Astronomy for Everybody/Part 4/Chapter 2

To set forth what is known of the major planets we shall take them up in the order of their distance from the sun. The first planet reached will then be Mercury. It is not only the nearest planet to the sun, but much the smallest of the eight; so small, indeed, that, but for its situation, it would hardly be called a major planet. Its diameter is about two fifths greater than that of the moon, but, the volumes of bodies being proportional to the cubes of their diameters, it has about three times the volume of the moon.

It has far the most eccentric orbit of all the major planets, though, in this respect, it is exceeded by some of the minor planets to be hereafter described. In consequence, its distance from the sun varies between wide limits. At perihelion it is less than twenty-nine millions of miles from the sun; at aphelion it goes out to a distance of more than forty-three millions of miles. It performs its revolution around the sun in a little less than three months; to speak more exactly, in eighty-eight days. It therefore makes more than four revolutions in a year.

Performing more than four revolutions around the sun while the earth is performing one, we readily see that it must pass conjunction with the sun at certain regular though somewhat unequal intervals. To show ​the exact nature of its apparent motion let the inner circle of the diagram represent the orbit of Mercury and the outer one that of the earth. When the earth is at E, and Mercury at M, the latter is in inferior conjunction with the sun. At the end of three months it will have returned to the point M, but it will not yet be in conjunc-

tion, because, in the meantime, the earth has moved forward in its orbit. When the earth reaches a certain point F, Mercury will have reached the point N and will again be in inferior conjunction. This revolution from one inferior conjunction to another is called the synodic revolution of the planet. In the case of Mercury this is somewhat less than one third more than the time of actual ​revolution; that is to say, the arc MN is a little less than one third of the circle.

Now suppose that when the earth is at E, Mercury, instead of being at M is near the highest point A of the orbit as represented in the figure. It will then be at its greatest apparent distance from the sun as we see it from the earth; or, in technical language, at its greatest east elongation. Being east of the sun it will

then set after the sun, by a time generally between an hour and a quarter and an hour and a half. This is the most convenient time for seeing it. If the sky is clear, it will readily be seen in the twilight from half an hour to an hour after sunset. At the opposite elongation, near C, it is west of the sun; then it rises before the sun and may be seen in the morning twilight.

The best time to make a telescopic study of Mercury is late in the afternoon, when it is near east elongation. ​or shortly after sunrise, if it rises before the sun. Supposing it east of the sun, it will probably be visible in the telescope at any time after noon, but the air is generally disturbed by the sun's rays so that it is hardly possible to make a good observation at that time. Late in the afternoon the air grows steadier, so that the planet can be better observed. But, after sunset, the planet is seen through a continually increasing extent of atmosphere, so that the seeming disturbance again begins to increase. Owing to these circumstances it is the most difficult of all the planets to study in a satisfactory way, and observers differ very much as to what can be seen on its surface.

The first observer who thought he could see any features on the surface of this planet was Schröter, a German. When Mercury presented the form of a crescent he fancied that its south horn seemed blunted at intervals. He attributed this to the shadow of a lofty mountain; and by observing the intervals between the blunted appearance he concluded that the planet revolved on its axis in twenty-four hours and five minutes. But Sir William Herschel, who observed at the same time with much more powerful instruments, could not see anything of the kind.

Until quite recently nearly all observers agreed with Herschel that no time of rotation could be certainly determined. But a few years since, Schiaparelli, observing with a fine telescope in the beautiful sky of northern Italy, noticed that the aspect of the planet seemed unchanged day after day. He was thus led to the conclu​sion that it always presents the same face to the sun, as the moon presents the same face to the earth. This view was shared by Mr. Lowell, observing at the Flagstaff Observatory. But the observation is too difficult to permit us to regard the fact as established. All that a conservative astronomer would be willing to say is that as yet we know nothing of the revolution of Mercury on its axis.

Drawings showing the face of Mercury have been made by several astronomers. As it is seen under all ordinary conditions no special features are well marked. Very different is the case at the Lowell Observatory in Flagstaff, Ariz. The most singular feature of its surface in the latter picture consists in the dark lines which cross it. These have not been seen by other observers, and, until they are established by independent evidence, astronomers will be sceptical as to their reality. The reason of this will be stated later in connection with the planet Mars.

Owing to the various positions of Mercury relative to the sun it presents phases like those of the moon. These depend upon the relation of the dark and the illuminated hemispheres relative to the direction in which we see the planet. The hemisphere which is turned away from the sun, being in darkness, is always invisible to us. At superior conjunction the illuminated hemisphere is turned toward us and the planet seems round, like a full moon. As it moves from east elongation to inferior conjunction, more and more of the dark hemisphere is turned toward us, and less and less of the illuminated one. But this ​disadvantage is counterbalanced by the fact that the planet continually comes nearer during the interval, so that we get a better view of whatever portion of the illuminated hemisphere may be visible to us. Its apparent form and size at different times during its synodic revolution go through a series of changes similar to those shown in the next chapter in the case of Venus.

