due to the light reflected from the earth. Rothmann was the first to point out the error of Copernicus respect.. ing the precession of the equinoxes, and the trepidation before spoken of: this was in a letter to Kepler, 1590.

Tycho Brahe.

The middle of the sixteenth century, we have seen, was rendered memorable by the publication of the great work of Copernicus: the close of it was adorned by the discoveries of Tycho Brahe. He was born in 1546, of a noble Danish family and while at the university of Copenhagen, the occurrence of a great solar eclipse, in 1560, is said to have excited in his mind an irresistable desire for astronomical pursuits. For these he quitted the legal profession; and going to Augsburg, he inspired with a similar love of science Peter Hainzell, an opulent citizen of that place, who, at his own expense, erected an observatory, in which Tycho continued his labours for several years. In 1570, he returned to Copenhagen, and in a retreat near that city had the good fortune to observe, through all its variations, the brilliant new star which appeared in 1572, in Cassiopeia, and disappeared in the following year. His high reputation now attracted the notice, and secured for him the liberal patronage, of Frederic, king of Denmark, who conferred on him a munificent pension, gave him the island of Huene in the Baltic, and erected upon it a splendid observatory, which was named Uranibourg. Here he continued for twenty years, amassing an invaluable collection of observations, made with instruments constructed upon a far larger and more accurate scale than had been hitherto attempted. The arcs were divided up to parts containing ten seconds: the best hitherto made did not extend to divisions of as many minutes.

After the death of Frederic, the enemies of Tycho induced the minister of his successor to withdraw from

him the donation of the late king. Thus he was driven to seek refuge in other countries: and, under the patronage of the emperor Rodolph II. (who appears to have valued him chiefly from the assistance his labours would give to the astrological art), he renewed his observations at Prague. But his health and spirits were broken; and he at length sunk under a lingering disease, in 1601.

In reviewing his scientific labours, we find that in the commencement of them, Tycho felt most sensibly as the capital deficiency in the observatory, the want of accurate measures of time: he successively tried a number of contrivances, and at length the imperfect clocks then constructed, but without any material success. Before the perfect construction of clocks, astronomers were necessitated to adopt various methods of finding the right ascensions, and thence the longitudes of the stars, by observations on their distances from the sun, which was effected by an intermediate comparison with the moon; or, according to Tycho's suggestion, with Venus. He endeavoured to improve upon the accuracy of preceding catalogues, by numerous determinations of this kind. He also invented the simple method of finding the latitude of a place by observing the meridian altitudes of a circumpolar star when above and below the pole. He investigated very fully the correction due to atmospheric refraction; though he ascribed it rather to vapours than to the atmosphere itself, and entirely exploded the trepidation which had so long disgraced the astronomical tables.

In comparing the positions given by Hipparchus with those in his own catalogue of the fixed stars, Tycho was led to the important discovery of the slow diminution of the obliquity of the earth's axis, which is so gradual, that it cannot be detected but by comparison of observations at very distant intervals: its existence was, therefore, long disputed; but modern observations have established it beyond question, and it has been shown to be a consequence of gravitation.

server.

In the theory of the moon, some important discoveries were also made by this indefatigable and skilful obTwo inequalities we have noticed as known to Ptolemy. Tycho discovered a third, called the "Variation." He also showed that there is a small periodical change in the inclination of the moon's orbit: and again, that a periodical change takes place in the motion of its nodes.

Tycho argued ably, from observations on the comet of 1577, against the received Aristotelian doctrine, that comets are only meteors formed in our atmosphere. He determined the horizontal parallax of the comet in question, which was such as to prove that it moved beyond the orbit of the moon. It hence followed that the celestial spaces could not be occupied by solid crystalline spheres. He was violently attacked on this point by the adherents of the school doctrine; and his adversaries completely exposed the weakness of their cause by having recourse to invective, and even personal calumny. The real nature of the cometary motions, however, remained undiscovered; though both Tycho and Mæstlin proposed theories involving epicycles.

The objections which Tycho unhappily entertained against the Copernican system were built partly on physical and partly on theological grounds. He, however, speaks of Copernicus in the highest terms of admiration; and even joins in the refutation of the objections urged against the diurnal motion of the earth by Ptolemy. The argument by which he himself was satisfied of the fallacy of the diurnal rotation was this; that if a stone be suffered to fall from the top of a high tower, it ought to reach the ground far behind the foot of the tower, which it does not. To this Rothmann replied, that a stone is a part of the whole mass, and therefore partakes in the earth's motion.

Against the earth's annual motion, Tycho urged the old and thrice refuted objection of parallax; for the complete answer to which he might have been referred back to Aristarchus. Another argument was derived

from the erroneous impression conveyed by the eye, that the stars have a sensible disk or magnitude; and to have such an apparent magnitude, if as distant as Copernicus would make them, Tycho calculated that their real size must be larger than the whole orbit of the earth. The answer is, we cannot say that this is not the case. But again, the use of the telescope has since shown that the assumption is incorrect, the stars having no real disks.

The theological objections which appeared to Tycho so formidable against admitting the motion of the earth, were deduced from certain passages in scripture, in which expressions occur attributing motion to the sun and rest to the earth. Considering the state of knowledge in his time, it is not a matter of surprise that such objections should have possessed considerable weight. On the one hand, it must be remembered, that the question of the celestial motions had not yet obtained any demonstrative solution: and, on the other, though the light of the Reformation had, before this period, been diffused throughout Europe, yet the general illumination of the age was not such as to allow of accurate discrimination in the use or application of the sacred writings. The philosophical theory could not command absolute conviction; and the use thus made of the authority of scripture was not perceived to be a misapplication. Hence it cannot surprise us, that the two appearing to be at variance, the former was made to give way. In truth, it can hardly be said, that even at the present day such questions are placed upon their right basis. The advance of illumination during several centuries, has hardly yet (among the generality of mankind) exhibited in their just light such contradictions between the letter of scripture and the results of science. But of this we shall see more instructive examples in the sequel. To return now to Tycho. Fully impressed, in his own mind, with the weight of these objections, he directed his thoughts to framing another system, by which they should be avoided. He supposed the five planets to revolve round the sun,

whilst the sun, carrying this system with him, revolved round the earth, at rest in the centre, as did also the moon, at a less distance. This system, no doubt, answered tolerably well the purpose of explaining the actual apparent motions, as far as the accuracy of observation had then been carried. It is, in fact, a modification of the principle of the epicycles: and had it been proposed prior to the system of Copernicus, it might justly have been regarded as a step in the progress of simplification and improvement. But coming, as it did, after the suggestion of that theory, which bears upon it the impress of the grandeur and simplicity of nature, it can only be regarded as a decidedly retrograde movement. It is a melancholy proof not only of the weakness, but of the perverseness of the human mind; not only of a backwardness in the pursuit of truth, but of an unhappy rejection of it when presented to our grasp.

The character of Tycho exhibits a remarkable mixture of acuteness and weakness. He was a firm supporter of astrology, and wrote in defence of it; he believed in omens, and even regulated his conduct in accordance with such warnings. At the same time, his conduct exhibits many instances of liberality and generosity; and, upon the whole, we cannot but regard him as fully entitled to the respect in which he has been generally held.

Reformation of the Calendar.

The period of which we have now been treating was distinguished by a transaction of some general interest, the reformation of the calendar by Pope Gregory XIII., in 1582. We have already noticed the devices adopted to keep the same nominal days and months to the same physical seasons, and the necessity for doing so, arising out of the circumstance, that the solar year is not measured by an exact number of days. The Julian correction, which intercalated a day every fourth year, made the calendar year about eleven minutes longer than