t5<> IN STARRY REALMS. 



sometimes may be as low as thirty-four millions of 

 miles. It will thus appear that the best oppositions 

 will exhibit the planet at a distance which is but little 

 more than half that of the unfavourable oppositions. The 

 most convenient method for expressing any particular 

 point of the earth's orbit is by the day at which the earth 

 is annually due there. On August 26th the earth is in 

 that part of its path which lies nearest to the orbit of 

 Mars. If, therefore, an opposition of the planet should 

 occur on or near to that date, the earth and tho planet 

 will then be separated by the least distance possible. 

 On the other hand, when the earth is passing through 

 that part of its path which it traverses every February 

 22nd, the conditions are reversed. Should an opposition of 

 the planet then occur, the distance between the two bodies 

 will attain a greater value than is possible when the 

 opposition occurs at any other date in the course of the 

 year. It therefore follows that the nearer the oppositions 

 are to August 26th, the better they are, and the nearer 

 they are to February 22nd, the worse they are. 



It is a noteworthy fact that the relations between the 

 periodic times of Mars and the earth are such that seven- 

 teen revolutions of Mars are accomplished in nearly thirty- 

 two years. There is a still more approximate relation 

 expressed in the fact that twenty-five revolutions of the 

 planet are almost exactly completed in forty-seven revolu- 

 tions of the earth. Hence it follows that the relative 

 positions which Mars and the earth occupy to-day they 

 will again regain in forty-seven years. Thus we sec. 

 that any favourable opposition of Mars will be succeeded 

 at intervals of thirty-two years and forty-seven years by 



