The Planet Mars. 467 



variations presented by Mars, which the other superior planets 

 do not exhibit, or at least not to any noteworthy extent. 

 These peculiarities will be understood when we come to examine 

 the nature of Mars' orbit. 



The planetary orbits it is well known, are elliptical. But 

 although several of these orbits differ very appreciably from 

 circles concentric with the sun, yet their difference from the 

 circular form is always very small indeed. For instance, the 

 orbit of Mars, which is one of the most eccentric, yet dif- 

 fers so little from the circular form that if it were accurately 

 traced down as an ellipse having a greater axis one foot in 

 length, a circle described around this ellipse would nowhere 

 separate from it by a distance exceeding one-eightieth part of 

 an inch ; though on the same scale, the distance of the sun from 

 the centre of the ellipse would be represented by a length of 

 more than half an inch (almost exactly "56 of an inch). Again, 

 the inclination of the planetary orbits to the ecliptic, though 

 quite sufficient to make the absolute distances by which the 

 planets may be separated from the plane of the ecliptic very 

 considerable, is yet so small in itself, that a representation of 

 the orbits in piano is not unsatisfactory — save only in the case 

 of Mercury, and some of the minor planets. It appears, then, 

 that circles traced on paper may be taken to represent ap- 

 proximately the orbits of planets, if only these circles are 

 properly centred. 



In Figs. 1 and 2, parts of the orbits of Mars and the 

 earth are traced down as accurately as possible, on the scale 

 of one-eighth of an inch to 4,000,000 miles. S is the posi- 

 tion of the sun, C is the centre of the earth's orbit. C the 

 centre of the orbit of Mars. The position of the earth at the 

 time of winter-solstice is at W. Sol., while in Fig. 2, the 

 position of the earth at the time of vernal equinox is at Y. Bq. 

 The two figures must be supposed to be parts of a single 

 delineation of the orbits, the scale of which would be some- 

 what too large for our pages. The reader is recommended, 

 however, to describe such circles on a larger sheet, taking the 

 centering from Fig. 1. He will then be able to appreciate 

 what is only partially indicated in Figs. 1 and 2, the remark- 

 able variation in the distance separating the two orbits. He 

 will find, indeed, that at the point of nearest approach (nearly 

 in direction C S) the distance separating the orbits is little 

 more than half the corresponding distance on the opposite 

 side — that is, in direction S C. 



The orbit of the earth i3 supposed to lie in the plane of the 

 paper, while the plane of Mars' orbit must be supposed to cut 

 the plane of the paper, in the line S £> the part of the orbit 

 which appears in our figures lying above the plane of the paper. 



