172 JN STARRY REALMS. 



a day and a quarter, which is not much longer than the 

 period of rotation of the planet. 



The names of the two satellites were suggested by a 

 passage in Homer, where Deimos and Phobos were sum- 

 moned, as the attendants of Mars, to yoke his steeds, 

 while he put on his armour. Deimos is the outer and 

 more conspicuous of the two, Phobos is the inner and 

 more interesting. 



Immediately after Professor Hall's great discovery, the 

 powerful telescopes all over the world were directed to 

 Mars, and many observers were fortunate enough to 

 obtain measurements of the position of the satellites, and 

 thus to contribute to our knowledge of their movements. 



One of the functions of the astronomer is to measure 

 and to weigh the masses of the celestial bodies. Now, 

 difficult as it may seem actually to weigh a mighty planet, 

 yet in many cases the task is a comparatively simple one. 

 It is true we do not seek to evaluate the actual mass of 

 the planet in tons. We are at present only striving after 

 a relative knowledge a knowledge of the mass of the 

 planet as compared with the mass of the earth or the sun. 

 Our task is an easy one when we have to deal with planets 

 attended by one or more satellites. It becomes a very 

 difficult one when the planet has no satellite that can be 

 observed. 



Let me endeavour to give some explanation of the 

 method we adopt when we seek to weigh a great planet 

 such, for example, as our earth against the sun. We 

 shall suppose that the earth revolves around the sun in a 

 nearly circular orbit, and that the moon revolves around 

 the earth in an orbit nearly circular also. We may 



