CHAPTER VIII.

VENUS.

Interest attaching to this Planet-The Unexpectedness of its Appearance-The Evening Star-Visibility in Daylight-Only Lighted 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-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 of the recent Transit of Venus at Dunsink-The Question of an Atmosphere to Venus - Dr. Copeland's Observations-Utility of such Researches-Other De. terminations of the Sun's Distance-Statistics about Venus.

Ir might, for one reason, be not inappropriate to have commenced our review of the planetary system by the description of the planet Venus. This planet 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 give 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 spectacles 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 beauties of Venus are hidden from mortal gaze. Its beauties are even enhanced by the caprice and the uncertainty that attends 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 correspondence with the character of the goddess of love. But to those who do not devote particular attention to the stars, the very unexpectedness of the appearance of Venus is one of its greatest charms. Venus has not been noticed, not been thought of, for many months. It is a beautiful spring 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 week or two later another beautiful sunset is seen, and now the planet is no longer merely a glittering point low down; it has risen high above the horizon, and continues a brilliant object long after the shades of night have descended. Again, a little longer, and Venus has gained its full brilliancy and splendour. All the heavenly hosteven 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 the west to the east. 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 Venus as compared with the other planets and with the fixed stars. Indeed, at this time Venus is from forty to sixty times as bright as the brightest star in the northern heavens.

The beautiful evening star is often such a very brilliant object, that it may seem difficult at first to realise that Venus is not self-luminous. Yet it is impossible to doubt that the planet is

really only a dark globe, and to that extent resembling our own earth. The brilliancy of the planet is not so very much greater than the brilliancy of the earth on a sunshiny day, and the splendour of Venus entirely arises from the reflected light of the sun, in the manner already explained with respect to the moon.

We cannot, however, distinguish the beautiful crescent shape of the planet by the unaided eye, which merely shows a brilliant point too small to possess sensible form. This is to be explained on physiological grounds. The optical contrivances in the eye form an image of the planet on the retina; this image is very small. Even when Venus is nearest to the earth, the diameter of the planet subtends an angle not much more than one minute of arc. On the retina of the eye a picture of Venus is thus drawn about one sixthousandth part of an inch in diameter. Great as may be the delicacy of the retina, it is not adequate to the perception of form in a picture so minute. The nervous structure, which has been described as the source of vision, forms too coarse a canvas for the reception of the details of this tiny picture. Hence it is that to the unaided eye the brilliant Venus appears merely as a bright spot. The unaided eye cannot tell what shape it has; still less can it reveal the true beauty of the crescent. If the diameter of Venus were several times as great as it actually is; were Venus, for instance, as large as Jupiter or some of the other great planets, then its crescent could be readily discerned by the unaided eye. It is curious to speculate on what might have been the history of astronomy had Venus only been as large as Jupiter. Were every one able to see the crescent form without a telescope, it would then have been an elementary and almost obvious truth that Venus was a dark body revolving round the sun. The analogy between Venus and our earth would have been at once perceived; and the great theory which was left to be discovered by Copernicus in comparatively modern times, might not improbably have been handed down to us with the other discoveries which have come from the ancient nations of the East.

In Fig. 41 we have three views of Venus under different aspects. The planet is so much closer to the earth when the crescent is seen,

that it appears to be part of a much larger circle than that made by Venus when more nearly full. This drawing shows the different aspects of the planet in their true relative proportions. It is very difficult to perceive distinctly any markings on the excessively brilliant surface. Sometimes observers have seen spots or other features, and occasionally the pointed extremities of the horns have been irregular, as if to show that the surface of Venus is not smooth. Attempts have even been made to prove from such

observations that there must be lofty mountains in Venus, but we cannot place much confidence in the results.

It so happens that our earth and Venus are very nearly equal in bulk. The difference is hardly perceptible, but the earth has a diameter a few miles greater than that of Venus. It is almost equally remarkable that the time of rotation of Venus on its axis seems to be very nearly equal to the time of rotation of the earth on its axis. The earth rotates once in a day, and Venus in about half an hour less than one of our days. There are also indications of the existence of an atmosphere around Venus, but we have no means of knowing at present what the gases may be of which that atmosphere is composed.

If there be oxygen in the atmosphere of Venus, then it would

seem possible that there might be life on Venus which was not of a very different character from life on the earth. No doubt the sun's heat on Venus is greatly in excess of the sun's heat with which we are acquainted, but this is a difficulty not insuperable. We see at present on the earth, life in very hot regions and life in very cold regions. Indeed, as we go into the tropics we find life more and more exuberant, so that should water be present on the surface of Venus and oxygen in its atmosphere, we might expect to find in that planet a luxuriant tropical life, of a kind perhaps analogous in some respects to life on the earth.

In our account of the planet Mercury, as well as in our brief description of the hypothetical planet Vulcan, it has been necessary to allude to the phenomena presented by the transit of a planet over the face of the sun. This phenomenon, always of interest and always meriting the attention of astronomers, is especially noticeable in the case of Venus. The transit of Venus rises in fact, to, an importance hardly surpassed by any other phenomenon in our system, and hence it will necessarily engage our attention in the present chapter. We have in recent years had the opportunity of witnessing two of these rare occurrences. No future transit can occur till after this generation shall have passed away; not, indeed, until the June of A.D. 2004. It is hardly too much to assert that the recent transit of 1882 and the previous one of 1874 have received a degree of attention never before accorded to any astro. nomical phenomenon.

The transit of Venus is hardly to be described as a very striking or beautiful spectacle. It is not nearly so fine a sight as a great comet or a shower of shooting stars. Why is it, then, that the transit of Venus is regarded as of such great scientific importance? It is because the transit enables us to solve one of the greatest problems which has ever engaged the mind of man. It is by the transit of Venus that we attempt to determine the scale on which our solar system is constructed. Truly this is a noble problem. Let us dwell upon it for a moment. In the centre of our system we have the sun-a majestic globe more than a million times as large as the earth. Circling round the sun we have