Popular Science Montlily 



907 



makes records of what it sees, the astronomer 

 works independently of time, to a certain 

 extent. By examining his photographic 

 plates, he some- 

 times makes dis- 

 coveries of phe- 

 nomena that 

 actually took 

 place years ago. 

 But he must com- 

 pare new photo- 

 graphs with old, 

 as a general rule. 

 Since Professor 

 Barnard disco\- 

 ered the fifth 

 satellite of Jupi- 

 ter, no moons 

 have been dis- 

 covered with the 

 eye, but several 

 have been de- 

 tected on photo- 

 graphic plates ex- 

 posed at wide 

 intervals apart. 

 Thus, by com- 

 paring two plates 

 on which Saturn 

 has been photo- 

 graphed at dif- 

 ferent intervals. 

 Professor Wil- 

 liam H. Pickering 

 in 1898 found a 

 ninth satellite of 

 Saturn. Since 

 then a tenth 

 satellite has been found for Saturn and 

 four new ones for Jupiter — all on photo- 

 graphic plates. 



Similarly, photography has proved an 

 invaluable aid 



1 • 1 • STAR 



auring solar 

 ■ clipses. A total 

 eclipse of the 

 sun never lasts 

 more than seven 

 minutes, and 

 usually much 

 less. So short 

 is the time for 

 observation that 

 each of the as- 

 tronomers who 

 constitute an 

 eclipse expe- 

 dition has his spe- 

 cific task during 



The sixteen-inch Metcalf telescope of the Harvard 

 College Observatory, showing an ordinary photo- 

 graphic telescope attached to a visual telescope for 

 following and measuring the movement of asteroids 



ASTEROID TRAIL 



Through the ordinary telescope the star appears as a 

 dot and the asteroid as a dash. Through the Metcalf 

 instnunent this is reversed as in picture on right 



the few precious minutes allotted to him. 

 Some of the men never even look up at the 

 sky; others carefully examine certain parts 

 of the sky around the sun. Indeed, it 

 may be said that the astronomers who 

 study an eclipse never really see it. 



It has long been a subject of debate 

 among astronomers whether or not there is 

 what is known as an intra-mercurial 

 planet — that is a planet much nearer 

 ^ the sun than Mercur>\ Certain ir- 



regularities in the orbit of Mer- 

 cury suggest that perhaps it 

 may be influ- 

 enced by some 

 such body. Mer- 

 cury itself can be 

 seen only at fa- 

 vorable times be- 

 cause of its near- 

 ness to the 

 dazzlingly bright 

 sun. An intra- 

 mercurial planet, 

 if,it existed, could 

 be seen only dur- 

 ing the darkness 

 of a total eclipse. 

 Hence it is part 

 of the duty of 

 every eclipse ex- 

 pedition to search 

 for an intra-mer- 

 curial planet. 

 Nowadays this is 

 done photo- 

 graphically. 

 The apparatus 

 emplo\ ed consists of a batter}- of cameras 

 mounted together and pointed so that they 

 cover the region of the sky immediately 

 surrounding the sun. Any object of ordi- 

 nary size would 

 have been re- 

 corded by them, 

 as the search has 

 been carried on 

 during several 

 recent eclipses, 

 but none has yet 

 appeared. 



Sometime s 

 the photo- 

 graphic plate 

 reveals phe- 

 nomena that 

 astonish even 

 practiced 

 astronomers. 



STAR TRAIL 



ASTEROID' 



