Here we take leave of the planet Mercury—an interesting and beautiful object, which stimulates our intellectual curiosity, while at the same time it eludes our attempts to make a closer acquaintance. There is, however, one point of attainable knowledge which we must mention in conclusion. It is a difficult, but not by any means an impossible, task to weigh Mercury in the celestial balance, and determine his mass in comparison with the other globes of our system. This is a delicate operation, but it leads us through some of the most interesting paths of astronomical discovery. The weight of the planet, as recently determined by Von Asten, is about one twenty-fourth part of the weight of the earth, but the result is more uncertain than the determinations of the mass of any of the other larger planets.
CHAPTER VIII.
VENUS.
Interest attaching to this Planet—The Unexpectedness of its Appearance—The Evening Star—Visibility in Daylight—Lighted only by the Sun—The Phases of Venus—Why the Crescent is not Visible to the Unaided Eye—Variations in the Apparent Size of the Planet—The Rotation of Venus—Resemblance of Venus to the Earth—The Transit of Venus—Why of such Especial Interest—The Scale of the Solar System—Orbits of the Earth and Venus not in the same Plane—Recurrence of the Transits in Pairs—Appearance of Venus in Transit—Transits of 1874 and 1882—The Early Transits of 1631 and 1639—The Observations of Horrocks and Crabtree—The Announcement of Halley—How the Track of the Planet differs from Different Places—Illustrations of Parallax—Voyage to Otaheite—The Result of Encke—Probable Value of the Sun's Distance—Observations at Dunsink of the Last Transit of Venus—The Question of an Atmosphere to Venus—Other Determinations of the Sun's Distance—Statistics about Venus.
It might, for one reason, have been not inappropriate to have commenced our review of the planetary system by the description of the planet Venus. This body is not especially remarkable for its size, for there are other planets hundreds of times larger. The orbit of Venus is no doubt larger than that of Mercury, but it is much smaller than that of the outer planets. Venus has not even the splendid retinue of minor attendants which gives such dignity and such interest to the mighty planets of our system. Yet the fact still remains that Venus is peerless among the planetary host. We speak not now of celestial bodies only seen in the telescope; we refer to the ordinary observation which detected Venus ages before telescopes were invented.
Who has not been delighted with the view of this glorious object? It is not to be seen at all times. For months together the star of evening is hidden from mortal gaze. Its beauties are even enhanced by the caprice and the uncertainty which attend its appearance. We do not say that there is any caprice in the movements of Venus, as known to those who diligently consult their almanacs. The movements of the lovely planet are there prescribed with a prosaic detail hardly in harmony with the character usually ascribed to the Goddess of Love. But to those who do not devote particular attention to the stars, the very unexpectedness of its appearance is one of its greatest charms. Venus has not been noticed, not been thought of, for many months. It is a beautifully clear evening; the sun has just set. The lover of nature turns to admire the sunset, as every lover of nature will. In the golden glory of the west a beauteous gem is seen to glitter; it is the evening star—the planet Venus. A few weeks later another beautiful sunset is seen, and now the planet is no longer a point low down in the western glow; it has risen high above the horizon, and continues a brilliant object long after the shades of night have descended. Again, a little later, and Venus has gained its full brilliancy and splendour. All the heavenly host—even Sirius and even Jupiter—must pale before the splendid lustre of Venus, the unrivalled queen of the firmament.
After weeks of splendour, the height of Venus at sunset diminishes, and its lustre begins gradually to decline. It sinks to invisibility, and is forgotten by the great majority of mankind; but the capricious goddess has only moved from one side of the sky to the other. Ere the sun rises, the morning star will be seen in the east. Its splendour gradually augments until it rivals the beauty of the evening star. Then again the planet draws near to the sun, and remains lost to view for many months, until the same cycle of changes recommences, after an interval of a year and seven months.
When Venus is at its brightest it can be easily seen in broad daylight with the unaided eye. This striking spectacle proclaims in an unmistakable manner the unrivalled supremacy of this planet as compared with its fellow-planets and with the fixed stars. Indeed, at this time Venus is from forty to sixty times more brilliant than any stellar object in the northern heavens.