136 A Short History of Astronomy [Cn. V. 



which interfered seriously with the common doctrine of 

 the solid crystalline spheres. He had further opportunities 

 of observing comets in 1580, 1582, 1585, 1590, and 1596, 

 and one of his pupils also took observations of a comet 

 seen in 1593. None of these comets attracted as much 

 general attention as that of 1577, but Tycho's observations, 

 as was natural, gradually improved in accuracy. 



104. The valuable results obtained by means of the new 

 star of 1572, and by the comets, suggested the propriety of 

 undertaking a complete treatise on astronomy embodying 

 these and other discoveries. According to the original 

 plan, there were to be three preliminary volumes devoted 

 respectively to the new star, to the comet of 1577, and to 

 the later comets, while the main treatise was to consist of 

 several more volumes dealing with the theories of the sun, 

 moon, and planets. Of this magnificent plan comparatively 

 little was ever executed. The first volume, called the 

 Astronomiae Instauratae Progymnasmata^ or Introduction 

 to the New Astronomy, was hardly begun till 1588, and, 

 although mostly printed by 1592, was never quite finished 

 during Tycho's lifetime, and was actually published by 

 Kepler in 1602. One question, in fact, led to another 

 in such a way that Tycho felt himself unable to give 

 a satisfactory account of the star of 1572 without 

 dealing with a number of preliminary topics, such as the 

 positions of the fixed stars, precession, and the annual 

 motion of the sun, each of which necessitated an 

 elaborate investigation. The second volume, dealing with 

 the comet of 1577, called De Mundi aetherei recentioribus 

 Phaenomenis Liber secundus (Second book about recent 

 appearances in the Celestial World), was finished long 

 before the first, and copies were sent to friends and 

 correspondents in 1588, though it was not regularly pub- 

 lished and on sale till 1603. The third volume was never 

 written, though some material was collected for it, and the 

 main treatise does not appear to have been touched. 



105. The book on the comet of 1577 is of special 

 interest, as containing an account of Tycho's system of the 

 world, which was a compromise between those of Ptolemy 

 and of Coppernicus. Tycho was too good an astronomer 

 not to realise many of the simplifications which the 



