A HISTORY OF SCIENCE 



in the minds of astronomers. The greatest variety 

 of opinions regarding them prevailed; they were 

 thought on the one hand to be divine messengers, 

 and on the other to be merely igneous phenomena of 

 the earth's atmosphere. Tycho Brahe declared that 

 a comet which he observed in the year 1577 had no 

 parallax, proving its extreme distance. The ob- 

 served course of the comet intersected the planetary 

 orbits, which fact gave a quietus to the long-mooted 

 question as to whether the Ptolemaic spheres were 

 transparent solids or merely imaginary; since the 

 comet was seen to intersect these alleged spheres, it 

 was obvious that they could not be the solid sub- 

 stance that they were commonly imagined to be, and 

 this fact in itself went far towards discrediting the 

 Ptolemaic system. It should be recalled, however, 

 that this supposition of tangible spheres for the 

 various planetary and stellar orbits was a mediaeval 

 interpretation of Ptolemy's theory rather than an 

 interpretation of Ptolemy himself , there being nothing 

 to show that the Alexandrian astronomer regarded his 

 cycles and epicycles as other than theoretical. 



An interesting practical discovery made by Tycho 

 was his method of determining the latitude of a place 

 by means of two observations made at an interval of 

 twelve hours. Hitherto it had been necessary to 

 observe the sun's angle on the equinoctial days, a 

 period of six months being therefore required. Tycho 

 measured the angle of elevation of some star situated 

 near the pole, when on the meridian, and then, twelve 

 hours later, measured the angle of elevation of the 

 same star when it again came to the meridian at the 



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