While Mercury is one of the five planets that can be seen with the naked eye, it must be confessed that he does not play any important part in the great spectacle of nature as we see it in the skies. But in a certain way this only adds to our interest in him. The very rarity of his appearances and the difficulty of finding him give a zest to the search, and a sense of achievement, when it is successful, that one does not have with regard to the other planets. It is something akin to the feeling one has when, after a long tramp to some secluded recess in the woods in search of the shy pink lady’s slipper, a splendid specimen of that lovely flower suddenly comes into view hanging gaily on its stalk, ready for the use of whatever fairy foot may tread its shady groves. Then, too, the spring o’ the year is the most likely time to see Mercury in the evening sky. He comes into his best position for this view The reason Mercury is so elusive is that he is always very near the sun, and in consequence his light is dimmed by the brighter light shed by that luminary until it is well below the horizon; and after the sun has set, the planet is so involved in the usual haziness of the atmosphere near the horizon that the conditions must be very favorable in order to see him. Though there are recorded observations of Mercury as far back as nearly three hundred years before Christ, yet some of the older of the modern astronomers, before the days of the perfected telescope, are said not to have seen him at all; and the most important observations of the planet nowadays are made in broad daylight, when it is higher up in the skies and free from the mists of the horizon. This can be done by means of a powerful tele WHEN AND WHERE TO FIND MERCURYMercury is never more than twenty-eight degrees from the sun, and is bright We can never see Mercury with the naked eye except when he is near one elongation or the other; and even then he is visible only about an hour after the sun is down in the evening or about an hour before it rises in the morning. Three times each year he appears in the evening for more or less than a week, according to the situation of the The mean synodic period of Mercury is about one hundred and sixteen days, or a little less than four months. That is, he returns to greatest eastern elongation and can be seen in the evening sky about every one hundred and sixteen days, and the same length of time elapses between his appearances in the morning sky at greatest western elongation. But this mean synodic period is made up of synodic periods varying in different revolutions from one hundred and five to one hundred and thirty-four days. So, though one may mark the dates at which the various positions of the planet occurred during any one revolution, one cannot so easily determine the exact time at which he will be found in the same positions at the next revolution; that is, whether the revolution will take place in less or more than one hundred and sixteen days. The earth and the But, lacking an almanac, one will generally find that Mercury will return to the same position relative to the earth and the sun within a few days of his mean synodic period. Three periods, however much they may vary individually, are almost always equal to three hundred and forty-eight days, or three times the mean period. This is seventeen days less than a year. Hence, if one is lucky enough to have seen Mercury at eastern elongation one spring, and will look the next year about seventeen days earlier, the planet will be found a little to the east (about fifteen degrees) of where he was when first seen the year before. He is there in the same position with relation to us and the sun that he had the preceding spring, but in a slightly different relation to us and the stars, because the sun lacks seventeen days of having completed its apparent yearly journey around the zodiac. It must still go through about one half of a constellation. When Mercury shows himself at eastern elongation, he may be seen in the west as an evening star for somewhere near a week, each evening drawing nearer to the sun. When he disappears from view he passes between us and the sun, and about four weeks later appears in the morning sky before the sun rises. Under favorable conditions he is again visible for a week or more; and then, again approaching the sun, he can be seen no more for about ten weeks, during which time he passes through superior conjunction on the other side of the sun from us and comes back to eastern elongation. Thus we can get, under very favorable conditions, six short views of Mercury during the year—three in the evening and three in the morning. So many views, however, are rarely secured by any but the professional observer. The circumstances may well be considered felicitous if one succeeds in getting a glimpse of him once or twice a year—at his favorable situation in the evening in the spring and the morning in the autumn. The sight of him, though, is truly worth a little inconvenience—even to the extent of facing a cold evening wind in the very early spring or getting out of a comfortable bed It is hardly possible to say exactly where one can find Mercury at all times during a long succession of revolutions. Moreover, it is not necessary. These computations are made anew each year by experts in the employ of the government, and the result is published in the Nautical Almanac. From there it finds its way into all almanacs, so it is easy of access to any one. In the almanacs Mercury is represented by the sign (?). It is a conventionalized form of the caduceus, or wand, carried by the god Mercury as a symbol of his power. The next seven eastern and western elongations of Mercury occurring after the publication of this book are as follows: DISTANCE AND BRIGHTNESSOf all the planets Mercury is nearest the sun. His average distance is thirty-six million