THE FLYING EARTH 115 



moon in terms of the earth's measure, demonstrating that the 

 diameter of the sun compared with that of the earth is greater 

 than 19 : 3 and less than 43 : 6, that is, between six and seven 

 times ; he therefore estimated the grandeur of the sun at three 

 hundred times that of the earth. And we have the word of 

 Aristotle that an approximate measure of the earth had been 

 made yet another century before Aristarchus had pointed his 

 astrolabes at the Alexandrian sky. It is an error to suppose 

 that it was a mere question of geometrical methods or measures 

 of mere dimensions ; for the dimensions were known, at 

 least in the right relative proportions, to the founders of the 

 two rival systems. 



The difficulty, as the learned Schiaparelli has been at the 

 length of an interesting memoir to prove, lay elsewhere. One 

 was the invention of a new system, which represented all the 

 known facts equally well. That was the theory of epicycles, 

 picturing the planets as gyrating through a circular path round 

 a circle. This system, worked out in detail by Hipparchus 

 from the mathematical developments of Apollonius of Perga, 

 was adopted by Ptolemy, and held the field until Tycho Brahe's 

 day. And it is curious to reflect that, within the confines of 

 pure astronomy, the absolute disproof could not come until 

 the establishment of the parallax of the stars, in our own time. 

 Even the simplicity of the Coppernican scheme could not prevail 

 over it until a new science was born. That was dynamical 

 mechanics, the mechanics of motion, unknown to Hipparchus' 

 and to Aristarchus' era. Though the one might think it absurd 

 that about the lesser earth the greater sun should turn, the 

 other evidently did not. 



The keen and restless-minded Eratosthenes could conceive 

 the sun as a body not three hundred, but twenty thousand 

 times the bulk of this globe we live on, set it at eighty or ninety 

 millions of miles away, and yet have no difficulty in revolving 

 such a colossus round an unmoving earth, once in each twenty- 

 four hours. The unthinkable speed it must attain seems to 

 have brought no dismay to his mind. He could calculate as 

 easily as you and I that this meant a body at least 200,000 

 miles in diameter, whizzing through space at the rate of 400,000 

 miles per minute, 23,000,000 miles per hour ; and still always 

 describing a perfect circle (so far as he could judge) about this 

 central speck of earth. 



