SECTION X. 

 ASTRONOMY. 



CHAPTER I. 

 HISTORY OF THE SCIENCE, AND ITS DEFINITIONS. 



ASTRONOMY is that science which teaches the distribution 

 and arrangement of the heavenly bodies ; their true and 

 apparent motions in space ; their magnitude, distance, 

 and physical condition ; and its history presents some of 

 the most brilliant examples of the development of the 

 human mind. 



Astronomy essentially owes its existence as a science, 

 ta well as its present degree of perfection, to continuous 

 observations of the heavenly bodies, from the first watch- 

 ing* of the Chaldean shepherds, to the refined instrumental 

 measurements of the astronomers of the nineteenth cen- 

 But if the accumulated observations of centuries, 

 wlu-ii the genius of man would naturally endeavour to 

 preserve, had not been made subservient for the predic- 

 tion of recurrences of the same phenomena. Astronomy 

 would exist only in name, and cease to hold its high posi- 

 tion among the accurate sciences. Its course, however, 

 lias not always been progressive : it is only within the last 

 few centuries that the errors of antiquity have been com- 

 pletely eradicated. To the invention of the telescope, 

 and its application to graduated instruments, we are in 

 a great measure indebted for this progress, which, in a 

 physical point of view, it ia impossible to over-estimate, 

 for it has permitted great and permanent advances to be 

 made in the accurate observations of the heavenly bodies. 

 By the latest improvements in these instruments, astro- 

 nomers have been enabled to note, with an incredible 

 degree of precision, the apparent places of the fixed stars 

 and planets. In this manner, as the accuracy of observa- 

 tion increased, the errors of the ancient theories exhibited 

 themselves more clearly, and the mind of man has been 

 enabled to explain, on sound and natural principles, the 

 new phenomena which have been revealed. The dis- 

 covery of the aberration of light and of the nutation of 

 the earth's axis, two of the greatest discoveries of any 

 age, have thus resulted, and afforded to Dr. Bradley the 

 means of investigating, by accurate observation of the 

 places of the stars, the preceding discovery by Roemer, 

 of the successive transmission of light, which he had 

 determined by observations of the eclipses of Jupiter's 

 satellites. 



The total overthrow of the Ptolemaic system is owing 

 to the same cause the observations of Tycho Brahe 

 having furnished means of clearly exhibiting to Kepler, 

 the errors of a circular hypothesis, in the same manner 

 as the errors of the Alphonsine tables, based also on the 

 Ptolemaic theory, pointed out to Copernicus the errors 

 of the ancient philosophy, when it was discovered, about 

 the epoch of 1500, that they differed more than two 

 degrees of the truth. Calculation has also lent its aid in i 

 reducing to law, and in grappling with difficulties, prob- 

 lems, which the happy invention of logarithms has 

 rendered accessible, and which, in other respects, without 

 this assistance, would be almost insurmountable. 



We find that, amongst the most ancieut nations of 

 antiquity, the appearance of the heavens was attentively 

 watched. And, in the first place, the motions of the sun 

 and moon commanded attention, in regard both to the 

 duties of the husbandman, and appointments for the 

 assemblage of large masses of the people. The Egyptians 

 md Chaldeans discovered that the motions of these bodies, 



VOL. L 



and the planets, were all performed within a certain com- 

 pass of the heavens, which was termed the Zodiac ; the 

 names of the constellations through which this belt 

 passed, having especial reference to the motions of the 

 sun. We also find that eclipses of the sun and moon 

 were recorded with superstitious veneration ; whilst, in 

 connection with the foolish study of judicial astrology, 

 the positions of the planets, with reference to bright 

 stars, were also carefully noted. The Egyptians and 

 Chaldeans were acquainted with the time of the revolu- 

 tions of the moon's nodes an epoch which they termed 

 the Saros which would give them the means of predict- 

 ing eclipses, recurring, as they naturally would, in the 

 same eras. The Chaldeans were also acquainted with 

 the lunar-solar period of 600 years, which Josephus 

 attributed to the ancient patriarchs, although Pliny cites 

 Hipparchus as its author. Their primitive observations 

 appear to have consisted in the heliacal rising of bright 

 stars, and probably in the use of the gnomon or sun- 

 dial. 



Thales, of Miletus, in Asia Minor, who had studied the 

 astronomy of the Egyptians, is supposed to hav pre- 

 dicted, by a knowledge of the period of 223 lunations, or 

 the Saros, the famous eclipse which occurred hout 

 585 B. c. , and which is recorded by Herodotus. 



Thales, on his return from Egypt, founded the Ionian 

 school (B c. 640). This school had for its followers 

 Anaximander (B.C. 610), Auaximenes (B.C. 530), and 

 Auaxagoras (B.C. 50i)). The first is known by a most 

 useful invention, viz., that of geographical charts. The 

 others added some fanciful notions, iu addition to those 

 of Thales, on the construction of the universe ; and they 

 all appear to have believed in the plurality of worlds. 

 The most enlightened disciple of this school, however, 

 was Pythagoras (B c. 580), who, having been advised by 

 Thales to travel into Egypt to perfect his studies, became 

 initiated in the secret mysteries of the priests in a greater 

 degree than any of his predecessors. He founded a school 

 in Italy on his return, where the doctrines of the Ionian 

 school were more fully promulgated. He appears to have 

 been acquainted with the earth's rotation on its axis, its 

 annual revolution, and its spherical figure doctrines 

 which, in imitation of the Egyptian priests, were kept 

 secret from the multitude. 



But the first approach to a scientific system of astro- 

 nomy was that of the Alexandrian school, where we find 

 observations carried on regularly, and a theory which, 

 though founded in error, rudely satisfied the observations 

 of the period. The most distinguished members of this 

 school were Aristarchus of Samos (B.C. 281), Eratosthenes 

 of Cyrene (B.C. 276), and Hipparchus of Bythinia 

 (B.C. 140). Aristarchus is famous for having made an 

 approximation to the distance of the sun from the earth, 

 by the angular measurement of the distance of the sun 

 and moon, at the time at which the moon is half illumi- 

 nated. Having found that the moon's angular distance 

 at this period was 87, he thence concluded that the sun 

 was distant eighteen or twenty times that of the moon. 

 This method, however ingenious, fails in consequence of 

 the roughness of the moon's surface. By careful obser- 

 vations of the sun's path, he also made an approximation 



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