ABSTRACT
Kepler devoted his Astronomia Nova to defining the force which moves the planets, or, more precisely, substituting a quasi-magnetic, physical force for the ‘animal’ force of the Mysterium Cosmo-graphicum; and to ascertaining the strict mathematical laws governing its action, as well as elaborating a new theory of planetary motion based on observational data provided by the labours of Tycho Brahe. There was an interval of ten years between the Astronomia Nova (published in 1609 though completed in 1607) and the Mysterium Cosmographicum. They were ten eventful years full of discovery. 1 The Mysterium Cosmographicum was not a success, notwithstanding the enthusiastic support of Maestlin, 2 who took a definite stand in favour of Copernicus against Tycho Brahr 3 in the excellent preface to the new edition of the Narratio Prima of Georg Joachim Rheticus, which, together with an appendix containing the calculations he had made for Kepler, was added to the printed copy of the Mysterium Cosmographicum. The work did not arouse the enthusiastic reception Kepler expected. 4 Nevertheless, it drew the attention of the great Danish astronomer to Kepler, and the sequel was to be of great importance. Tycho Brahe whilst at Wandsbeck, where he had taken refuge after having been obliged to quit Denmark, received a letter (dated 13 December 1597 5 ) from Kepler in 1598 in which the latter asked his opinion on the Mysterium Cosmographicum. Tycho Brahe made a very gracious, though slightly ironic, reply, saying that it had given him much pleasure to receive and peruse the Mysterium Cosmographicum. Whilst he criticized the work—he expressed the opinion that astronomy should proceed a posteriori, and try to find concordant relationships only after having established the facts—he nonetheless recognized the usefulness of Kepler's speculations as well as the cleverness of the author whom he invited to pay him a visit for the purpose of discussing these matters and comparing the theories with his accumulated observational data. 6 He wrote as follows:
‘Your book entitled Prodromus Dissertationum Cosmographicaum, I have already seen and perused as far as my other occasions permitted me to do. It has really given me more than an ordinary measure of pleasure. Your shrewd intelligence and keen mind shine clearly therein; it was an original and ingenious idea to relate, as you have done, the distances and periods of the planets to the symmetrical properties of the regular solids. There seems to be a reasonably good agreement on the whole, and the slight discrepencies with respect to the proportions given by Copernicus are of no great importance, for they themselves disagree considerably with the phenomena. I heartily commend the ardour you have shown in making these enquiries. Yet I should not care to say that you are right in everything. By using the true values for the eccentricities of the planets as obtained by myself over many years, it would be possible to make more accurate verification; but as I am at present much occupied in preparing and publishing my astronomical works, which I was unable to finish in Denmark, I have not the time to make such a comparison. Perhaps it will be possible some other time.’
