THE P LAN ET MARS.

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comparatively trivial. It will, in fact, suffice, if we calcu-late when Mars will be in that part of its orbit in which itmost closely approaches the Sun , and then notice whether theEarth will at the same time be nearly between the two.*Such a position of Mars relatively to the Earth and the Sun ,when it is as nearly as possible in the straight line joining them,but upon the opposite side of the Earth to the Sun , we shalldenote by the technical term Opposition.

Having premised thus much, let us investigate how oftenespecially near approaches of the Earth and Mars will recur,when the latter is in Opposition to the Sun , supposing theEarth ’s orbit to be circular, and that it is only the ovalnessof that of Mars which need be considered.

We first of all notice, that, the period of Mars round the Sun being 687 terrestrial days, or about 43 days less than two ofour years, the planet will be at its nearest to the Sun , or, inmore technical language, in its Perihelion, once in every 687days. Let us imagine the Earth to pass exactly between theSun and Mars very nearly at a date when Mars is so situated.Then the two will make an especially close approach to oneanother. This was the case, for instance, in the autumn ofthe year 1877, as is indicated in Fig . LX., in which thepoint, p, is the Perihelion of the orbit of Mars , while the linemarked 1877 joins the places of Mars and of the Earth earlyin the month of September of that year.

Mars having therefore approached particularly near to theEarth, when seen in Opposition to the Sun , in 1877, what wehave next to determine is :—How soon can it and the Earth again occupy similar, or nearly similar, relative positions ?

To do so, we must next observe that, while the Earth

* If the Earth could also at the same time be very nearly at itsgreatest distance from the Sun , the above-mentioned 36,000,000 of milesmight become more nearly 34,000,000. In these days, the distance isoccasionally less than 36,000,000 ; as in 1877, when it was about35,000,000 of miles; but it would take an almost inconceivably longcourse of ages, before the exceedingly slow movements of the Periheliaof the two orbits (P and p in Fig. LX.) -would allow the Earth to be inits Aphelion, and Mars in its Perihelion precisely together; so that thenearest approach of the two ever possible might exactly occur.