MARS AS A WORLD. 157 



oppositions which are also favourable. As an illustration 

 of the use that may be made of these numbers, we shall 

 predict the occurrence of an approaching favourable oppo- 

 sition. There was one in 1877, there must, therefore, 

 have been a good opposition thirty-two years previously, 

 that is, in 1845. It must similarly be followed in 

 forty-seven years by another good opposition, which 

 brings us to 1892. We may therefore anticipate in the 

 autumn of 1892 another good view of our neighbouring 

 planet. 



We have already spoken of the rotation of Mars, and it 

 follows that this planet has an axis about which that rota- 

 tion is performed ; the planet being in this respect, as in so- 

 many others, a body analogous to our earth. The plane of 

 the orbit of Mars is inclined at a small angle to that of the 

 earth. This angle is under two degrees. It varies slightly, 

 and is at present decreasing with extreme slowness. The 

 value for the year 1890 is 1 51' 1", and the decline in its 

 value is at the rate of two or three seconds per century. 

 So far as the mere aspect of Mars from our earth is con- 

 cerned, this small angle of inclination possesses an inap- 

 preciable influence. The case would be very different, 

 however, from the point of view of an inhabitant of Mars. 

 If there were absolutely no inclination between the two 

 orbits, then, whenever an opposition of Mars took place, 

 it would happen that the earth would, of course, be seen 

 actually projected on the disc of the sun when viewed 

 from Mars. As, however, there is some inclination, it 

 will usually happen that the Martial inhabitant will not 

 see the earth in front of the sun during opposition, for 

 the same reason that we do not see Venus on the sun's 



