The Ways of the Planets

by Martha Evans Martin

If its color is not a sufficient mark by which to identify Mars, a still further difference between it and the stars is its markedly rapid movement. A single night will make a sufficient change in its position to show the planet as a wanderer. On an average, it travels over about four-tenths of a degree in the heavens in one day. This equals more than half the diameter of the moon, a change of position sufficiently great to be easily detected.

WHEN AND WHERE MARS MAY BE SEEN

Unlike Mercury and Venus, which are never far from the sun, and can be seen only for a comparatively short time either early in the morning or in the evening, and are never very high up in the skies, Mars may be situated so that it can be seen at any time of the night, and also at any distance from the sun. When it is in opposition it rises just as the sun sets, and is then in view all night. At this time it is nearer, larger, and brighter than at any other time in the particular revolution it is then making, and, consequently, is in the best position to be viewed by us that it will have during that revolution.

Oppositions differ, however, in different revolutions, and some show us the planet more splendidly brilliant than it appears at others. The oppositions at which Mars shows most brilliant take place, fortunately, in the summer and early autumn—the seasons which are most agreeable for outdoor observation. He is then traveling through that region of the sky, sparse in stars, that lies between Sagittarius and Aries; and, since the ecliptic there runs rather low in the sky, he can easily be observed at any time in the night without any neck-breaking postures.

These favorable oppositions occur in the summer because the earth is in line in the latter part of August with that point in the orbit of Mars where the planet makes its nearest approach to the sun. Oppositions never occur when Mars is exactly at that point; but they do occur when he is very near it, and at such times we see him in his greatest glory. This happens once every fifteen or seventeen years. But at any summer or early-autumn opposition Mars is not very far from this nearest point to the sun, so that at any oppositions during these seasons he is very brilliant and almost as bright as when he is at his best.

The earth is in line in the winter with that part of Mars’s orbit which carries him farthest from the sun, and at opposition then he is much less bright than at the summer oppositions. He is at the same time in those constellations which pass nearly overhead in the sky, and cannot be quite so comfortably seen at all times in the night as he can be in the summer. The very best and most brilliant oppositions occur in the latter part of August or in the early part of September; the least favorable ones occur in February. The others vary in brilliancy according to their distance from these favorable and unfavorable dates, all the summer ones being quite brilliant, and all the winter ones much less so. At any opposition, though, however unfavorable, the planet is much nearer to us and much brighter than when in conjunction.

It is worth one’s while, even at some inconvenience, to see Mars at whatever time he is in opposition, for he is a delight to the observer, and always notable in the part of the skies through which he is then passing. There are some aspects of the planet that are so charming at a winter opposition that it is a positive loss not to have seen him at such times. He is more isolated and conspicuous in the summer; but he fits well in that gay company of winter stars that shine more brilliantly than any others, and we can easily feel something akin to family pride as we watch him moving so graciously among them.

Mars makes a complete circuit of the skies, and comes back into the same position with relation to the sun and the earth on an average every seven hundred and eighty days, which makes his synodic period longer than that of any other planet. Owing to the great eccentricity of his orbit, and his consequent unequal motion in the various parts of it, the synodic period varies as much as thirty-five or thirty-six days. One cannot say, therefore, without computation of some length, just exactly how many days will elapse between any two single oppositions.

For mere purposes of naked-eye observation the variations in the synodic period of Mars do not make any difference, for the planet is in view practically all night for many nights before and after opposition, with changes of brightness too small to be noticed by an untrained eye. For at least two months at the time of opposition it has almost the same aspect to us. At that time it is always in the east early in the evening, and shines all night. For nearly nine months afterward it is visible and conspicuous in the evening sky, appearing each evening nearer and nearer to the western horizon, until finally, in a little more than a year after opposition, it passes behind the sun and becomes a morning star. But, as it then rises before the sun and passes across the heavens in the daytime, it is invisible to us. It is pleasant, however, at such times to know that as the sun passes across the skies in its daily journey Mars is up there, within a certain distance from it, making the same journey with it, beaming down upon us with the same lively light that it shows at night, and could be as well seen at any time but for the too dazzling rays of the sun.