miles. He is nearly eighty times nearer than Neptune, the outermost planet, and more than two and one-half times nearer than we are. But his orbit departs so far from being a circle that his distance from the sun varies as much as fifteen million miles. When he is nearest the sun, or in perihelion, he is only twenty-eight million miles from it; when he is farthest, or in aphelion, his distance is forty-three million miles. There is even greater variation in his distance from us. The difference between his least possible and his greatest possible distance from us is as much as eighty-nine millions of miles. For the earth has an elliptical orbit as well as Mercury, and when we are at perihelion, which occurs in the winter, we are three millions of miles nearer to the sun than we are in mid-summer. If Mercury chances to be then at his greatest distance from the sun, and also at inferior conjunction, or between us and the sun, he is only forty-seven millions of miles from us. If, when we are farthest from the sun, he These changes in distance from the earth have much to do with Mercury’s changes in apparent brightness to us. At his brightest, when he appears at greatest elongation and we can see him without a telescope, he is brighter than Arcturus, the brilliant first-magnitude star in BoÖtes, that swings over us nightly from early spring to late autumn. When seen with the naked eye, he is also red in color, somewhat like Arcturus; but through a telescope he is dull silver, like the moon, or even more ashy in his paleness. As he goes farther and farther from us he becomes dimmer and dimmer and can be followed only with a telescope until, even with this aid to vision, he is lost in the rays of the sun at superior conjunction. His apparent diameter as mathematically measured varies from five seconds, when he is farthest away, to thirteen seconds, when he is nearest. When he is at his nearest possible distance from us, light travels from Mercury to us in a little more than four minutes. At his Another cause of Mercury’s apparent change in brightness is due to the fact that, in common with Venus, he goes through phases from crescent to full like the moon. This is, as we have seen, a result of his shining only by reflected light and of his orbit’s being between ours and the sun. If he shone by his own light, he would be at his nearest approach to us a very brilliant body indeed. As it is, his dark side is turned toward us when he is nearest, and when his full face is illuminated he is on the far side of the sun. We see half of his face when he is at greatest elongation; but he is brightest when we see less than half, because he is then nearer to us, and the difference in distance more than compensates for the difference in illumination. These phases cannot be seen with the naked eye, but it requires only a small telescope to show them, and a very charming little As a mere place from which to view the other bright bodies Mercury would be far superior to the earth. He not only has the sun nearly seven times larger in appearance at its mean distance than we see it, but, being himself nearest the sun, all the other planets are outer planets in relation to him, and all have their discs fully illuminated. The earth and the moon, as seen from Mercury, would show as a splendid pair of stars circling about each other, the earth more brilliant than any first-magnitude star, and the moon of the third magnitude, or about as bright as Phecda, the star at the bottom of the bowl of the Big Dipper, just under the beginning of the handle. The earth would show a disc of about twenty Venus would appear from Mercury as much as four times as large as she seems to us—a veritable little moon, and always full, her size varying slightly as Mercury speeded back and forth from the farthest to the nearest point in his orbit, changing the extreme of the distance between them from one hundred and ten million to less than twenty-four million miles. If Mercury needed a moon, he could well find some consolation for his lack of it in the presence of the lovely Venus in his sky. MERCURY’S SIZE AND THE CONSEQUENCES OF ITMercury is the smallest of all the major planets. His diameter is about three thousand miles. It is only about nine hundred miles greater than that of our moon. The surface of Mercury is only one-seventh that of the earth, and his volume only one-twentieth. Jupiter and Saturn each have a satellite that is considerably larger. Mercury would make a splendid satellite or a giant asteroid, but as a planet seems hardly to have had a fair chance in life. For being a small planet means something more than being constructed on smaller lines than some others are. It means a difference in physical development. It means less power to hold the gases that compose an atmosphere, which is the cover that shields the planets from the too burning rays of the sun and keeps their internal heat from radiating too quickly into space. It means less power to resist the tidal friction that the parent body uses as a brake to retard rotation. It means a shorter time of activity in life, and a long, dull, monotonous old age. The nucleus that was detached from the great spiral, or the portion of nebula that was Mercury’s force of gravity is small. It is smaller than that of any of the other planets. It is a little less than one-quarter that of the earth. The same weight of feathers that would compose a pillow here would make a whole feather bed on Mercury. Any object weighing one hundred pounds here would weigh only twenty-four there. The materials composing our earth and all the planets are held together only by the force of gravity. The air we breathe would dart off into space with almost incredible fleetness if the earth had not sufficient gravitative force to hold it. Its particles are So a planet with a small mass starts out in life with a disadvantage. It not only