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THE NATIONAL GEOGRAPHIC MAGAZINE 



DIAGRAM SHOWING THE USUAL METHOD 

 OF MOUNTING A BIG %-EESC'OPE 



The big telescopes are so mounted that the 

 principal axis is on the meridian and parallel 

 to the axis of the earth. Then, as the earth 

 moves from west to, east, a clock movement 

 carries the barrel of the telescope in the oppo- 

 site direction, so that it always points at the 

 same spot in the sky as long as an observation 

 is being made. The other — or declination 

 axis— is at right angles to that of the earth, 

 and is used to train the instrument on the path 

 of the star under observation. 



star and keep it there requires that it be 

 mounted on two bearings, one at right 

 angle to the other. 



To understand the function of these 

 two bearings, imagine yourself on a 

 merry-go-round, looking through a spy- 

 glass at a house away off in the distance. 

 In order to keep the house in the field 

 of vision, you would "have to move the 

 big end of the glass backward as you 

 traveled forward. The earth is the 

 merry-go-round and the star is the house 

 in tlie distance. 



So there lias to be one bearing that 

 will permit the line of vision in the tele- 

 scope to move backward just as fast as 

 the earth moves forward. Our terres- 

 trial merry-go-round is rotating at the 

 rate of about 1,040 miles an hour at the 



Equator, but the sun and the stars are 

 so distant that we seem to pass them 

 very slowly, though their speed as well 

 as their brightness is magnified in the 

 telescope. 



To keep the telescope moving back- 

 ward as the earth flies forward is at once 

 a very big and extremely delicate task. 

 Imagine swinging a huge instrument 64 

 feet long and weighing, with its movable 

 parts, 22 tons, through the air with such 

 nicety of poise that the spider thread in 

 the eyepiece, which is 1/6000 of an inch 

 in diameter, is kept constantly cutting in 

 two a star image that is 1/2500 inch in 

 diameter. 



Yet that is what is done at the Yerkes 

 Observatory with the big telescope. ^ In 

 the case of the Mount Wilson 100-inch 

 reflector, the parts to be moved weigh 100 

 tons. In all the instruments the move- 

 ment is made by a huge clockwork that 

 carries the big barrel as steadily as ever 

 an hour-hand of a full-jeweled watch was 

 driven by its mechanism. 



"shooting" the stars 



But if we imagine ourselves in the 

 merry-go-round and looking at the house 

 in the distance through a spyglass, we not 

 only have to turn it backward as we move 

 forward in order to keep the house in 

 view, but we cannot see it at all if the 

 glass be pointed too high or too low. 

 However, when we get our spyglass at 

 the proper elevation we do not have to 

 raise or lower it thereafter. 



So also with the big telescope. The 

 astronomer has to put it in the nightly 

 path of the star across the sky before he 

 can follow it in its journey. To do this 

 requires a second bearing, or axle. 



The observer consults his star tables to 

 see exactly how far above the Pole the 

 star's path is. He then moves the lever 

 of an electric motor, and the great tube 

 begins to rise until it is trained on that 

 path. A big graduated circle, distinctly 

 marked and numbered, tells the approxi- 

 mate position. For the exact position, it 

 is adjusted with a slow motion, the ad- 

 justment being determined by a very fine 

 circle, the marks on which are read 

 through microscopes. 



The astronomer now consults his star 

 tables again and finds the star's position 



