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HISTORY OF PHYSICAL ASTRONOMY.
This method, however ingenious, laboured under the disadvantage ofbeing practicable only on rare occasions. The other method contrivedby Hipparchus for attaining the same end was more general in its appli-cation. When the sun was still above the horizon, he determined thedifference of the longitudes of the sun and moon with the astrolabe, andas soon as the star became visible after sunset, he ascertained, in a similarmanner, the difference of the longitudes of the moon and star. It is clearthat from these two observations he obtained the difference of the longi-tudes of the sun and star, taking into account the correction due for themotion of the moon during the intermediate interval; and as the place ofthe sun with respect to the equinox was known by the solar tables, hehence deduced the absolute longitude of the star. When the longitudeof one star was accurately found by this process, the longitude of anyother star might be directly derived from it by simultaneous observationsof both stars with the astrolabe. It was thus that the genius of Hippar-chus triumphed over a difficulty which long appeared to be insurmount-able, and thereby enabled him to effect with complete success one of themost stupendous undertakings recorded in the annals of science.
Ptolemy , although the most eminent astronomer of antiquity, afterHipparchus , does not rank high as an observer. It has been alreadymentioned that his catalogue of the stars has been strongly suspected tobe no other than the catalogue of Hipparchus , reduced to his ow'n time bythe application to all the longitudes, of what he conceived to be the truequantity of the alteration occasioned by the precession of the equinoxes.An examination of his labours on other subjects of astronomy tends toconfirm this impression. He nowhere enters into that minuteness ofdescription which indicates a bond fide observer anxious to persuade hisreaders of the excellent qualities of his instruments, and of the precautionswhich he employed to ensure the accuracy of his results. But what ismost to be deplored is his suppression of all the ancient observations,including those of Hipparchus , with the exception of the few whichsquared with his own calculations. The loss of these precious materialshas involved many interesting points of the ancient astronomy in impene-trable obscurity.
An account has been given of the circular instrument employed by theancient astronomers in determining the meridional altitude of the sun.Ptolemy rejected the complete circle, adopting in its stead a quadrant,which he considered to be preferable. The vertically of the instrumentwas established by means of the plumb-line. A small cylinder fixed at thecentre threw its shadow upon a similar body which moved along the dividedlimb, and thereby indicated the altitude of the sun *. The Greek astro-nomers appear to have divided their instruments only to every ten minutes.It is remarkable that Ptolemy does not describe any method for placinghis instruments in the plane of the meridian.
Ptolemy ’s substitution of the quadrant for the complete circle was a
ing stars. Even if the eclipse occurred a few days before or after the equinox, the placeof tiie sun, and consequently that of the moon, might still be determined within the limitsof the errors of the ancient observations, by supposing the sun to revolve uniformly roundthe earth in 365j days. It is possible that Timoeharis may have found in this mannerthat the star Spica Virginis preceded the autumnal equinox 8 3 in his time, but it ismore probable that Hipparchus , who cited this fact as a proof of the precession of theequinoctial points, was in reality the individual who first established its existence by a dis-cussion of the eclipses of the moon observed by Timoeharis, and the use (f his own solartheory.