10 HISTORY OF ASTRONOMICAL DISCOVERY. 



whole terrestrial circumference was 250,000 stadia. The great uncertainty that exists, as 

 to the value of the stadium in question, prohibits any appreciation of the measurement : but 

 several important errors were committed in the practical application of a right principle. 

 No allowance was made for the solar parallax, and instead of Syene being under the 

 tropic of Cancer and on the meridian of Alexandria, it is about 50" north of the former, 

 and nearly 3 east of the latter. The principle of the method employed is, however, pre- 

 cisely the same as that which has been acted upon in modern times ; and our more accu- 

 rate results in determining the magnitude of the earth are owing to greater nicety in 

 observation, attention to all the elements which the solution of the problem requires, and 

 more perfect instruments for the measurement of linear and angular distances. The 

 inventor of the method was born at Gyrene, in the year 276 B. c. The third Ptolemy 

 invited him to his capital, giving him the charge of its library; but becoming weary of 

 life at the advanced age of eighty, he died by voluntary starvation, and was succeeded in 

 his office of librarian by the author of the Argonautics. 



We now come to the greatest astronomical name in antiquity that of Hipparchus 



who may be properly regarded, on account of the plans he pursued and the results he ob- 

 tained, as the father and founder of real astronomy. The invention of spherical trigonometry 

 is supposed to be due to him, and undoubtedly the first application of it is, by which the 

 places of the celestial bodies may be fixed, and the variations of their movements exhi- 

 bited with precision. He approximated also closely to the true length of the tropical year, 

 which had been previously held to be 365^ days. This he discovered to be an error in 

 excess, by comparing one of his own observations of the summer solstice, with another 

 made by Aristarchus of Samos, 145 years before. His own determination of 365 days, 

 5 hours, 55 minutes, 12 seconds, exhibits a value greater than the truth by 6' 13" only, 

 as according to Laplace, the length of the tropical year at that time must have been about 

 4"*2 shorter than in the present age. The error can occasion no surprise. It must be 

 remembered, in behalf of the ancients generally to use the words of Delambre, that their 

 astrolabes were nothing but armillary spheres, of no great diameter, and with very small 

 subdivisions of a degree ; and that they had neither telescope, vernier, nor micrometer. 

 " What should we do," he goes on to remark, " even now if deprived of these helps, and if 

 we knew neither the refraction nor the true altitude of the pole, on which point, even at 

 Alexandria, and with armillse of every sort, an error of a quarter of a degree was com- 

 mitted?" At this day we dispute about a fraction of a second ; they could not then answer 

 for any fraction of a degree, and might be wrong by a whole diameter of the sun and 

 moon. The appearance of a new star in the time of Hipparchus is said to have induced 

 him to make a catalogue of the fixed stars, in order that posterity might be able to recog- 

 nise any changes that might take place in the appearance of the heavens. He was well 

 aware of the importance of such a catalogue, especially for observations of the moon and 

 planets ; and in executing the task he rendered essential service to astronomy, and made 

 his ,most remarkable discovery. Comparing the place of the star Spica Virginis, as de- 

 termined by himself, with that assigned to it about 170 years previously by two distin- 

 guished Alexandrians, he found that this star was six degrees distant from the autumnal 

 equinox, whereas the before-mentioned astronomers had found it eight degrees from the 

 equinox. He saw that there must have been either a movement of the star in longitude 

 during the interval, or a contrary movement of the equinoctial point in the heavens. The 

 same phenomenon was observed in relation to other stars ; that while their latitudes had 

 been retained unaltered, they had advanced in longitude ; and hence the retrogradation of 

 the equinoctial points along the ecliptic was inferred, the cause of which remained a secret 

 till the age of Newton. 



The catalogue formed by Hipparchus contained 1080 stars. The labour it involved, 



