The Birmingham age-herald., 1915-08-29

therefore almost surely notice several spots upon its surface and the proba- bility is that a few of these interest Ing objects will be seen which are of an unusually large size.
 MEASURING THE DISTANCE OF THE
 STARS
 When the great astronomer Coper- nicus more than 350 years ago, an nounced his discovery that the earth is not fixed but that 1t Is moving around the sun, this new and star ting idea was opposed, even by scien- lists. on the strictly logical ground that such a motion of the earth must certainly cause an apparent displace- ment of the stars on the sky. Just as when one Is walking through a wood or riding on & railway he sees all ob- jects apparently moving past him in a direction exactly opposite to that in which he is going. The nearer ob- jects because they aresnearer, appear to move the most rapidly, while those which are very distant may seem to be moving very little indeed.
 In exactly the same way. if the earth is really passing around a circular path, any star should seem to be nov Ing in direction exactly opposite to the instant of observation. little re- flcction will show the reader that the total effect of this apparent motion of the star will be to cause 1t to ap- parently describe small closed path In the heavens, completing its circuit about this path in exactly. one year.
 Three centuries ago most careful ob- servations were begun with the most ac- curate instruments that had then ever been constructed in order to see if any such displacement could be found. but apparently no trace of it existed. The req son Is that the stars are so very far away that the greatest displacement of even the nearest of them is quite too small to have been detected with these early instruments. Tn fact. very nearly all of the stars are so distant that even in our most accurate instruments they seem not to be displaced in the slightest de- gree while we on the earth change our position 1S6,O00.O0O of miles in going from one side of our sun to the other. If In the course of all this journey a star Is displaced in its position on the sky by the thickness of the finest spider thread we can measure the displacement and So find the distance away of the star. This work illustrates probably better than any other the extraordinary accuracy of mod ern astronomical observations.
 There are several different methods em- ployed for measuring these excessively minute displaccments. The most direct one Is to actually measure with the sn- called Meridian Circle the successive pos.- tions of the star on the sky-that is, Its distance above or below the Celestial equator and its angular distance from the vernal equinox. If 1t Is found that the point of the sphere occupied by the star does not remain constant but that it apparently moves along a little elliptic curve, we know that this apparent mo- ton is wholly due to the nearness of the star. The nearer the star Is to us the greater will be it's displacement; the com putation of its absolute distance from the amount of It's displacement is very sim pie matter
 Another method now very extensively employed is to measure the position of the nearer star with reference to one or more stars which are so infinitely far away that these so-called comparison stars will not seem to move at all as we move around the sun. This can either be done by direct measures at the tele. scope or. as is now more usual, by pho tography. If at several different dates d very small region of the heavens about the star whose distance is to be meas. ured is photographed and if the pilates ara than carefully measured under n