THE PLANET MA.RS IN JANUARY 1867. 
25 
however, sends us more light, probably because bis atmosphere 
bears large belts and masses of clouds capable of reflecting 
light very perfectly, and also preventing the loss of light which 
would accrue in the double passage through the planet’s atmo- 
sphere. The studies of our leading astronomers and physicists 
leave little doubt that the light by which we see Mars has 
suffered diminution in this way to a very considerable extent. 
Oppositions of Mars near perihelion occur at intervals of 
fifteen and seventeen years successively. Sometimes it happens, 
as in 1860 and 1862, that two successive oppositions occur at 
nearly equal distances from perihelion ; it follows that the next 
opposition near perihelion (in 1877) will fall midway between 
these positions, or very much nearer perihelion than either of 
the two others ; in other words. Mars will be very favourably 
situated for observation in 1877. Much of the superiority of 
perihelion-oppositions is, however, lost in our northern latitudes, 
since these oppositions occur in August; and the sun being high 
by day, it follows, of course, that the ecliptic (near which Mars is 
always situate) is low by night. On the other hand, it is clear from 
the figure that, if Mars is in opposition in midwinter, when of 
course he has a considerable altitude at night, he is too near 
aphelion to be favourably seen. On the whole, it follows that 
the most favourable position for Mars to come to opposition in 
is towards autumn (when he is near 8 of fig. 1). At this season, 
while not very far from perihelion, he attains an altitude of 
from 55° to 60° on the meridian. Such an opposition took place 
in 1862, when very admirable views of Mars were obtained by 
Messrs. Dawes, Lockyer, and Phillips, and by others of our best 
observers. The opposition of 1864 was also a very favour- 
able one. 
But another circumstance remains to be considered. The 
planet, rotating on an axis considerably inclined to the orbit of 
motion (and also to the ecliptic), presents at different seasons 
different aspects, not only with reference to the sun but also to 
the observer on earth. At one time his north pole is bowed 
down towards the sun, at another his south pole ; and the same 
relations, only in a somewhat more complex order, are main- 
tained with respect to the earth. If, then, the astronomer would 
rightly study the peculiarities of our neighbour Mars, he must 
examine the planet at oppositions occurring in every part of his 
orbit. On this account, therefore, the opposition of Mars, 
which takes place on January 10, 1867, although occurring 
when he is not very far from aphelion, is not to be on that 
account neglected. It happens, indeed, that, for the study of 
the northern hemisphere of Mars, this opposition occurs, on the 
whole (that is, combining considerations of distance, altitude. 
