LESSONS IN ASTRONOMY. 



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LESSONS IN ASTRONOMY. IL 



BARLY ASTRONOMICAL INSTRUMENTS HISTORY OF THE 



BOIKNt K (.mfi/iidO COPERNICUS AND HIS 8YHTEM 



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iii.: i>f tli.' various systems devised by the ancients to 

 nut for tin' grand mechanism of the heavens, we mast bear 



nun. I, and make allowance for, the very imperfect nature and 

 iri.-tiim of tlui instruments they possessed; and when thin 



.ln.', instead of wondering at the errors they mode, we shall 

 often bo surprised at the accuracy of their observations ; some 

 of these which still exist being sufficiently accurate to be at 

 times of service to astronomers in the present day. 



The simplest and probably the most annient astronomical 

 instrument consisted of a vertical pillar set upon an even sur- 

 face, so that, by observing the shadow, the direction of the sun 

 and its altitude at any period might be measured ; by noticing, 

 also, the direction in which the shortest shadow was cast by the 

 pillar, they could ascertain the north and south points of the 

 heavens. It is believed by many that the obelisks and stone 

 illors which were common among Eastern nations were con- 

 structed for some such purpose, and that they were frequently 

 surmounted by a ball, in order that the position of the shadow 

 might be more easily marked. Some of these obelisks were 

 afterwards removed to Borne for the same purpose. These 

 instruments were called gnomons. 



The telescope, which has made such astounding revelations to 

 men of modern times, and which boa so greatly extended their 

 knowledge of the universe, was quite unknown in early ages. 

 Instruments for measuring time were also very imperfect, 

 although, as will be seen further on, the importance of noticing 

 the exact moment of the occurrence of any of the celestial 

 phenomena is very great. Various forms of the sun-dial were 

 in use, but these could only be of service when the sun was 

 shining, and even then could not give very accurate indications. 

 Other instruments were therefore planned, and the one most 

 commonly employed was the clepsydra, or water-clock, in which 

 the hour was shown by the amount of water that had passed 

 through an aperture. Sand was afterwards used in the place of 

 water, as its flow was found to be more regular and even. 



Bather strangely, we have come back very recently to a 

 method of measuring minute intervals of time similar to this old 

 plan. A vessel is provided with a small aperture from which a 

 fine stream of mercury is issuing, and when it is required to note 

 any brief interval as, for instance, that occupied in the passage 

 of a planet between two lines situated in the field of view of a 

 telescope the mercury is diverted into a separate vessel at the 

 moment of the disc of the planet coming into contact with the 

 first line, and allowed to flow on until it has passed the second, 

 when the stream is allowed to flow as at first. The amount of 

 mercury in the vessel is then accurately weighed, and by com- 

 paring it with the amount which is known to flow out in a given 

 interval say, for instance, five seconds the exact duration of 

 the passage can be noted. 



A few other rude instruments were also occasionally employed, 

 but their construction was very imperfect, and we are not there- 

 fore surprised at the slow progress of the science. Among the 

 Romans, too, science never found a congenial home ; glory in 

 war being the object of their ambition, rather than the peaceful 

 yet glorious triumphs achieved by intellect. After the age of 

 Ptolemy little progress appears to have been made, and even 

 known truths were to a great extent forgotten. His system was 

 indeed universally received for many centuries, more especially 

 as it was supported by the authority of Aristotle ; and fresh 

 additions to it, in the shape of eccentrics and epicycles, were 

 made ; but few, if any, new discoveries appear to have been 

 effected, and no noteworthy name appears on the pages of 

 history. 



After the fall of the Boman empire the science found a homo 

 among the Arabians, who, in the eighth century, seem to have 

 devoted much attention to its study, and to have made con- 

 siderable advances in it. By them the length of the solar year 

 was calculated to within a very little of its true amount ; the 

 obliquity of the ecliptic was also measured ; and at a place in 

 the desert, near Palmyra, the length of a degree was ascertained 

 with very creditable accuracy. The Ptolemaic system was, how- 

 ever, firmly received, though many of the more thoughtful and 

 careful observers seem to have been far from satisfied with it, 



84 N.E. 



and expressed their wonder at its manifest disproportion*. 8tik 

 uch is the hold that preconceived notion* obtain over the 

 human mind, especially when those views are supported by 

 priestly authority and made matters of religion, that for 

 centuries no one seems to have referred to the old theory of 

 Anaxagoras, or proposed any new one to clear np the difficulty. 



