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.