478 



SCIENCE 



[N. S. Vol. XLIII. No. 1110 



less it should turn out that the law of giavitation 

 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 determination 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 

 opportunity for the determination of the 

 solar parallax, as at the very next opposi- 

 tion, in November, 1900, it would approach 

 to within 30,000,000 miles of the earth. At 

 the meeting of the Astrographie Chart 

 Congress in Paris in July, 1900, it was 

 resolved to seize this opportunity and or- 

 ganize an international parallax campaign. 

 Fifty-eight observatories took part in the 

 various observations called for by the gen- 

 eral plan. The meridian instruments deter- 

 mined 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 con- 

 tinuing the measures throughout the entire 

 night; and the photographic equatorials 

 obtained permanent records of the position 

 of. Eros among the surrounding stars. In 

 addition long series of observations had to 

 be made to determine the positions of the 

 stars to which Eros was referred. 



Wlien several years had elapsed after the 

 completion of the observations, and no gen- 

 eral discussion of all the material had been 

 provided for, Professor Arthur R. Hinks, of 

 Cambridge, England, volunteered for the 

 work. The undertaking was truly monu- 

 mental. 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 instru- 

 ments and from the photographic plates. 

 This done, with these results as a basis, a 

 larger catalogue of about 6,000 stars had to 

 be formed from measures on the photo- 

 graphic plates. He was then ready to com- 

 mence the discussion of the observations of 

 Eros itself. From 1901 to 1910 there ap- 

 peared in the Monthly Notices of the Royal 

 Astronomical Society eight articles cover- 

 ing 135 pages giving the results of his 

 labors. 



From a discussion of all the photographic 

 observations he obtained a solar parallax 

 of 8".807 ± 0".0027, a probable error equiv- 

 alent to an uncertainty of about 30,000 

 miles in the distance to the sun. 



From a discussion of all the micrometrie 

 observations he obtained 8".806 ± 0".004. 



The observations with the meridian in- 

 struments gave 8".837 ± 0".0185, a deter- 

 mination relatively much weaker than 

 either of the others. 



A parallax of 8".80, the value adopted 

 for all the national almanacs twenty years 

 ago, corresponds to a distance of 92,900,000 

 miles. At present it seems improbable 

 that another parallax campaign will be 

 undertaken, before 1931, when Eros ap- 

 proaches still nearer to the earth, its least 

 distance at that time being about 15,000,- 

 000 miles. 



APPEOXIMATE DISTANCES FKOM EAETH TO SUN AS 

 ACCEPTED AT VAEIODS TIMES 

 Date. Distance, Miles. 



275 B.C. to A.D. 1620 4,-500,000 



1620 Kepler 13,500,000 



1672 Mamsteed 81,500,000 



1916 92,900,000 



When Copernicus proposed that the sun 

 is the center of the solar system and aU the 

 planets including the earth revolve around 

 the sun, it was at once seen that such a mo- 



