Page:Popular Science Monthly Volume 90.djvu/923

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���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

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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.

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��ASTEROID'

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