232 THE WORLD MACHINE 



Some idea of the extraordinary accuracy of modern observa- 

 tions may be gained from the curious discovery of an actual 

 variation in the latitude of several observatories. This variation 

 of latitude is apparently due to a minute change in the position 

 of the earth's axis, so that it describes a circuit around its 

 mean position in the course of about a year and a quarter, 

 though never varying from the mean centre more than thirty 

 feet. This, it may be remarked incidentally, is the seventh 

 of the known periodical movements of the earth, four having 

 been discovered since Coppernicus left Europe vertiginous by 

 announcing three. 



The solar parallax now agreed upon fixes the mean solar 

 distance at very close to ninety-three millions of miles that is 

 to say, approximately four hundred times the distance of the 

 moon. Cassini had computed the distance at but six million 

 miles less. His error was not great. But reflect upon the fear- 

 ful wrench it brought to all the world conceptions which men 

 had treasured from practically the beginning of the intelligent 

 consideration of nature. A sun six or seven times the diameter 

 of the earth was unbelievable enough ; the measurements of 

 Cassini made its diameter more than a hundred times. To our 

 eyes the moon and sun appear of exactly the same size ; could 

 the sun be brought as near to the earth as the moon, its apparent 

 diameter would be more than two hundred degrees ; it would 

 seem to us as large as 160,000 moons ; it would fill the entire 

 heavens, and there would practically be no night. As the 

 amount of heat which the sun may shed upon a planet depends 

 upon the square of the distance, it follows that with the sun 

 at the distance of the moon, the earth would be 160,000 times 

 as hot as now. Nothing living could exist for a second ; it 

 would shrivel in a flash, and the earth itself return to the in- 

 candescent mass from which it sprang. 



The determination of the sun's true distance first made it 

 possible to gain a correct idea as to the size and the dimensions 

 of the solar system. Coppernicus was able to fix, with an 

 accuracy that is still admirable, the relative distances and some- 

 what of the relative sizes of the six planets known to him ; it 

 is obvious that he could have but little idea of their absolute 

 measures. So long as the sun was assumed to be but twenty 

 times the distance of the moon, it followed that, for example, 

 Venus, at three-tenths of that distance, or six times the distance 



