174 



eichelberger: distances of heavenly bodies 



In other words, the Yerkes 40-inch equatorial used photographi- 

 cally determines stellar parallaxes with one-tenth the labor re- 

 quired with an 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 thirteen stars had 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 fifteen years ago. What may we not 

 expect when the 100-inch reflector gets to work on Mt. 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 nine 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 eleven stais 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 Parallax Determinations 



DATE 



1838 



1838 



1880-1898 



1888-1912 



1893-1905 

 1910 

 1915 

 1915 

 1915 



INSTRUMENT 



o 

 o 



Dorpat refractor 



Konigsberg heliometer 



Cape heliometer 



Yale heliometer 



Washburn meridian circle 



Yerkes refractor 



Yerkes refractor 



Leander McCormick refractor 

 Mt. Wilson 60-inch reflector. . . 



