II. Orbit

Thoroughly to appreciate, then, the physical condition of Mars, we must begin at this beginning of our knowledge of the planet, since every detail will be found to play its part in the final result. I shall therefore give in a word or two the general facts known about the planet, before taking up the observations which make the subject matter of this book. The first of these general facts is the path the planet describes about the Sun. Who first found out that the ruddy star we call Mars was not like the rest of the company about him we do not know ; possibly some, to fame unknown, Chaldean shepherd alone with the night upon the great Chaldean plains. With the stars for sole companions while his sheep slept, he must, as he watched them night after night , have early recognized that they always kept the same configuration. They rose and set, but they all rose and set together. But one night he thought he noticed that one of them had changed its place with reference to the rest. A few nights later he became sure of it. One of the immovable had patently moved. That memorable though unremembered night marked the birth of our acquaintance with the rest of the universe.

Whether the midnight pioneer was Chaldean or Assyrian or of some other race, certain it is that to the Egyptians we owe the first systematic study of the motions of this and of four other roving stars, and to the Greeks whom they taught, the name by which we know them, that of planets, meaning merely wanderers. Since then, as we know, many others of like habit have been added to the list. Now, from observations of the apparent places of a planet, it is possible to determine the relative path of the planet in space as compared with the path of the Earth. This Kepler did from observations of Tycho Brahe's, and showed the wanderers to belong to a system of bodies, all revolving about the Sun in various elliptic orbits, the Sun being at the focus of each ellipse. He also found that the line connecting each planet with the Sun passed over equal areas in equal times, and thirdly, that the squares of the times were as the cubes of the major axes of the orbits. From these three "laws" Newton deduced the fact that the force controlling the planets was directed toward the Sun, that it varied inversely as the square of the distance, and that it was the same in origin for all. This is the so-called law of gravitation, and this is the way in which it was discovered. We do not yet know why gravity so acts, but it is interesting to note that it follows the simple law of geometrical expansion, diminishing in exact ratio to the space it fills, just like electricity or light. It may, therefore, also be a wave motion.

Thus all the wanderers proved to be associated in common dependence on the Sun, and among the members of the solar family thus recognized Mars was found to hold the position next exterior to the Earth, and the path he followed in his circuit of the Sun to be situated with regard to the Earth's as in the following diagram.

On consulting the diagram we shall at once perceive why it is that every fifteen years Mars becomes so unusually bright as to seem, to one who has not kept track of him, a new and startling star. His orbit, it will be seen, is an ellipse of some eccentricity, and deviates in consequence considerably from a circle. The point marked Perihelion denotes the point where the planet is nearest the Sun; the point marked Aphelion, the point where the planet is the most remote from the Sun. In like manner the points marked Perihelion and Aphelion on the inner circle show the corresponding points of the Earth's orbit, which is much more nearly circular. Now as the two planets revolve in different periods of time, Mars taking 686.98 of our days to complete his circuit, and the Earth 365.26 days to complete hers, the one planet will overtake the other only once every two years and two months or so. Meanwhile they are at great distances apart. But even when they do meet, they do not always meet equally near. For the one orbital period is not an exact multiple of the other, and as the orbits are both ellipses, it is evident that these meetings of the two planets will occur at different points of their orbits, and, therefore, at different distances.