eichelberger: distances of heavenly bodies 165 



The nearest approach of Venus to the Earth is during her 

 transit across the face of the Sun, and these occasions, four 

 during the last two centuries, have been utilized to determine 

 the solar parallax. Here as in the case of Mars two different 

 methods may be used, either by combining observations at two 

 stations widely separated in latitude, or at two stations widely 

 separated in longitude. (Diagrams shown.) 



The methods just described for obtaining the solar parallax, 

 the geometrical methods, were made available, as has been said, 

 by the discovery of Kepler's laws of planetary motion. New- 

 ton's discovery of the law of gravitation gave rise to another 

 group of methods, designated as gravitational methods. The 

 best of these is probably that in which the distance of the Sun 

 from the Earth is determined from the mass of the Earth, which, 

 in turn, is determined 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. Professor 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 hence will have superseded all the others, — unless indeed it should 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 unknown are interfering with the move- 

 ments 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 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 satel- 

 lites, or with the constant of aberration derived from observa- 

 tions 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 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 re- 

 solved to seize this opportunity and organize an international 



