328 POPULAR SCIENCE MONTHLY. 



the northwest frontier of Persia under Nasr-ed-Din as chief astronomer. 

 Here the Ilkhanic tables were prepared. Ulugh-Beg, prince of 

 Samarkand, a grandson of Tamerlane, founded a great observatory in 

 1420, on the hill of Kolik, where a hundred observers and calculators 

 were employed. Albategnius, another Arab prince, possessed ad- 

 mirable instruments, as we have seen. Astronomy was in favor with 

 princes and caliphs, and flourished accordingly. We have seen that 

 some of the instruments of Albategnius read to one minute of arc 

 (1') and were very likely divided to 2'. The observatory of Nisapur, 

 in Khorassan, had in A. D. 851 a huge armilla reading to 1'. In 

 992 Al Chogandi set up at Bagdad a sextant of sixty feet radius. In 

 1260 the observatory of Meraga possessed, among many other in- 

 struments, a mural quadrant of twelve feet radius. Ulugh-Beg had 

 a quadrant (perhaps a species of sun-dial) that had a radius of 180 

 feet. Colossal instruments of the sort permitted accurate readings 

 of angles because the space corresponding to an arc of one minute 

 was correspondingly large. Until the invention of the telescope 

 accuracy was only to be attained by the use of large circles, and the 

 Arabian school anticipated Tycho Brahe in the use of such instru- 

 ments by several centuries. Some of the Arabian observers employed 

 free-swinging pendulums to measure short intervals of time; and the 

 science of gnomonics — the theory of sun-dialing — was extensively de- 

 veloped by them. 



This is the place to describe the system by which Ptolemy explained 

 the world. It will be sufficient to explain the two main problems 

 that any system of astronomy was bound to consider, and to leave 

 details to one side. These two chief problems were: (1) How to 

 account for the rising and setting of the sun, moon, stars and planets 

 — how to explain the general diurnal motion of all celestial bodies; 

 (2) how to explain the motions of the planets among the stars. These 

 motions are, in general, towards the east — but are varied by occasional 

 westward motions, and interrupted by periods of no motion at: the 

 'stations.' As we have seen, Ptolemy declared the earth to be a 

 sphere fixed in the center of the heavens. The sphere of the fixed 

 stars was at an immense distance, so that the earth was a mere point 

 in respect of the distance of the stars and the stars revolved about 

 the earth. All the observed phenomena of the rising and setting of 

 the stars are satisfactorily explained in this way. Ptolemy perfectly 

 understood that they could also be explained by the hypothesis of a 

 rotating earth, but he concluded that it was easier to attribute motion 

 to bodies like the stars which seem to be of the nature of fire, than 

 to the solid earth. The sun, moon and planets share in the diurnal 

 motion of the stars. It will be seen that no mechanical conception 

 of the diurnal motion is attainable in this way without the assump- 

 tion of crystal spheres. Ptolemy sought an analytic device by which 



