206 



A Short History of Astronomy [Cu. vin. 



M 



and others in France, led to a reasonably accurate estimate 

 of the distance of Mars and hence of that of the sun. 

 Mars was at the time in opposition (chapter IL, 43), so 



that it was nearer to the earth 

 than at other times (as shewn 

 in fig. 68), and therefore 

 favourably situated for such 

 observations. The principle 

 of the method is extremely 

 simple and substantially iden- 

 tical with that long used in 

 the case of the moon (chap- 

 ter IL, 49). One observer 

 is, say, at Paris (P, in fig. 69), 

 and observes the direction in 



FIG. 68. Mars in opposition. which Mars appears, i.e. the 



direction of the line P M ; the 



other at Cayenne (c) observes similarly the direction of 

 the line CM. The line CP, joining Paris and Cayenne, is 

 known geographically ; the shape of the triangle c P M and 



FIG. 69. The parallax of a planet. 



the length of one of its sides being thus known, the 

 lengths of the other sides are readily calculated. 



The result of an investigation of this sort is often most 

 conveniently expressed by means of a certain angle, from 



