HISTORY OF PHYSICAL ASTRONOMY.

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retrograde step in practical astronomy. This circumstance, however,was generally overlooked by all his successors down to the time of Roemer,who has the merit of being the first to restore the use of the circle inastronomical observations. Indeed, it is only in our own day that thequadrant has been definitively abandoned by astronomers as an essentiallyimperfect instrument for determining the apparent positions of the celes-tial bodies.

Although Ptolemy does not seem to have been an observer of the firstorder, he possesses many claims to the admiration of posterity. Abundantproofs of his sagacity are to be found in his great work, the MathematicaSy ntaxis, but his discovery of the inequality in the moon’s longitude, termedthe evection, by a comparison of his own observations and those of Hip-parchus with the computed places of the moon, would alone entitle himto be ranked with the greatest astronomers of ancient or modern times.

Very little is known respecting the mode in which time was measuredby .the ancient astronomers. In the daytime this object seems to havebeen effected by means of dials. During the absence of the sun theyused clepsydras of various sorts, but all such contrivances were found tobe very imperfect, on account of the unequal flow of the water. In someinstances the time of an observation was indicated by the point of thezodiac which was on the meridian. This method, when practicable, wasdoubtless preferable to any other with which the ancient astronomers wereacquainted.

The Arabian astronomers do not seem to have effected any essential im-provement in the methods of observation. Their instruments, however,were generally larger and better constructed than those of the Greek astro-nomers, and they appear to have taken greater precaution to ensure theaccuracy of their results. The astrolabe, as used by them, was, in someinstances, a complicated instrument, since it carried circles representingthe equator, the ecliptic, and the other principal circles of the celestialsphere. In this form it acquired the appellation of an armillary sphere.Instruments of a similar construction continued to be used in makingobservations of the celestial bodies down to the beginning of the seven-teenth century.

The Arabians usually indicated the time of an observation by meansof the apparent altitude of a star whose position was known. It hasbeen mentioned that Ptolemy has given no account of the method heemployed in tracing a meridian line. The Arabian astronomers effectedthis object by equal altitudes of the sun when he was east and w T est of themeridian*. It does not appear that they took into account the effect ofthe motion of the sun during the interval included between the two ob-servations.

Upon the revival of science in Europe towards the close of the fifteenthcentury, the cultivation of astronomy was for some time confined almostexclusively to Germany. Waltherus, a native of Nuremburg , to whomwe have already had occasion to allude, may be considered the earliestindividual of modern times whose observations have contributed to theadvancement of astronomical science. He first introduced the practice ofdetermining the apparent place of a planet by observations of its altitude,and its distance from two stars whose places had been already ascertained.

* Delambre, Hist. Ast. Moyen Age, p. 129. The astronomers of India appear alsoto have practised this method by tracing upon the sand a circle around the foot of thegnomon.