TJw Planet Mars. 469 



The short lines surmounted by arrow-heads, represent the dis- 

 tance at which different parts of the orbit lie above the plane 

 of the ecliptic* 



The figures representing the earth and Mars, are not drawn 

 to scale, but purposely enlarged. The meridian lines upon 

 them serve to indicate the position of the polar axes of the two 

 planets. They are such that whereas the earth, when at V. Eq., 

 has its polar axis square to the line from the sun, Mars is 

 similarly circumstanced when near N or W. The axis of the 

 earth must be supposed to be inclined at an angle of 667/ de- 

 grees to the plane of the paper ; that of Mars, at an angle of 

 about 60 degrees. 



The two planets move round their orbits in the same direc- 

 tion (E' E E" or M'MM") at different rates; the earth taking 

 one year to complete her circuit, Mars taking 1 year 321 2 days 

 to complete his. Thus if they start together, the earth will go 

 twice round, while Mars has gone little more than once round ; 

 and on an average the earth will make up this small arc by 

 which Mars is in advance, in 49 \ days. Thus the interval 

 between successive oppositions is on an average 2 years 49J 

 days. But the rate at which Mars travels is so variable, owing 

 to the eccentricity of his orbit, that this interval of 49 £ days 

 is sometimes largely exceeded, at other times as largely fallen 

 short of. For Mars in perihelion travels nearly half as fast again 

 as he does in aphelion : the earth also moves at a variable 

 rate, though her variation is much less marked than that of 

 Mars ; it happens, further, that the earth is moving at a rate 

 less than her mean rate, when opposite that part of Mars' orbit 

 in which he travels fastest, and vice versa. Hence it is that the 

 intervals between successive oppositions differ so noticeably, 

 Eor example: in 1860, Mars came to opposition on the 17th 

 of July; in 1862, he was in opposition on October 5th, 80 

 days later; in 1864, on December 1st, 56 days later; in 1867, 

 he will come to opposition on January 10th, only 41 days 

 later; in 1869, on February 13th, only 33 days later; and in 

 1871, a yet smaller number of days will separate the date of 

 opposition from that of 1869. 



A word or two as to the absolute dimensions of the two orbits. 

 The mean distance of the earth from the sun, is 91,650,000 

 miles ; her greatest and least distances 93,190,000 miles, and 

 90, 110,000 miles, respectively; thus the eccentricity, or C S, 

 is 1,540,000 miles. The mean distance of Mars from the sun, 

 or the distance CM, is 139,650,000 miles; his aphelion and 

 perihelion distances being, respectively, 152,670,000, and 



* Only the part of such lines between the two cross-lines is to be taken as 

 representing this distance. 



