HISTORY OF PHYSICAL ASTRONOMY.

437

andrian School for the sound principles of observation which they appearto have practised, as well as for the care with which they determined someelements of fundamental importance, it is to Hipparchus alone that theestablishment of astronomy, as a science of calculation based upon ob-served facts, is to be attributed. The mode in which he availed himselfof the results of observation, in the execution of this great work, is in thehighest degree interesting. One of the most important elements of astro-nomical science is the length of the tropical year. Hitherto it had beensupposed to consist exactly of 365J days. Hipparchus , however, havingcompared a solstice observed by himself with one observed by Aristarchus 147 years earlier, found that the sun arrived in the same place 12 hourssooner than he ought to have done, if the year had consisted exactly of366^ days. He, therefore, concluded that the true length of the year wasless than this quantity by T J B tli part, and consequently he determined itto be equal to 3G5 d 5 h 55 ra 12 s . This is the earliest example, in thehistory of astronomy, of the correction of a fundamental fact of the sci-ence by the comparison of two distant observations. The result obtainedby Hipparchus on this occasion exceeds the true length of the tropicalyear by about six minutes; but the magnitude of the error is not to bewondered at, when the difficulty of determining the exact instant of thesolstice is taken into account*.

Another beautiful example of the simplicity of the means employed byHipparchus in establishing the basis of his theories, is exhibited in liis re-searches on the elements of the solar orbit. Having found that an intervalof 04 d 12' !1 elapsed between the vernal equinox and the summer solstice,while only 92 d 12 h were included between the summer solstice and theautumnal equinox, he from these two facts deduced the eccentricity of thesolar orbit, and also the place of the apogee. His mode of deriving theanalogous elements of the lunar orbit from three observed eclipses ofthe moon, is also equally worthy of admiration.

This illustrious astronomer did not content himself with merely ob-serving the moon on those occasions during which she was eclipsed, anddetermining her place in the heavens by means of the sun, which was thenopposite to her. He also observed her position in other parts of herorbit by means of the astrolabe, an instrument of which he is supposed,with great probability, to have been the original inventorf. Ptolemy

* The equinoxes admit of more accurate determination than the solstices, but therewere no early observations of this kind accessible to Hipparchus . He did not fail, how-ever, to observe several equinoxes with great care, in order that the results might serve asuseful materials, at some future period, for the advancement of astronomical science. Pto-lemy, coming 160 years after Hipparchus , might have deduced a further correction to thelength of the tropical year, but lie has sadly damaged his reputation as an observer by hisresearches on this subject. Lalande, however, by comparing 9 equinoxes determined byHipparchus , with the observations of modern astronomers, has obtained, 365 d o h 48 ni 48 sfor the length of the tropical year; a result which does not perhaps differ one second fromthe true value. (Mem. Acad, des Sciences, 1782, p. 249.)

f Certain passages in the works of Pliny and several other ancient writers, concurin supporting this assertion. But, indeed, apart from all positive statements, there is astrong presumption that Hipparchus was the inventor of the astrolabe, since it is almostimpossible to conceive that it could be of any service to astronomy previous to his time.Thus, one of the objects of the astrolabe was to determine tire longitude of the moonrelative to the sun in different parts of her orbit; but no such observations were made bythe earlier astronomers, since they were unacquainted with any theory of the moon’smotion, and therefore had no obvious motive for observing that body except in eclipses.Another object of the astrolabe was to determine the difference of the right ascensions