The diameter of the 

 earth's orbit is 200,000,000 miles : yet we can detect 

 no difference in their apparent places, viewed from 

 the opposite points of this diameter : a change of place 

 amounting only to a second would be detected by the 

 accuracy of modem observations: geometrical con- 

 siderations, therefore-, prove that the nearest star can- 

 not be less than twenty billions of miles distant from 

 us. After considering (lie apparent motions of the 

 heavenly bodies, and the real motions which give 

 rise to these appearances, physical astronomy rises to 

 the explanation of the cause, and the investigation 

 of the laws, of the celestial phenomena. Applying 

 the laws of motion to the heavenly bodies, it discovers 

 a force operating throughout, which is called attraction, 

 the amount ot which is directly as the quantity or 

 matter, and inversely as the squares of the distances. 

 By the application of this general principle, it descends 

 to those more refined inequalities, which, owing to their 

 minuteness, or the length of their periods, would es- 

 cape or mislead the observer unassisted by theory. 

 (For a view of the planetary system, the compara- 

 tive magnitudes, distances, &c. of the planets, see 

 the plate accompanying this work, entitled The 

 Planetary System.) 



ASTRONOMY, HISTORY OF. The history of this science 

 begins with the most remote antiquity. The starry 

 heavens must have been one of the first and most 

 striking objects which attracted the attention of man, 

 and his immediate wants compelled him to attend to 

 the revolution of the seasons, the changes of the 

 moon, &c. The most ancient astronomical observa- 

 tions known to us are Chinese. Such a one, men- 

 tioned by Montucla (see p. 455 of his work, vol i., 

 quoted below), viz. a conjunction of Saturn, Jupiter, 

 Mars, Mercury, and the moon, occurs almost 2500 

 years before our era. The Chaldeans also boast of 

 some very ancient astronomical observations, but 

 Ptolemy (q. v.) only mentions two lunar eclipses ob- 

 served by them, about 700 B. C. Still less import- 

 ance does he ascribe to the astronomical knowledge 

 of the Egyptians, although the placing of their 

 pyramids in a position exactly facing the four car- 

 dinal points of the compass, the zodiacs discovered in 

 Egypt (see Zodiac), and other circumstances, are by 

 no means calculated to give us such a disadvanta- 

 geous idea of it. The theory of Bailly, a later historian 

 of astronomy, respecting a nation settled in Middle 

 Asia, and possessed of profound astronomical know- 

 ledge, seems as unfounded as our acquaintance with 

 Indian astronomy is slight. The science made greater 

 progress in Greece, and the Greek philosopher Thales 

 (q. v.), born 640 B. C., calculated a solar eclipse. 

 Pythagoras, also, seems to have been possessed of 

 astronomical knowledge. After him, the Athenian 

 Melon (433 B. C.) introduced the famous lunar cycle 

 of nineteen years, at the end of which time the new 

 moon appears on the same day of the year as at the 

 beginning of it, since nineteen solar years constitute 

 v<^ nearly 235 lunations a discovery which was 

 then regarded as so important, that the calculation 

 was engraved in letters of gold, whence the number, 

 which marks the year of the cycle, is still called 

 golden. Great progress was made in astronomy un- 

 der the Ptolemies, and we find Timocharis and Aris- 

 tyllus employed, about 300 years B. C., in making 

 iiseth. planetary observations. But they were far 

 surpassed, in philosophical spirit, by Aristerchus 

 (q. v.) of Samos, born 267 years B. C., who, accord- 



ing to the indubitable evidence of Archimedes (see 

 t lie remarkable passage in the beginning of Aven- 

 arius), taught the double motion ot the earth around 

 its axis and around the sun ;* and, about 100 years 

 after him, Hipparchus (q. v.) determined more ex- 

 actly the length of the solar year, the eccentricity ot 

 the sun's orbit, the precession of the equinoxes, and 

 even undertook a catalogue of the fixed stars; ausiu, 

 as Pliny (Hist. Natur., lib. 2, cap. 26) expresses him- 

 self, rem etiam Deo improbam, annumerare posteris 

 Stellas, ccelo in hert'tlititti'in cunctis rclicto. From the 

 time of Hipparchus, a chasm exists in the history ot 

 astronomy, till the commencement of the 2d century 

 after Christ, when Ptolemy (q. v.) compiled a com- 

 plete system of astronomy, in thirteen books, which 

 is best known under the name of Almagest, given it 

 by the Arabians, who translated it into their language 

 in 827, and which, as the Ptolemasan system of the 

 world, notwithstanding its many errors, has main- 

 tained its value down to the latest times. Ptolemy 

 supposed the earth to be the immovable centre of 

 the universe, round which all the heavenly bodies 

 revolved as crystalline spheres, and the planets in the 

 order represented by the following cut. 



Thus the moon ((1) occupies the place nearest to 

 the earth, and the six other planets in their order; 

 namely, Mercury () ; Venus (2); the Sun(O); 

 Mars (3); Jupiter (2>); Saturn (5). These are 

 succeeded by the sphere of the fixed stars, otherwise 

 called the firmament, or the eighth sphere, and two 

 other spheres called crystalline, all which were put 

 in motion by the outermost sphere, called the Primum 

 Mobile, or prime mover. Whether what is called 

 the Egyptian System, be of greater antiquity than 

 Ptolemy's, may not be easily determined ; out it will 

 be seen from the following cut, that it more resem- 

 bles the Tychonic than the Ptolemaic system. 



* Aristarchug nays exprewly, 1st c., that /he earth re- 

 olv*s in an ubliijuc circle atouud the 91111, und that Hie 



