472 The Planet Mars. 



arities presented by this interesting planet— the only one 

 whose true surface we are able to examine satisfactorily — we 

 are not to content ourselves with observing him in this or that 

 position among the signs. For then we should only know of 

 his appearance at this or that Martial season. We must 

 examine him at every opposition, and endeavour, if possible, to 

 connect the appearances presented at different oppositions with 

 the seasonal changes which may fairly be assumed to be taking 

 place upon his surface. 



The seasons on Mars are due to an axial inclination not 

 differing greatly from that of our own earth. From a series of 

 observations by Sir W. Herschel it would appear to follow that 

 the point of Mars' orbit corresponding to our vernal equinox 

 (northern hemisphere) is at present in longitude 78°, — N in 

 Fig. 1 . He made the inclination of the axis to the plane of 

 Mars' orbit 28° 42', to the plane of the elliptic 30° 3o'. The 

 determination of these elements is not a problem which we can 

 expect to have exactly solved. Accordingly, we find that 

 Dr. Oudemans, applying observations taken in 1830 — 1837 by 

 Bessel, with the Konigsberg heliometer, obtained results 

 differing appreciably from Herschel's. He assigns an inclina- 

 tion I5 less than that given by Herschel, and he places the 

 Martial vernal equinox as at ISi', Fig. 1. We learn also that 

 this determination does not correspond quite accurately with 

 some pictures of Mars taken by good observers.* 



Fig. 3 indicates the polar presentation of Mars (supposed 

 to be seen in an inverting telescope) at opposition, on Dr. 

 Oudemans' assumption as to the inclination of the axis. The 

 arrow represents the direction in which the planet is actually 

 moving, while m m' represents a declination- circle through his 

 centre, and ee a declination -parallel, so that the apparent 

 motion of Mars across the field of the telescope will take 

 place in direction ee. The polar presentation will gradually 

 diminish as the planet retrogrades (that is, until the middle of 

 February), and then gradually increase. In the beginning of 

 April, when Mars has nearly reached his maximum phase of 

 gibbosity, he will appear as shown at Fig. 3. At this time he 

 will nearly have reached the summer-solstice of his northern 

 hemisphere, so that the southern polar snows may be expected 

 to be much more extensive than they had been three months 

 before. Ihe planet will, of course, appear much smaller his 

 distance being so much greater — see in Fig. 2 the longer line 

 marked 1867. 



* It appears to me that the two pictures by Dc la Hue, reproduced in Guillcmm'a 

 Heavens, require a somewhat larger inclination than that assigned by Dr. 

 Oudemans. 



