64 PLEASANT WA YS IN SCIENCE. 



determinable by noticing Mars's position with respect to 

 stars which chance to be close to him. For this purpose the 

 heliometer is specially suitable, because, having first a view 

 of Mars and some companion stars as they actually are 

 placed, the observer can, by suitably displacing the movable 

 half-glass, bring the star into apparent contact with the 

 planet, first on one side of its disc, and then on the other 

 side the mean of the two resulting measures giving, of 

 course, the distance between the star and the centre of the 

 disc. 



This method requires that there shall be two observers, 

 one at a northern station, as Greenwich, or Paris, or Wash- 

 ington, the other at a southern station, as Cape Town, 

 Cordoba, or Melbourne. The base-line is practically a 

 north-and-south line; for though the two stations may not 

 lie in the same, or nearly the same, longitude, the displace- 

 ment determined is in reality that due to their difference of 

 latitude only, a correction being made for their difference 

 of longitude. 



The other method depends, not on displacement of 

 two observers north and south, or difference of latitude, but 

 on displacement east and west Moreover, it does not 

 require that there shall be two observers at stations far 

 apart, but uses the observations made at one and the same 

 stations at different times. The earth, by turning on her 

 axis, carries the observer from the west to the east of an 

 imaginary line joining the earth's centre and the centre of 

 Mars. When on the west of that line, or in the early evening, 

 he sees Mars displaced towards the east of the planet's true 

 position. After nine or ten hours the observer is carried as 

 far to the east of that line, and sees Mars displaced towards 

 the west of his true position. Of course Mars has moved in 

 the interval He is, in fact, in the midst of his retrograde 

 career. But the astronomer knows perfectly well how to 

 take that motion into account Thus, by observing the two 

 displacements, or the total displacement of Mars from east 

 to west on account of the earth's rotation, one and the same 



