2oo A Short History of Astronomy [CH. vin., $ 154 



Christiaan Huygens (1629-1695), a native of the Hague. 

 Huygens possessed remarkable ability, both practical and 

 theoretical, in several different directions, and his contribu- 

 tions to astronomy were only a small part of his services 

 to science. Having acquired the art of grinding lenses 

 with unusual accuracy, he was able to construct telescopes 

 of much greater power than his predecessors. By the help of 

 one of these instruments he discovered in 1655 a satellite of 

 Saturn (Titan). With one of those remnants of mediaeval 

 mysticism from which even the soberest minds of the century 

 freed themselves with the greatest difficulty, he asserted that, 

 as the total number of planets and satellites now reached the 

 perfect number 12, no more remained to be discovered a 

 prophecy which has been abundantly falsified since ( 160; 

 chapter XIL, 253, 255 ; chapter xin., 289, 294, 295). 



Using a still finer telescope, and aided by his acuteness 

 in interpreting his observations, Huygens made the much 

 more interesting discovery that the puzzling appearances 

 seen round Saturn were due to a thin ring (fig. 64) inclined at 

 a considerable angle (estimated by him at 31) to the plane 

 of the ecliptic, and therefore also to the plane in which 

 Saturn's path round the sun lies. This result was first 

 announced according to the curious custom of the time 

 by an anagram, in the same pamphlet in which the dis- 

 covery of the satellite was published, De Saturni Luna 

 Observatio Nova (1656); and three years afterwards (1659) 

 the larger Systema Saturnium appeared, in which the in- 

 terpretation of the anagram was given, and the varying 

 appearances seen both by himself and by earlier observers 

 were explained with admirable lucidity and thoroughness. 

 The ring being extremely thin is invisible either when 

 its edge is presented to the observer or when it is pre- 

 sented to the sun, because in the latter position the rest 

 of the ring catches no light. Twice in the course of 

 Saturn's revolution round the sun (at B and D in fig. 66), 

 i.e. at intervals of about 15 years, the plane of the ring 

 passes for a short time through or very close both to the 

 earth and to the sun, and at these two periods the ring is 

 consequently invisible (fig. 65). Near these positions (as at 

 Q, R, s, T) the ring appears much foreshortened, and pre- 

 sents the appearance of two arms projecting from the body 



