164 EICHELBERGER : DISTANCES OF HEAVENLY BODIES 



the Sun their relative distances from that body, and thus to 

 determine the distance of the Sun from the Earth, by determin- 

 ing the distance or parallax of one of the planets. 



From observations of Mars, Kepler obtained the distance of 

 the Sun from the Earth as about three times that accepted up to 

 his time. His value, however, was but one-seventh of the true 

 distance. About fifty years later Flamsteed and Cassini work- 

 ing independently and using the same method as that employed 

 by Kepler obtained for the first time approximately the correct 

 value of the distance of the Sun from the Earth. In a letter, 

 dated November 16, 1672, to the Publisher of the Philosophical 

 Transactions, Flamsteed says: 



September last I went to Townley. The first week that I intended 

 to have observed c? there with Mr. Townley, I twice observ'd him, 

 but could not make two Observations, as I intended, in one night. 

 The first night after my return, I had the good hap to measure his 

 distances from two Stars the same night; whereby I find, that the 

 Parallax was very small; certainly not 30 seconds: So that I believe 

 the Sun's Parallax is not more than 10 seconds. Of this Observation 

 I intend to write a small Tract, when I shall gain leisure; in which I 

 shall demonstrate both the Diameter and Distances of all the Planets 

 by Observations; for which I am now pretty well fitted. 



During the two and a half centuries since Flamsteed's de- 

 termination there have been more than a hundred determinations 

 of the solar parallax by various methods. In the method used 

 by Flamsteed, the rotation of the Earth is depended upon to 

 change the relative position of the observer, the center of the 

 Earth, and Mars. (Diagram shown.) Another method is to 

 establish two stations widely separated in latitude, and in ap- 

 proximately the same longitude. At one station, the zenith 

 distance of Mars will be determined as it crosses the meridian 

 north of the zenith; at the other station, the zenith distance will 

 be determined as it crosses the meridian south of the zenith. 

 The sum of the two zenith distances minus the difference in 

 latitude between the two stations will give the displacement of 

 Mars due to parallax. These two methods have been success- 

 fully applied to several of the asteroids whose distances from the 

 Sun are very nearly that of Mars. 



