ERA OF COPERNICUS, TYCHO BRAKE, KEPPLER, AND GALILEO. 25 



the planets cut by them in every direction, the inference was clear that the planetary 

 spheres had no material existence. It may seem idle now to notice so absurd a notion 

 as that of the planets moving in solid transparent spheres, but it was not so then ; and 

 even Newton deemed it necessary to argue against the monstrous doctrine in the 

 Principia. The labours of Tycho were also directed towards forming a catalogue of the 

 stars, and he determined the relative and absolute positions of 777, a work inferior as to 

 numbers to preceding catalogues, but vastly superior on account of its scrupulous exact- 

 ness. He amassed, likewise, a regular series of observations on the planets, which after- 

 wards, in the hands of Keppler, materially contributed to the framing of his famous laws. 

 To him, also, we owe the discovery of that inequality of the moon called the variation, 

 that of the inequalities of the motion of the nodes, and of the inclination of the lunar 

 orbit ; and by a comparison of his own observations with previous ones, he was the first 

 to announce the slow diminution of the obliquity of the ecliptic, which the. theory of uni- 

 versal gravitation now teaches, while he commenced the systematic application to the 

 observed altitudes of the sun, moon, and stars, of the correction required for the refraction 

 of light. In practical astronomy the refraction of the atmosphere is an important ele- 

 ment, for, owing to it, all the heavenly bodies appear to us considerably higher than they 

 really are. Let a b, a b, a b, a b, be strata, or layers of the atmosphere, increasing in 

 density towards mn, the surface of the earth. A ray of light from the star s, impinging 



on the atmosphere, will be refracted, or bent, so as to move 

 in the curve rrr A; and as an object is seen in the direc- 

 tion of the ray that meets the eye, the star which is actually 

 at s, will seem, to a spectator at A, to be in the direction c. 

 This refraction, which always acts in a vertical direction, 

 elevates objects above their real place, and hence a body at 

 D, below the horizon H o, will be raised and seen as if at o. 

 Ptolemy, as has been already noticed, was acquainted with 

 the refraction of the atmosphere, and mentions it in a trea- 

 tise on optics, but as it is not alluded to in his astronomical work, it is presumed that he 

 had not then discovered it, and made no practical use of it in observation. The Arab 

 astronomers, likewise, were aware of the fact through him, but to Tycho Brahe the 

 honour belongs of calculating its effects, correcting altitudes by them in a systematic 

 manner, and forming the first table of refractions. He estimated its amount at 34' with 

 reference to bodies in the horizon, which is nearly correct, but erroneously supposed that 

 it did not exist at elevations greater than forty-five degrees. This was owing to the 

 effects of refraction above that altitude being insensible to his instruments. 



The urgent recommendation of Tycho to his own sovereign by the Landgrave William, 

 induced the King of Denmark to offer him an asylum in his own country, and an annual 

 provision for the prosecution of his scientific pursuits. Embracing the offer, he received a 

 grant of the small island of Hoene in the Baltic, opposite Landscrona, and an annual allow- 

 ance of two thousand dollars, with the proceeds of a fief in Norway, and a canonry in the 

 church. Here he laid the foundation of a house for himself and an observatory, which 

 ultimately grew into a fantastic castle, resembling rather the abode of an eastern magician 

 than the home of a sober astronomer. Uraniberg, or the castle of the heavens, expressed 

 the large views, feudal spirit, and undisciplined mind of its architect and tenant. He 

 was an admirable observer, had high thoughts of the dignity of his possession, never 

 forgot his own nobility, was abundantly superstitious likewise, and unskilful at system- 

 atising. These are characteristics plainly stamped upon the edifice which he constructed. 

 Its front elevation, shown in the vignette, extended sixty feet in length, and seventy -five in 

 height. It was surrounded by a wall twenty-two feet high, in the form of a square, each 



