MARS AS A WORLD. 155 



night, the planet must be on the meridian, and at its 

 highest in the sky. 



The oppositions of Mars succeed each other at intervals 

 of 780 days. It therefore follows that about every two 

 years and two months the planet occupies a favourable 

 position for being observed. It must, however, be noticed 

 that all oppositions are not equally advantageous. Owing 



Fig. 14. View of Mars, Sept. 10th, llh. 20m., 1887. 



to the high eccentricity in the shape of the orbit of 

 Mars it lies in one region of its path much closer to 

 the earth's circuit than it is when elsewhere. To ex- 

 press the results with numerical accuracy, we may take 

 the average distance from the earth to the sun to be 

 about ninety-three millions of miles. Then the distance 

 between Mars and the earth at opposition may some- 

 times be as much as sixty-three millions of miles, and 



