XXI.] THE SUN. ' 365 



its axis perpendicular to the plane of the earth's orbit. But, 

 as a matter of fact, the spots travel in this direction only at 

 certain seasons, and, at other times of the year, they may be 

 seen moving in curved lines : the direction of the curve 

 being sometimes towards the north, and sometimes towards 

 fhe south. This change of direction is represented, though 

 very much exaggerated, in Fig. 1 20 ; where the first figure 

 indicates the apparent direction of movement in March, the 

 second in June, the third in September, and the fourth in 

 December. These varying directions of the paths of the 

 spots at different periods are easily explained by supposing 

 that the sun's axis is not perpendicular, but is oblique to the 

 plane of the ecliptic ; so that, sometimes, the sun's axis is 

 inclined towards us, and, at other times, is directed from us. 

 The rotation of the sun therefore takes place, like the rota- 

 tion of the earth, around an inclined axis. The inclination 

 of the sun's axis, however, is very much less than that of 

 the earth's axis ; being, in fact, only about 7 1° from the 

 vertical. 



Observations on the motion of the sun-spots have also 

 established the fact that the sun is not strictly a fixed body, 

 around which the earth revolves, but that it has a motion of 

 its own through space. The earth, indeed, not only revolves 

 in a nearly circular path round the sun, but this path is being 

 carried along with the sun at an enormous speed. Hence, 

 the actual path which the earth describes in the heavens 

 must be compounded of these two motions, and will 

 probably be a spiral. 



As so much of our knowledge of the sun has' been 

 derived from a study of its spots, it is worth while to inquire 

 a litde more closely into their nature. Reference to Fig. 119 

 will show that a spot is not equally dark throughout ; the 

 fringed margin, represented by a half-shade in the figure, is 



