THE PLANET MAES IN JANUARY 1867. 
23 
efficiently exerted — where Newton long since told men that such 
an instrument should be placed — far above those denser atmo- 
spheric strata whose disturbances never cease, and are magnified 
and aggravated by every increase of telescopic power. When 
this is done, we may look in Mars for that which has long been 
sought for fruitlessly upon the lunar surface — the signs of life, of 
change, of progress, of decay. In one point, indeed, Mars has 
already supplied such evidence ; since, as we shall presently see, 
he exhibits, in regular succession, appearances corresponding to 
changes well known to be taking place regularly upon our 
earth. 
There is another circumstance which tends to heighten the 
interest with which the astronomer regards this small planet. 
Its motions, watched for maiiy long }mars by Tycho Brahe, and 
studied for twenty years by the ingenious Kepler, were the 
means of overthrowing for ever the elaborate system of errors 
and hypotheses known as Ptolemaic astronomy. They afforded 
also to Newton the first hint on which he founded the law of 
universal gravitation. The figure of Mars’ orbit, and the rela- 
tion which that orbit bears to the orbit of our earth, rendered 
the planet the most fitting, one may almost say the only 
fitting, member of the solar system for the purposes Kepler had 
in view. 
As it is necessary for the right understanding of the appear- 
ances presented by Mars at successive returns to opposition 
that the nature of his orbit should be rightly understood, I 
shall solicit the reader’s patience while I run as briefly as pos- 
sible through the points of chief importance. This is the more 
necessary because no popular work on astronomy (that I at 
least have ever met with) presents with any approach to 
accuracy this very important feature of the solar system. Even 
that admirable and interesting work, Guillemin’s u Heavens,” 
deals very inadequatel} 7 , though at some length, with this 
question. 
In fig. 1, represents the orbit of the earth, and 
MjMgMgM^j that of Mars. M is the perihelion of Mars’ orbit, 
which, it will be observed, is noticeably eccentric^, the centre, 
being 13,000,000 miles from the sun); E is the perihelion of the 
earth’s less eccentric orbit, whose centre is at C 2 . The arrows 
indicate the direction in which both planets revolve around the 
sun. The plane of Mars’ orbit is inclined at an angle of 1° 51' 5 " 
to that of the earth, the points marked & and £3 being those 
at which the orbit of Mars intersects the plane of the earth’s 
orbit ; at M and M' Mars attains his greatest distance from the 
plane of the earth’s orbit, the short arrow indicating, as nearly 
as possible, on the scale of our figure, the distance at which 
Mars is above and below the plane of the ecliptic at these two 
