CHAPTER V
MERCURY
The planet nearest to the sun is not one which has proved itself particularly attractive to observers in the past; and the reasons for its comparative unattractiveness are sufficiently obvious. Owing to the narrow limits of his orbit, he never departs further from the sun either East or West than between 27° and 28°, and the longest period for which he can be seen before sunrise or after sunset is two hours. It follows that, when seen, he is never very far from the horizon, and is therefore enveloped in the denser layers of our atmosphere, and presents the appearance sadly familiar to astronomers under the name of 'boiling,' the outlines of the planet being tremulous and confused. Of course, observers who have powerful instruments provided with graduated circles can find and follow him during the day, and it is in daylight that nearly all the best observations have been secured. But with humbler appliances observation is much restricted; and, in fact, probably many observers have never seen the planet at all.
Views of Mercury, however, such as they are, are by no means so difficult to secure as is sometimes supposed. Denning remarks that he has seen the planet on about sixty-five occasions with the naked eye—that in May, 1876, he saw it on thirteen different evenings, and on ten occasions between April 22 and May 11, 1890; and he states it as his opinion that anyone who will make it a practice to obtain naked-eye views should succeed from about twelve to fifteen times in the year. During the spring of 1905, to take a recent example, Mercury was quite a conspicuous object for some time in the Western sky, close to the horizon, and there was no difficulty whatever in obtaining several views of him both with the telescope and with the naked eye, though the disc was too much disturbed by atmospheric tremors for anything to be made of it telescopically. In his little book, 'Half-hours with the Telescope,' Proctor gives a method of finding the planet which would no doubt prove quite satisfactory in practice, but is somewhat needlessly elaborate. Anyone who takes the pains to note those dates when Mercury is most favourably placed for observation—dates easily ascertained from Whitaker or any other good almanac—and to carefully scan the sky near the horizon after sunset either with the naked eye, or, better, with a good binocular, will scarcely fail to detect the little planet which an old English writer more graphically than gracefully calls 'a squinting lacquey of the sun.'
Mercury is about 3,000 miles in diameter, and circles round the sun at a mean distance of 36,000,000 miles. His orbit is very eccentric, so that when nearest to the sun this distance is reduced to 28,500,000, while when furthest away from him it rises to 43,500,000. The proportion of sunlight which falls upon the planet must therefore vary considerably at different points of his orbit. In fact, when he is nearest to the sun he receives nine times as much light and heat as would be received by an equal area of the earth; but when the conditions are reversed, only four times the same amount. The bulk of the planet is about one-nineteenth that of the earth, but its weight is only one-thirtieth, so that its materials are proportionately less dense than those of our own globe. It is about 3½ times as dense as water, the corresponding figure for the earth being rather more than 5½.
Further, it is apparent that the materials of which Mercury's globe is composed reflect light very feebly. It has been calculated that the planet reflects only 17 per cent. of the light which falls upon it, 83 per cent. being absorbed; and this fact obviously carries with it the conclusion that the atmosphere of this little world cannot be of any great density. For clouds in full sunlight are almost as brilliantly white as new-fallen snow, and if Mercury were surrounded with a heavily cloud-laden atmosphere, he would reflect nearly five times the amount of light which he at present sends out into space.
As his orbit falls entirely within that of our own earth, Mercury, like his neighbour Venus, exhibits phases. When nearest to us the planet is 'new,' when furthest from us it is 'full,' while at the stages intermediate between these points it presents an aspect like that of the moon at its first and third quarters. It may thus be seen going through the complete series from a thin crescent up to a completely rounded disc. The smallness of its apparent diameter, and the poor conditions under which it is generally seen, make the observation of these phases by no means so easy as in the case of Venus; yet a small instrument will show them fairly well. Observers seem generally to agree that the surface has a dull rosy tint, and a few faint markings have, by patient observation, been detected upon it (Fig. 20); but these are far beyond the power of small telescopes. Careful attention to them and to the rate of their apparent motion across the disc has led to the remarkable conclusion that Mercury takes as long to rotate upon his axis as he does to complete his annual revolution in his orbit; in other words, that his day and his year are of the same length—namely, eighty-eight of our days. This conclusion, when announced in 1882 by the well-known Italian observer Schiaparelli, was received with considerable hesitation. It has, however, been confirmed by many observers, notably by Lowell at Flagstaff Observatory, Arizona, and is now generally received, though some eminent astronomers still maintain that really nothing is certainly known as to the period of rotation.
FIG. 20.—MERCURY AS A MORNING STAR. W. F. DENNING, 10-INCH REFLECTOR.