At length, however, about the year 1472, there was born one, 

 Nicholas Copernicus, who, leaving all the speculations of former 

 observers, inquired for himself into the motions of the celestial 

 bodies. He first examined all the ancient observations he could 

 find, and then commenced for himself a system of close and 

 careful study of the heavens. He compared the actual places 

 occupied by the sun and planets with those which, according to 

 former theories, they ought to occupy, and thus obtained a 

 better knowledge of their irregularities and variations than any 

 astronomer before his time. He continued thin course for many 

 years, and at length arrived at the conclusion that Mercury and 

 Venus revolved around the sun, instead of round the earth. He 

 gradually extended his reasoning further, and at last started 

 his celebrated theory, which regarded the sun as the centre of 

 the system, with the earth and the other planets all revolving 

 in regular order around it. By this grand idea all the compli- 

 cated and bewildering schemes which had puzzled so many 

 observers were at one stroke swept away. Instead of the 

 cumbrous machinery of crystal spheres revolving one within the 

 other, the utmost simplicity is seen to prevail ; order and regu- 

 larity take the place of almost inextricable confusion ; and as 

 the observer transfers his station of observation from the earth 

 to the sun, the planets, which had previously appeared to 

 wander on in ever-varying directions among the stars now 

 retracing their steps, and then, after an interval of rest, starting 

 afresh are seen to be steadily moving on in elliptic orbits 

 around the central luminary of the system. The movements of 

 the inferior planets Mercury and Venus, the reason why they 

 were never seen very far removed from the sun, the retrograde 

 motions of the planets, and their irregular movements, were 

 all clearly explained by this grand yet simple theory. 



We can with difficulty form an idea of the prejudice with 

 which this scheme would be received; the earth was by it 

 degraded from its central place, and reduced to the rank of one 

 of the planets ; and that which men had always been wont to 

 regard as fixed and immovable, was now declared to be in rapid 

 flight around the sun, and, at the same time, to be ever whirling 

 round on its own axis. He himself foresaw the effects of this 

 prejudice, and hence he seems to have been long before he fully 

 accepted the theory, and then to have waited still longer before 

 he ventured to make it public. His work on the subject, entitled 

 " On the Bevolution of the Heavenly Bodies," was finished in 

 the year 1530, but he delayed publishing it for several years, 

 although a few friends, to whom he had communicated and 

 explained his views, at once adopted them and urged him to do 

 so. At last, however, he gave his consent to its being printed, 

 but his dedication almost takes the form of an apology for 

 venturing to suggest such views, and his ideas were put forward 

 rather in the shape of an hypothesis than of a definite system. 



We must not, however, suppose that Copernicus formed a 

 complete system to account for all the motions of the planets ; 

 his life was too short for this task. His work was rather to 

 indicate the true theory of the universe, leaving it for others to 

 trace out more accurately the exact curves in which the planets 

 moved, and to ascertain their various distances, sizes, and rates 

 of motion. This work was token up by Kepler, who has some- 

 times been called the " Legislator of the Heavens," as it was he 

 who first laid down the laws and rules which govern the move- 

 ments of the heavenly bodies. We shall notice more about this 

 celebrated astronomer shortly, but must first look at the labours 

 of another distinguished man who preceded him Tycho Brahe. 

 He was of Danish extraction, and was born very shortly after 

 the death of Copernicus. It is said that his attention was first 

 directed to the science of astronomy by an eclipse which 

 happened at the time predicted, in the year 1560, and incited 

 him to learn something of the wonderful science which enabled 

 such predictions to be made. When at the University of Leipsi(> 

 much of his night was often devoted to observation of the stars, 

 and thus he soon attained considerable proficiency ; but there 

 is one thing which tends rather to lower him in our estimation, 

 and that is his partial rejection of the Coperaican system, and 

 the proposal of a new one, in which the earth occupied the 



