CHARTING THE UNIVERSE 



This test is calledi measuring the star's parallax. The 

 test is made by noting the position of a star in re- 

 lation to other stars, preferably by photography at 

 intervals of just six months. For purposes of com- 

 parison, some astronomers prefer to take successive 

 negatives on the same plate, a method introduced 

 by Professor Kapteyn. Three exposures are neces- 

 sary at intervals of six months, that allowance may 

 be made for the earth's progressive motion as well 

 as for its orbital swing. The delicacy of the test will 

 be appreciated when it is stated that the half-yearly 

 shift of position of the star that is our nearest neigh- 

 bor in space is only three-quarters of a second of arc. 

 Reduced to comprehensible terms, this is equivalent 

 to a shift of position of about one inch in an object 

 at a distance of five miles. Only seventeen stars are 

 known to have a parallax exceeding about one-fourth 

 this amount. To test such minute changes, the larg- 

 est telescopes are needed; the Lick refractor with 

 its sixty-foot barrel being peculiarly effective. 



Professor Eddington, of the Royal Observatory 

 of England, has recently pointed out that there is a 

 good deal of uncertainty about the precise value of 

 the parallax in the case of a good many of the stars 

 that have been tested. But it gives us an illuminating 

 sense of the marvelous accuracy of modern measure- 

 ments to note his interpretation of "uncertainty" in 

 the present sense. He explains that the measure- 

 ments in a good many cases may be wrong by a 

 matter of five-hundredths of a second, which 

 represents a variation of three-fifths of an inch at 

 forty miles distance. Meanwhile he assures us that 



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