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National Geographic : 1919 Aug
Contents
THE NATIONAL GEOGRAPHIC MAGAZINE DIAGRAM SHOWING THE USUAL METHOD OF MOUNTING A BIG TELESCOPE 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 the distance. So there has 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 I/6000oo 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 Ioo-inch reflector, the parts to be moved weigh Ioo 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
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