has little power to grow by drawing in particles from its environment, but also has little power to hold such as by their nature are volatile and swift of motion, as the molecules of gases are. The mass of Mercury is not exactly known. The only way we have of measuring the masses of the planets is by their influence through gravitation on other bodies near them. When a planet has In these ways the mass of Mercury has been found, with reasonable belief in its accuracy, to be about three one-hundredths that of the earth. Yet there are, indeed, considerable differences regarding it among astronomers. The exact figures are not important to any but the close student. It is certain that the mass of Mercury is very small—so small that the planet probably WHAT THE SUN DOES FOR MERCURYIt is probable that Mercury has no alternations of light and darkness, causing day and night such as we know them. That is, the planet does not rotate on its axis in such a way as to turn first one side and then the other toward the sun as the earth does. In this, as in some other things, Mercury must accept the fate that overtakes many other small bodies which revolve around large ones—that of our moon, for instance, and the satellites of some of the other planets. Working under the law of gravitation, which gives such power to the large bodies, the sun has so retarded the rotation of Mercury that the planet now makes but one rotation on its axis during one circuit around that central body, and so keeps always the same face toward the sun. Some astronomers do not regard this as having been wholly proved; but all the later observations of Mercury strongly indicate that it is the fact, and it is coming to be more and more regarded as established. But, even if this is the predicament into which Mercury has come, the planet is prob Since Mercury is less than two-thirds as far from the sun at perihelion as he is at aphelion, there is a corresponding variation in his rate of speed. When he is nearest the sun, at perihelion, he darts along at the rate of thirty-five miles a second; at aphelion, when he is farthest from the sun, he travels only twenty-three miles a second. Twenty-three miles in one second is not exactly a snail’s pace, terrestrially considered, and it is faster than the earth moves at any time; but the planet was named Mercury because of his swiftness, and we would not expect much lagging even when he is moving at his slowest gait. This difference in speed in different These librations result in there being two strips on the surface of Mercury—one on each side—which undoubtedly have a day and night, varying in length in the different parts of the strips. The part that lies nearest the illuminated side of the planet has alternate periods of sunlight and darkness, each of considerable duration, while that part nearest the dark side has merely a glimmer of sunlight every eighty-eight days, which is Mercury’s sidereal year, or the time required for him to make one revolution around the sun. These two strips on which the light varies comprise about one-eighth of the surface of Mercury. One half of his entire surface is always light, and of the other It is possible that on those parts of Mercury where the sunlight and darkness are unstable there may be something resembling a tolerable temperature. They are something more than a thousand miles in breadth, and perhaps near the center of them the sun may give heat sufficient to enliven and yet not burn. More than likely, they are alternately scorched and frozen. For it takes more than the mere presence of sunlight to make a climate tolerable. Atmosphere is what is necessary, and we have seen that Mercury has probably lost practically all his atmosphere long, long ago. An atmosphere absorbs much of the radiant energy that comes from the sun before it reaches the more solid parts of a planet, and it also acts as a blanket in preventing the too rapid escape of such heat as the planet may have acquired. Thus it has the doubly beneficent office of tempering the rays that would otherwise be scorching and of hindering a radiation that would leave the planet stiffened and frozen. Stiffened and frozen is what the dark side On the other side of Mercury the heat is beyond anything we have any notion of. With an equal atmosphere it would receive from the sun six thousand times as much light and heat as Neptune on an equal space, and, on an average, seven times as much as the earth. At Mercury’s distance from the sun his hot side would be more than 300° above zero, if there were absolutely no atmospheric protection. Even though tempered by a thin atmosphere, as it may be, the heat on this side is still probably enough to boil away any water that might be there and to change some other substances from what we regard as their normal state. Stability, at least, is a quality of the hot It is the atmosphere also which causes twilight, as well as the gradual changing from heat to cold. With no atmosphere, we would drop from full daylight to the darkness of starlight at the setting of the sun. So, with the thin air that Mercury probably has (if he has any), the two zones which are alternately light and dark, and hot and cold, are not much better off than the parts which are permanently either light or dark. They are plunged alternately from the temperature and light of the hot side of Mercury to the temperature of the cold side, with few gradations to prepare them for such extremes. Thus the only part of the planet that might be expected to have any variations of sea 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 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, 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? TRANSITSOccasionally 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 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 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. |