Tycho introduced Kepler to the acquaintance of' the Emperor Rodolph, which led to his appointment as mathe matician to tit,: empelor, a title which he retained under the successive reigns of Matthias and of Ferdinand. Not withstanding this kindness, Kepler complained of an un willingness on the part of his friend, either to promote his interest, or to make hint acquainted with his discoveries and improvements. The death of Tycho, however, in 1601, put an end to those feelings of discontent, which might soon have ripened into a serious quarrel.
The mind of Kepler was deeply tinged with the errors of astrology. In 1602, he published at Prague, his Nova Disscrtatiuncula de fundamentis Astrologic certioribus, ad cosmotheoriam spectans, in which he speaks of the possi bility of predicting future events by the aspects of the pla nets. And in another work on the same subject, which appeared at Frankfort in 1610, entitled Tertium Interve nfens, he exhorts theologians, physicians, and philosophers, but principally Philip Feselius, to take care that they do not injure their own interests, by rejecting the errors of astrology ; and he attempts to shew, that an influenco may be communicated from the planets to the earth, through the medium of the light which they reflect.
In 1604, he published at Frankfort. his Paralillomena ad Vitcllonenz, quibus Astronomic pars Optica traditur, potissi nzim de artlficzosa observatione et estimatione diametrorum, deliquiorumque Solis et Lune. This work contains the me thod of calculating eclipses, which is in use at the present day. In 1605, he published at Prague, his Elzistola ad re rum celestium anzatores universos, respecting the eclipse in the month of October 1605 ; which was followed in the same year, by his Epistola de Cometa 1604, which was the prelude to his great work on the star in the foot of Serpentarius. In 1606, he printed at Frankfort, his Silva Chronologica ; and in the same year appeared his work,. entitled De Stella Novo in pede Serpentarii et qui sub ejus exortum de novo iniit triguno ignco. Libellus Astronomicis, Physicis Metalzhysicis, et Jistrologicis disputationibus en doxis et paradoxis litmus. In 1608. Kepler published at Halle, a history of the new comet of 1607, with a discourse on the nature and motion of comets, and their indications.
In looking at the sun in a camera obscura, he maintained that he had seen Mercury on the sun's disc. He published the same error at Leipsic, in 1609, in a pamphlet, entitled Phenomenon singulare, seu Mercurius in Sole ; but in his Ephemerides for 1616, he acknowledges his mistake, and congratulates himself on having been the first that observ ed the spots of the sun His famous work, entitled Astro nonzia nova sive Physica cclestis tradita conz mentariis de nzotibus sten& Martis ex observationibus Ty chonis Brahe, appeared at Prague in 1609. This work, as we have already shewn, (sec our article ASTRONOMY, Vol.
p. 596,) conducted its author to the great discovery of the true form of the planetary orbits. In 1611, appeared his Dissertatio cum Nuncio Sidereo nuper ad mortales misso a Galileo, in which he announces to Galileo, the great satis faction which his discoveries had given him. He express es his hope, that he may discover other satellites round Sa turn and Mars. He states his astonishment, that tele scopes had not been made formerly, particularly after what Baptist's Porta had said of them in his Magia Naturalis ; and, in addition to many interesting observations, he conjec tures that Jupiter had a motion of rotation about his axis.
In 1611, he published at Augsburg, his Dioptricee; item examen pralationis Jo PENAE Galli in Optica Euclidis ; De usu Optices en Philosophia. This work was reprinted at London, in ;653. Descartes is said to have borrowed free ly from it. \Vhen Descartes was charged with this, he acknowledged that Kepler was his first master in optics, and that lie understood more of the subject than all his predecessors. In the same year appeared his Strena. sets do Nive scxangula, in which he seems to have been the first that discovered the tendency of water to crystallize in an gles pf 6o°.
In 1615. Kepler published his Eclogee chronicc ex episto lis doctissinzorunz aliquot virorum et suis nzzauis ; and in the same year at Limz, his Nova Stereometria doliorum vina riorum, which he is said to have in consequence of a revenue officer having made a false measurement of his %%Me at the time of his marriage His Ephemerides Novx Mot. Celest. ab anno 1617, appeared at Lintz in 1617.