364 A Short History of Astronomy [CH. xm. 



the old methods of time-observation being supplemented 

 by photography and by direct micrometric measurements 

 of the positions of Venus while transiting. 



The method of finding the distance of the sun by means 

 of observation of Mars in opposition (chapter vni., 161) 

 has been employed on several occasions with considerable 

 success, notably by Dr. Gill at Ascension in 1877. A 

 method originally used by Flamsteed, but revived in 1857 

 by Sir George Biddell Airy (1801-1892), the late Astronomer 

 Royal, was adopted on this occasion. For the determination 

 of the parallax of a planet observations have to be made from 

 two different positions at a known distance apart; commonly 

 these are taken to be at two different observatories, as 

 far as possible removed from one another in latitude. 

 Airy pointed out that the same object could be attained if 

 only one observatory were used, but observations taken at 

 an interval of some hours, as the rotation of the earth on 

 its axis would in that time produce a known displacement 

 of the observer's position and so provide the necessary 

 base line. The apparent shift of the planet's position 

 could be most easily ascertained by measuring (with the 

 micrometer) its distances from neighbouring fixed stars. 

 This method (known as the diurnal method) has the great 

 advantage, among others, of being simple in application, a 

 single observer and instrument being all that is needed. 



The diurnal method has also been applied with great 

 success to certain of the minor planets ( 294). Revolving 

 as they do between Mars and Jupiter, they are all farther 

 off from us than the former ; but there is the compensating 

 advantage that as a minor planet, unlike Mars, is, as a 

 rule, too small to shew any appreciable disc, its angular 

 distance from a neighbouring star is more easily measured. 

 The employment of the minor planets in this way was first 

 suggested by Professor Galle of Berlin in 1872, and recent 

 observations of the minor planets Victoria, Sappho, and Iris 

 in 1888-89, m ade at a number of observatories under the 

 general direction of Dr. Gill, have led to some of the most 

 satisfactory determinations of the sun's distance. 



282. It was known to the mathematical astronomers of 

 the i8th century that the distance of the sun could be 

 obtained from a knowledge of various perturbations of 



