172 ANNUAL REPORT SMITHSONIAN INSTITUTION. 1916. 



mined from the perturbative effect of the earth upon Venus and 

 Mars. This method is long and laborious, but its importance lies 

 in the fact that the accuracy of the result increases with the time. 

 Prof. C. A. Young says 



this is the " method of the future," and two or three hundred years hence will 

 have superseded all the others, unless, indeed, it should appear that bodies at 

 present unlj;nown are interfering with the inoveraents of our neighboring 

 planets, or unless it should turn out that the law of gravitation is not quite 

 so simple as it is now supposed to be. 



A third group of methods of determining the distance of the sun 

 from the earth, called the physical methods, depends upon the de- 

 termination of the velocity of light in conjunction either with the 

 time it takes light to travel from the sun to the earth obtained 

 from observations of the eclipses of Jupiter's satellites or with the 

 constant of aberration derived from observations of the stars. 



In August, 1898, Dr. Witt, of Berlin, discovered an asteroid^ 

 since named Eros, which was soon seen to offer exceptional oppor- 

 tunity for the determination of the solar parallax, as at the very 

 next opposition, in November, 1900, it would approach to within 

 30,000,000 miles of the earth. At the meeting of the Astrographic 

 Chart Congress in Paris in July, 1900, it was resolved to seize this 

 o^Dportunity and organize an international parallax campaign. 

 Fifty-eight observatories took part in the various observations called 

 for by the general plan. The meridian instruments determined the 

 absolute position of Eros from night to night as it crossed the 

 meridians of the various observatories; the large visual refractors 

 measured the distance of Eros from the faint stars near it, at times 

 continuing the measures throughout the entire night ; and the photo- 

 graphic equatorials obtained permanent records of the position of 

 Eros among the surrounding stars. In addition long series of obser- 

 vations had to be made to determine the positions of the stars to 

 which Eros was referred. 



When several years had elapsed after the completion of the obser- 

 A'ations, and no general discussion of all the material had been 

 provided for. Prof. Arthur K. Hinks, of Cambridge, England, vol- 

 unteered for the work. The undertaking was truly monumental. 

 He first formed a catalogue of the 671 stars which had been selected 

 by the Paris congress for observation as marking out the path of 

 Eros from a discussion of the results obtained by the meridian 

 instruments and from the photographic plates. This done, with 

 these results as a basis, a larger catalogue of about G,000 stars had 

 to be formed from measures on the photographic plates. He was 

 then ready to commence the discussion of the observations of Eros 

 itself. From 1901 to 1910 there appeared in the Monthly Notices of 



