DISTANCES OF HEAVENLY BODIES EICHELBERGER. 



179 



In other words, the Yerkes iO-inch equatorial used photographi- 

 cally determines stellar parallaxes with one-tenth the labor required 

 with a heliometer and with twice the accuracy. 



In July, 1913, stellar parallax work was undertaken with the 

 60-inch reflector of the Mount Wilson Solar Observatory, and at the 

 meeting of the American Astronomical Society at San Francisco in 

 August, 1915, a report on that work was made. The parallaxes of 

 13 stars have been determined, with a maximum probable error of 

 0".010 and an average probable error of less than 0",006, giving 

 twice the accuracy of the Schlesinger results with the Yerkes 40-inch 

 and from three to five times that obtained 15 years ago. What may 

 we not expect when the 100-inch reflector gets to work on Mount 

 Wilson? 



At the meeting of the American Astronomical Society, to which 

 reference has just been made, two other observatories reported upon 

 their stellar parallax work. Lee and Joy, of the Yerkes Observatory, 

 reported the parallaxes of 9 stars with a maximum probable error 

 of 0".014 and an average probable error of 0".010 ; and Mitchell, of 

 Leander McCormick Observatory, reported the parallaxes of 11 stars 

 with a maximum probable error of 0''.012 and an average probable 

 error of 0".009. 



The progress made in the accuracy of parallax results is shown at 

 a glance in Table IV. 



Table IV. — The accuracy of stellar paraUax determinations. 



From these results it appears that any star whose parallax is as 

 much as 0".02, i. e., whose distance from the earth is less than 

 10,000,000 times that from the earth to the sun, should give a positive 

 result when subjected to the treatment now employed in parallax 

 investigations, and as 8 or 10 observatories are devoting their ener- 

 gies to stellar parallax work at present, the combined programs 

 containing over 1,000 different stars, we ought soon to have lists of 

 at least a few thousand stars whose parallaxes are known, where 

 our present lists contain but a few hundred. 



