=94] 



The Minor Planets 



379 



of any of the discs which stem reliable are those of 

 Professor E. . Barnard^ made at the Lick Observatory 

 in 1894 and 1895 ; according to these the three largest 

 minor planets, Ceres, Pallas, and Vesta, have diameters 

 of nearly 500 miles, about 300 and about 250 miles 

 respectively. Thd r sizes compared with the moon are 

 shewn on the diagram (fig. 90). An alternative method 

 the only one available except for a few of the very largest 



FIG. 90. Comparative sizes of three minor planets and the moon. 



of the minor planets is to measure the amount of light re- 

 ceived, and hence to deduce the size, on the assumption that 

 the reflective power is the same as that of some known planet. 

 This method gives diameters of about 300 miles for the 

 brightest and of about a dozen miles for the faintest known. 

 Leverrier calculated from the perturbations of Mars that 

 the total mass of all known or unknown bodies between 

 Mars and Jupiter could not exceed a fourth that of the earth ; 

 but such knowledge of the sizes as we can derive from 



