DISTANCE AND GRANDEUR OF THE SUN 95 



elusions as this. In the face of such consistent results it is 

 with difficulty that we can credit the statement that he regarded 

 the moon as a body larger than the earth. The report is the 

 more incredible since he had so closely computed its distance, 

 and since its visual diameter is so readily measurable. On the 

 other hand, his computation as to the height of the earth's 

 atmosphere, the first we know of, would command respectful 

 attention for any physical investigation he might make. 



But for either of these estimates, which reached so singularly 

 near the truth, what sort of accuracy may we presuppose ? 

 What idea can we gain of their value ? 



The large armils erected at Alexandria for Eratosthenes by 

 the enlightened Ptolemy Euergetes had a scale, as we know, 

 divided down to the sixth of a degree. A moment's considera- 

 tion suffices to show that, to be of any value for stellar measure- 

 ment, they must have been at least fifteen or twenty feet in 

 diameter. They were doubtless made with exceeding accuracy, 

 for some of the observations made with them have been of 

 value in verifying the conclusions of modern astronomers. Pro- 

 bably they were better instruments than any known in 

 Christendom until Tycho's day. But even with these they 

 could make but rather rough approximations. From a careful 

 discussion of the evidence, Bailly l concludes that the accuracy 

 of the ancients did not surpass five minutes of arc, or one- 

 twelfth of a degree. The parallax of the sun, as we know now, 

 is less than ten seconds of arc, or not much more than one- 

 fiftieth of the limit within which these giants of the past could 

 be sure. How then could they guess so well ? 



Baffled by difficulties, the ingenuity of man will sometimes 

 turn in unexpected ways. The ancients may have found a 

 path that is lost to us. We cannot say. What we know is 

 the sun's parallax could have been measured correctly by no 

 instrument then known. 



Galileo, with his telescope, sought it in vain. All he could 

 know was that it was unobservably small, and the sun therefore 

 almost infinitely far away. It was not until after twenty-three- 

 year-old William Gascoigne, who fell at Marston Moor, had 

 slipped a wire-netting across the focus of the telescope, that 

 the observation was possible. Galileo, Descartes, Kepler, all 

 were dead when it had, at last, in 1670, been made. 

 1 Histoire de I'Astronomte Moderne, i. 457. 



