1^6] Kepler s Epitome and his Book on Comets 193 



which has attracted so much attention in the last half- 

 century (chapter xm., 3 01 )- 



146. The treatise on Comets (1619) contained an account 

 of a comet seen in 1607, afterwards famous as Halley's 

 comet (chapter x., 200), and of three comets seen in 1618. 

 Following Tycho, Kepler held firmly the view that comets 

 were celestial not terrestrial bodies, and accounted for their 

 appearance and disappearance by supposing that they moved 

 in straight lines, and therefore after having once passed 

 near the earth receded indefinitely into space ; he does 

 not appear to have made any serious attempt to test this 

 theory by comparison with observation, being evidently 

 of opinion that the path of a body which would never 

 reappear was not a suitable object for serious study. He 

 agreed with the observation made by Fracastor and Apian 

 (chapter in., 69) that comets' tails point away from the 

 sun, and explained this by the supposition that the tail is 

 formed by rays of the sun which penetrate the body of 

 the comet and carry away with ' them some portion of its 

 substance, a theory which, allowance being made for the 

 change in our views as to the nature of light, is a curiously 

 correct anticipation of modern theories of comets' tails 

 (chapter xm., 304). 



In a book intended to have a popular sale it was 

 necessary to make the most of the " meaning " of the 

 appearance of a comet, and of its influence on human 

 affairs, and as Kepler was writing when the Thirty Years' 

 War had just begun, while religious persecutions and wars 

 had been going on in Europe almost without interruption 

 during his lifetime, it was not difficult to find sensational 

 events which had happened soon after or shortly before 

 the appearance of the comets referred to. Kepler himself 

 Was evidently not inclined to attach much importance to 

 such coincidences ; he thought that possibly actual contact 

 with a comet's tail might produce pestilence, but beyond 

 that was not prepared to do more than endorse the pious if 

 somewhat neutral opinion that one of the uses of a comet is 

 to remind us that we are mortal. His belief that comets are 

 very numerous is expressed in the curious form : " There 

 are as many arguments to prove the annual motion of the 

 earth round the sun as there are comets in the heavens." 



