Page:Mars - Lowell.djvu/35

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liancy of a body how the body is situated, both with regard to the source of light and with regard to the observer. Now it so chances that at the meetings of Mars with the Earth these two factors attain their maximum effects nearly together, and similarly with their minimum. For at the times when we are closest to Mars, Mars is nearly at his closest to the Sun, and reversely when we meet him at the opposite part of his orbit. It thus comes about that at some meetings,—oppositions, they are called, because Mars then is in the opposite part of the sky from the Sun,—the planet appears four and one half times as bright as at others. Here, then, we have the explanation of the planet's great changes in appearance, changes so great as to deceive any one who has not followed its wanderings, into the belief that it is some new and portentous apparition.

Important as is the ellipse in which Mars moves with regard to his visibility by us, it is considerably more important as regards the physical condition of the planet itself. For the Sun being situated at one of the foci of his orbit, the motion of the planet sweeps him now near to, now far from that dispense of light and warmth; and the amount of both which the planet receives varies just like gravity with his distance from their source. Now the eccentricity of the orbit of Mars is such that