THE PLANET MAES IN JANUARY 1867. 
27 
little attention has been bestowed upon it by writers on astro- 
nomy will be evident from this, — the numbers given by Herschel 
are repeated not only without comment, but even without those 
changes which the variations in the orbit of the planet render 
necessary. Given the position of Mars’ axis with respect to his 
orbit, and the position of his orbit with respect to the earth’s, 
then the position of his axis with respect to the earth’s orbit 
follows at once. If either of the data vary, the result will vary. 
Now, the second datum has varied largely since Herschel’s time; 
but no corresponding variation in the angle 30° 18' (named 
above) has been introduced into our works on astronomy. 
The diameter of Mars is differently estimated by different 
astronomers. In Madler’s u Elements,” 4,070 miles is assigned 
as the planet’s equatorial diameter. Most observers assign a 
larger diameter : Hind, in his “ Astronomy,” giving the planet 
a diameter of 4,500 miles. These estimates are, of course, 
founded on the old estimate of the sun’s distance. It seems 
probable that 4,150 miles on that estimate, or 4,000 miles, if 
the modern reduced estimate of the sun’s distance is accepted, 
is not very far from the true diameter of the planet. In other 
words, the linear dimensions of Mars are about one-half those 
of the earth, or twice those of the moon ; more roughly, his 
surface is about one-fourth that of the earth, or four times that 
of the moon; and, yet more roughly, his volume about one- 
eighth that of the earth, or eight times that of the moon. 
Herschel determined the compression of Mars at ; modern 
observers greatly reduce this quantity. Professor Kaiser, of 
Leyden, makes the compression t A- ; Main, of the Kadcliffe 
Observatory, deduced in 1862, but in some earlier measure- 
ments made the polar greater than the equatorial diameter. 
Mr. Dawes, apptying two modes of measurement, found, from 
the first, no compression ; from the second, he found the polar 
greater than the equatorial diameter. Is it going too far to say 
that the oblateness of Mars’ figure is not yet determined satis- 
factorily ? Probably it is too small for measurement. 
Herschel made Mars’ rotation-period 24 h. 39 m. 35 s. ; 
Madler gives 24 h. 37 m. 23*7 s.; and Professor Kaiser con- 
siders 24 h. 37 m. 22*3 s. the true value. 
Let us now turn to the opposition of January 10, 1867. 
Referring to fig. 1, we see that the opposition takes place when 
Mars is not very far from aphelion. The earth, also, being 
very near perihelion, is drawn as far away from Mars’ orbit as 
this cause permits her to be. Mars is nearly at his greatest 
distance from the plane of the ecliptic. Thus the distance of 
Mars from the earth is very much greater than at the oppositions 
of 1864, 1862, and 1860. In fact, the absolute distance of 
Mars will be no less than 58,500,000 miles, much nearer to his 
