Mercury (planet)

“ We have seen how Mercury revolves in a path inside that of Venus, and it is therefore nearer to the sun. Indeed, Mercury is so close to the sun that it is generally overpowered by his brilliance and cannot be seen at all. Like every other planet, Mercury is lighted by the sun’s rays, and shows phases in the telescope just as the moon does (Fig. 48) . In this figure the different apparent sizes of the planet at different parts of its path are shown. Of course the nearer Mercury is to the earth the larger does it seem. ”

“ Mercury is a globe composed, like our earth, of materials possessing in themselves no source of illumination. One hemisphere of the planet must necessarily be turned towards the sun, and this side is accordingly lighted up brilliantly by the solar rays. When we look at Mercury we see nothing of the non-illuminated side, and the crescent is due to the foreshortened view which we obtain of the illuminated part. The planet is such a small object that, in the glitter of the naked-eye view, the shape of the luminous body cannot be defined. ”

“ Enough seems now to have been said to indicate clearly the hopelessness of looking for any analogies between Mercury and the earth which would warrant the conclusion that the former planet is capable of supporting inhabitants or forms of life resembling those that swarm upon the latter. If we would still believe that Mercury is a habitable globe we must depend entirely upon the imagination for pictures of creatures able to endure its extremes of heat and cold, of light and darkness, of instability, swift vicissitude, and violent contrast. ”

“ But, even if this is the predicament into which Mercury has come, the planet is probably not in so bad a plight as many another body to which the same sort of thing has happened. The extreme eccentricity of his orbit, which has given him the true mercurial temperament, resulting in sprightliness, agility, and changeableness, is accountable for some mitigating circumstances. The sun may hold him so that he cannot turn his face away from that luminary ”

“ 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. ”

“ Mercury rotates synchronously with its revolution—that is, once in 88 days—on an axis sensibly perpendicular to its orbital plane. No certain signs of a Mercurian atmosphere are visible. The globe is seamed and furrowed with long narrow markings, explicable as cracks in cooling. It is, and always was, a dead world. From micrometrical measures, moreover, the inferences were drawn that the planet's mass has a probable value about 1/20 that of the earth, while its mean density falls considerably short of the terrestrial standard.
The theory of Mercury's movements has always given trouble. ”

“ The inhabitants of Mercury in those six pregnant weeks see the sun expand in the sky to more than two and a half times its former magnitude, while the solar heat poured upon them swiftly augments from something more than four and a half times to above eleven times the amount received upon the earth! Then, immediately, the retreat of the planet begins, the sun visibly shrinks, as a receding balloon becomes smaller in the eyes of its watchers, the heat falls off as rapidly as it had previously increased, until, the aphelion point being reached, the process is again reversed. ”

“ Mercury is the only planet that in color even suggests Mars, and for Mercury it can never be mistaken after one has once seen the two planets. Mercury, we know, is always very near the sun; but when visible at all is, even in that unfavorable situation, always as bright as a first-magnitude star. Mars is near the sun, to our view, only when it is approaching conjunction, and it is then so far from us that it always appears as a rather small star, and, while never insignificant, is, in this situation, quite inconspicuous even as compared with the rarely visible Mercury. ”

“ In either position it never twinkles as Mercury sometimes does, but shines so steadily and softly that at times its disc can almost be seen with the naked eye, and it has such brilliancy that its light can often be seen in the daytime, if one knows when and how to look for the planet. At its brightest it frequently throws a light sufficiently strong to cast a shadow, as one may easily prove by holding a book or some other opaque object between Venus and a white background, such as the wall of a white house. ”

“ Mercury is enveloped in a very dense, thick atmosphere, which doubtless sensibly tempers the solar heat, for the Sun exhibits to the Mercurians a luminous disk about seven times more extensive than that with which we are familiar on the Earth, and when Mercury is at perihelion (that is, nearest to the Sun) , his inhabitants receive ten times more light and heat than we obtain at midsummer. In all probability, it would be impossible for us to set foot on this planet without being shattered by a sunstroke. ”

“ 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. ”

“ 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 three-eighths are always dark. ”

“ When this planet is at its utmost distance from the sun the intensity of solar radiation is even then more than four times greater than the greatest heat which ever reaches the earth. But when Mercury, in the course of its remarkable changes of distance, draws in to the warmest part of its orbit, it is exposed to a terrific scorching. The intensity of the sun's heat must then be not less than nine times as great as the greatest radiation to which we are exposed. ”

“ So far as we know at present, Mercury is the nearest planet to the Sun. The circumstances under which it presents itself to us and a brief general account of its movements have already been stated. In the present chapter, therefore (and this remark applies in substance to each of 58 the succeeding chapters appropriated to particular planets) , I shall limit myself to such topics as seem to be of interest to an observer armed with a telescope. Mercury, as already mentioned, exhibits from time to time phases which may be said to be the same as those of the moon ”

