154 IN STARRY REALMS. 



the various features of it are disclosed under every variety 

 of aspect and foreshortening as they advance from the 

 margin, and then cross its surface to recede again from 

 view by the progress of its perpetual rotation. We have 

 not, however, to rely solely on the rotation of the planet 

 to present us with a view of its different aspects. It 

 fortunately happens that sometimes the north pole 

 of the planet and sometimes the south pole is tilted to- 

 wards us. 



To understand the different circumstances under which 

 the planet is thus presented, it will be necessary to enter 

 a little into the nature of its movements as represented in 

 Fig. 13. This drawing shows two orbits which, at a 

 superficial glance, appear to be circles, but which, on a 

 more careful examination, are found to be ellipses. The 

 inner of the two represents the path of the earth, the 

 outer shows the orbit of Mars. At the centre the sun is 

 placed. The earth completes its motion round the sun in 

 365 days, while Mars requires 687 days for its journey. 

 It will therefore happen, sometimes, that the earth, as it 

 were, overtakes Mars, and comes between the planet and 

 the sun. This phenomenon is called the opposition of 

 Mars ; and it is on the occasion of an opposition of the 

 planet that the greatest facilities for making a telescopic 

 scrutiny of its surface are enjoyed. This will be obvious 

 from a glance at the figure, which shows that at the 

 moment of opposition the earth must be closer to Mars 

 than the bodies can generally be at other times. Remem- 

 bering also that our point of view is on the earth, which, 

 in such a case, lies between Mars and the sun, it follows 

 that at the best hours of observation, in the middle of the 



