LECTURE X.

THE PLANET MARS.

“ The planets of each system representKind neighbours.”

Young.

“ It timor, et major Martis jam apparet imago.”

Vikg., JEn., 8, 557.

We must wiug our flight beyond the orbit of the Earth , to adistance which may vary from about 36,000,000 to about62,000,000 miles, in order to reach the orbit of Mars , ournearest neighbour as we journey away from the Sun. Thismay seem to he a rather indefinite statement, hut we shallpresently see, that it simply affirms the orbit of Mars to he soconsiderably oval in shape, that the distance between it andthe nearest portion of the approximately circular orbit of theEarth is in one part nearly 1| times as great as in the oppositepart. And this peculiarity of the path of Mars has so muchto do with the occurrence of those occasions when we canbest observe the planet, and with what we shall have to■discuss with regard to it, that we desire, at the very com-mencement of this Lecture, to draw special attention to it.

Let us then see, by the quotation of a few numbers from thetables given in Lecture V., how far we are justified in thestatement we have just made.

We there indicated that the Earth revolves at a mean dis-tance from the Sun of about 93,000,000 miles, and that theplanet Mars has a mean distance of about 142,000,000 miles.It follows that, if the Earth and Mars should be at any time inthe same straight line, or very nearly so, and on the same side•of the Sun, they would, if at their respective mean distances