“ We live at a time when many have been formed, and many are still forming. Our own solar system is to be regarded as completed, supposing its perfection to consist in the formation of a series of planets, for there are mathematical reasons for concluding that Mercury is the nearest planet to the sun, which can, according to the laws of the system, exist. ”

“ It is well known that mathematical theory, based upon Newton's law of gravitation, has not yet fully accounted for the motion of Mercury. The perihelion of its orbit moves forward at least 40′′ in a century more than theory calls for. The most plausible way of accounting for this progression has been the supposition that an undiscovered planet, or a group of small planets, exists within the orbit of Mercury. The search for such objects has been a well-defined eclipse problem; the sun-lit sky prevents effective search by every-day methods. ”

“ The question seems to be settled, with considerable certainty, that nothing exists inside of Mercury large enough to be dignified by the name of planet. There may be, and there probably are, for the perturbations of Mercury indicate it, multitudes of small masses circulating around the sun like the planets, being fragments of comets or condensations of primitive matter, whose combined luster is seen in the zodiacal light. ”

“ Mercury presents itself to the sun in different phases of its cycle in a similar manner. It constantly directs one of its diameters, not toward the focus of its elliptical orbit which is occupied by the sun, but toward the second focus. These two foci being distant from one another not less than a fifth of the whole diameter of the orbit of Mercury, the libration of the planet is enormous. ”

“ Underlying that hypothesis was the assumption that all the various appearances arose from the oscillations of a single body, and hence the discovery of Mercury was established on a basis as firm as the discovery of Jupiter or of Venus.
In the latitudes of the British Islands it is generally possible to see Mercury some time during the course of the year. It is not practicable to lay down, within reasonable limits, any general rule for finding the dates at which the search should be made; but the student who is determined to see the planet will generally succeed with a little patience. ”

“ The general impression gained is that the tiny planet is barren, rough and mountainous, bleak, seared and desolate, with one side sweltering in terrific heat and the other numbed with dark and cold. Since there is no appreciable atmosphere on Mercury, there must be no wind—no warm currents of air to flow from the hot side to the cold side, nor from the cold side to the hot side,—thus making what is already drear seem motionless and dead. ”

“ Very little is to be seen on the minute disk of this planet, except that it goes through all the phases of the moon—crescent, gibbous, full, gibbous, crescent. Whether Mercury turns round on its axis or not, cannot be said to be known, because the markings that are suspected on its surface are too indefinite to permit exact observation. More than likely the planet presents always the same side or face to the sun, so that it turns round on its axis once, while traveling once around the sun in its orbit. ”

“ Unfortunately, our comparative proximity to this beautiful planet does not much aid us in learning anything about her personal peculiarities. Shining only by reflected light, and being, like Mercury, situated nearer to the sun than the earth is, when she comes around to the same side of the sun on which we are, her unillumined side is turned toward us, and at the point of very closest approach she is absolutely invisible to the naked eye. ”

“ In a spiritual sense, at any rate, we know that men who live near to Jesus dwell in the divine flame of love.
Mercury is a comparatively small planet; its diameter is about 2960 miles, while that of the earth is 7975. Mercury rushes round the sun in eighty-eight days, traveling at the rate of nearly 110,000 miles in an hour, while the earth traverses only 65,000 miles in the same time. Fancy crossing the Atlantic in about two or three minutes! It is an instance of the wisdom of God that Mercury appears to be the densest of the planets. ”

“ The distortion in the orbit of the planet Mercury has been accounted for by the urgent suggestion that there must be some planet, as yet undiscovered, that disturbs the regularity of Mercury’s movements, but whose orbit is so near to the sun as to baffle all ordinary efforts to see it. ”

“ Mercury's orbit is highly eccentric. At perihelion, tidal force tries to pull it apart along the planet-to-sun line, while at aphelion the tidal force is less and the little world's own gravity tries to bring it back to a spherical shape. The real change in form is not great, but a large force working through even a small amount of distance can mean a good deal of energy.
If the energy can't leak out—and Mercury's rocks conduct heat no better than those of Earth—the temperature must rise. ”

“ Mercury's distance from the sun averages 36 million miles, the diameter of the planet is 3,000 miles, and his orbital speed is 30 miles per second, the swiftest of all the planets. No moon of Mercury is known to exist, although many times diligently searched for, especially during transits of the planet. ”

“ It 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. ”

“ A planet half as large as Mercury would have a diameter about four-fifths of Mercury's, and at equal distance would present a disc about two-thirds of Mercury's in apparent size. But supposing the planet to be half as far from the sun as Mercury (and theory required that the planet should be rather nearer the sun) , its surface would be illuminated four times as brightly as that of Mercury. ”

“ The result is that, neglecting the effects of atmospheric refraction, which would tend to expand the borders of the domain of sunlight, about one quarter of the entire surface of Mercury is, with regard to day and night, in a condition resembling that of our polar regions, where there is but one day and one night in the course of a year—and on Mercury a year is [Pg 40] eighty-eight days. One half of the remaining three quarters of the planet's surface is bathed in perpetual sunshine and the other half is a region of eternal night. ”