no A Short History of Astronomy [CH. iv. 



equinox, the earth has reached D (fig. 41), the sun is again 

 in the plane of the equator, and the day is everywhere equal 

 to the night. 



83. Coppernicus devotes the first eleven chapters of the 

 first book to this preliminary sketch of his system; the 

 remainder of this book he fills with some mathematical 

 propositions and tables, which, as previously mentioned 

 ( 74), had already been separately printed by Rheticus. 

 The second book contains chiefly a number of the usual 

 results relating to the celestial sphere and its apparent 

 daily motion, treated much as by earlier writers, but with 

 greater mathematical skill. Incidentally Coppernicus gives 

 his measurement of the obliquity of the ecliptic, and infers 

 from a comparison with earlier observations that the 

 obliquity had decreased, which was in fact the case, though 

 to a much less extent than his imperfect observations 

 indicated. The book ends with a catalogue of stars, which 

 is Ptolemy's catalogue, occasionally corrected by fresh 

 observations, and rearranged so as to avoid the effects of 

 precession.* When, as frequently happened, the Greek 

 and Latin versions of the Almagest gave, owing to copyists' 

 or printers' errors, different results, Coppernicus appears to 

 have followed sometimes the Latin and sometimes the 

 Greek version, without in general attempting to ascertain 

 by fresh observations which was right. 



84. The third book begins with an elabprate discussion 

 of the precession of the equinoxes (chapter IL, 42). From 

 a comparison of results obtained by Timocharis, by later 

 Greek astronomers, and by Albategnius, Coppernicus ififers 

 that the amount of precession has varied, but that its 

 average value is 5o"'2 annually (almost exactly the true 

 value), and accepts accordingly Tabit ben Korra's unhappy 

 suggestion of the trepidation (chapter in., 58). An 

 examination of the data used by Coppernicus shews that 

 the erroneous or fraudulent observations of Ptolemy 

 (chapter n., 50) are chiefly responsible for the perpetua- 

 tion of this mistake. 



* Coppernicus, instead of giving longitudes as measured from the 

 first point of Aries (or vernal equinoctial point, chapter i., 'n, 13), 

 which moves on account of precession, measured the longitudes from 

 a standard fixed star (a Arietis) not far from this point, 



