The only changes in climate which may have an appreciable effect are mainly those caused by the eccentricity of Mercury’s orbit, which carries him so near the sun at certain times and so comparatively far away at others. When he is nearest the sun he receives more than twice as much heat and light as when he is farthest away. At aphelion he receives four times as much heat and light as the earth. At perihelion the amount of heat and light is increased to more than nine times that of the earth. Since it takes Mercury a little more than twelve weeks to make one revolution around the sun, he passes from nearest distance to farthest, or the reverse, every six weeks. And thus, as viewed from the planet, the sun expands gradually for six weeks until it has increased its diameter two and one-half times, and the next six weeks it diminishes in the same proportion. At such times, of course, the amount of heat is more or less according to the planet’s distance from the sun; but all the time it is very great.
Moreover, it is believed that the axis on which Mercury rotates stands perpendicular to his orbit. This being the case, there would be on Mercury no change of seasons such as the earth has. The earth’s axis is inclined a little more than twenty-three degrees to its orbit, and from this we get the sun’s rays in a great variety of directions and different degrees of obliquity, causing the seasons, as we know them, in grateful variation. With the axis perpendicular, as it probably is in the case of Mercury, the sun’s rays fall on the face of the planet always with the same degree of directness, the only relief from their greatest heat being when the planet backs away from the sun every six weeks, and when in his librations he turns first one sun-burned cheek and then the other toward the coolness of space.
Thus we must regard the smallest of our family of planets, Mercury, as always the dwarf among us, with never a fair chance to develop a rich and luscious life according to our ideas of such a life. Beaten by the sun’s hard rays, and with no sufficient atmospheric protection; pulling always at his tether, but held firmly with his face to the center; circling at times with mercurial swiftness and thus cheating the sun into sending its rays farther toward the dark, cold side of him than it otherwise would, and with all his defects from a human point of view, we may still regard him as a right merry, roguish little planet, after all. He may be prematurely aged, he may have missed many experiences that the larger planets are having, he may have a long time to wait for the final change that will reunite us all; but he is not lying in sluggish inactivity until it comes.
In view of the fact that he is the only planet that twinkles, may it not suggest, when we see his ruddy face peering through the thick atmospheric mists near our horizon, that the impish little body is winking at us, and that it may be with planets as it is with people: they may not always be in an unfortunate plight because their fate is different from ours?
TRANSITS
Occasionally Mercury passes at inferior conjunction between us and the disc of the sun, appearing like a black spot against the sun, and thus makes what we call a transit. Because the planet is so small, his transit across the sun cannot be seen with the naked eye; but it is an interesting phenomenon to those who can view it with a telescope, though, apparently, astronomers do not regard it as having any great scientific importance. It is during a transit, however, that we watch for confirmation of the theories concerning Mercury’s atmosphere, which, if it were a reality, would show a diffused light about the planet; and until this question is settled beyond any dispute it will always come up at the time of a transit of Mercury. At nearly every transit some observer sees these indications of an atmosphere; but the better the telescope, the less they seem to be seen. Hence it is probable that there is an illusion somewhere either of eye, or instrument, or mind, and that the majority opinion, which accords to Mercury practically no atmosphere, is about the correct one.
These transits occur at intervals of seven, thirteen, or forty-six years, according to the position of the earth. They would occur every time that Mercury passed inferior conjunction if the earth’s orbit and that of Mercury were in exactly the same plane. But the orbit of Mercury, we have seen, is tilted out of the plane of the ecliptic, which marks our orbit, seven degrees, so that the only time the earth and the planet are anywhere nearly in the same plane is when they are at or near the points where their orbits cross each other.
The earth is near the two points where Mercury crosses the ecliptic about May 8th and November 9th, so that transits can occur only near these dates. Mercury passes these points four times every year, or once in each revolution around the sun. But the earth is not always there at the same time, and it is because of this that transits occur only in periods of seven, thirteen, or forty-six years. They occur more frequently in November than in May. The last transit was in November, 1907. The next will be on November 7, 1914, and there will not be another in November until 1927, an interval of thirteen years. But at the point where the May transits occur there will be one on May 7, 1924.