The question whether Mercury has an atmosphere is also one on which opinions differ, the prevailing opinion being in the negative. It seems quite certain that, if it has one, it is too rare to reflect the light of the sun.

It will be readily seen that, if an inferior planet revolved around the sun in the same plane as the earth, we should see it pass over the sun's disk at every inferior conjunction. But no two planets revolve in the same plane. Of all the major planets the orbit of Mercury has the largest inclination to that of the earth. In consequence, when in inferior conjunction, it commonly passes a greater or less distance to the north or to the south of the sun. If, however, it chances to be near one of its nodes at the time in question, we shall see it as a black spot passing across the sun's disk. This phenomenon is called a transit of Mercury. Such transits occur at intervals ranging between three and thirteen years. They are observed with much interest by astronomers because it is possible to determine with great precision the time at which the planet enters upon the solar disk, and leaves it again. Knowing these times, ​valuable information is afforded respecting the exact law of motion of the planet.

The first observation of a transit of Mercury was made by Gassendi on November 7, 1631. His observation is not, however, of any scientific value at the present time, owing to the imperfection of his instruments. A somewhat better but not good observation was made by Halley, of England, in 1677, during a visit to the island of St. Helena. Since that time the transits have been observed with a fair degree of regularity. The following table shows the transits that will be visible during the next fifty years, with the regions of the earth in which each may be seen:

Observations of transits of Mercury since 1677 have brought out one of the most perplexing facts of astron​omy. The orbit of this planet is found to be slowly changing its position, its perihelion moving forward by about forty-three seconds per century farther than it ought to move in consequence of the attraction of all the known planets. This deviation was discovered in 1845 by Le Verrier, celebrated as having computed the position of Neptune before it had ever been recognised in the telescope. He attributed it to the attraction of a planet, or group of planets, between Mercury and the sun. His announcement set people to looking for the supposed planet. About 1860, a Dr. Lescarbault, a country physician of France, who possessed a small telescope, thought he had seen this planet passing over the disk of the sun. But it was soon proved that he must have been mistaken. Another more experienced astronomer, who was looking at the sun on the same day, failed to see anything except an ordinary spot. It was probably this which misled the physician-astronomer. Now, for forty years, the sun has been carefully scrutinised and photographed from day to day at several stations without anything of the sort being seen.

Still, it is possible that little planets so minute as to escape detection in passing over the sun's disk may revolve in the region in question. If so, their light would be completely obscured by that of the sky, so that they might not ordinarily be visible. But there is still a chance that, during a total eclipse of the sun, when the light is cut off from the sky, they could be seen. Observers have, from time to time, looked for them during total eclipses. In one instance something of the sort was ​supposed to be found. During the eclipse of 1878, Professor Watson, of Ann Arbor, and Professor Lewis Swift, both able and experienced observers, thought that they had detected some such bodies. But critical examination left no doubt that what Watson saw was a pair of fixed stars which had always been in that place. How it was with the observations of Professor Swift has never been certainly ascertained, because he was not able to lay down the position with such certainty that positive conclusions could be drawn.

Notwithstanding such failures, observers have repeated the search during several of the principal total eclipses. The writer did so during the eclipse of 1869, and again during that of 1878, the search being made with a small telescope. In recent times the powerful agency of photography has been invoked by Professors Pickering and Campbell during the eclipses of 1900 and 1901. Campbell's results during the latter eclipse were the most decisive yet reached. With his photographic telescope some fifty stars were photographed, some as faint as the eighth magnitude, but they were all found to be known objects. It therefore seems certain that there can be no intramercurial much brighter than the eighth magnitude. It would take hundreds of thousands of such planets as this to produce the observed motion of Mercury. So great a number of these bodies would produce a far brighter illumination of the sky than any that we see. The result therefore seems to be conclusive against the view that the motion of the perihelion of Mercury can be produced by intramercurial planets. In addition to all these difficul​ties in supposing the planet to exist we have the difficulty that, if it did exist, it would produce a similar though smaller change in the position of the nodes of either Mercury or Venus, or both.

Altogether, the evidence seems conclusive against the reality of any bodies whose attraction could produce the observed deviation, which still remains unexplained. The most recent supposition on the subject is that the force of gravitation deviates slightly from the law of the inverse square. But this requires farther investigation.