VENUS

Next in order to Mercury, proceeding outwards from the sun, comes the planet Venus, the twin-sister, so to speak, of the earth, and familiar more or less to everybody as the Morning and Evening Star. The diameter of Venus, according to Barnard's measures with the 36-inch telescope of the Lick Observatory, is 7,826 miles; she is therefore a little smaller than our own world. Her distance from the sun is a trifle more than 67,000,000 miles, and her orbit, in strong contrast with that of Mercury, departs very slightly from the circular. Her density is a little less than that of the earth.

There is no doubt that, to the unaided eye, Venus is by far the most beautiful of all the planets, and that none of the fixed stars can for a moment vie with her in brilliancy. In this respect she is handicapped by her position as an inferior planet, for she never travels further away from the sun than 48°, and, even under the most favourable circumstances, cannot be seen for much more than four hours after sunset. Thus we never have the opportunity of seeing her, as Mars and Jupiter can be seen, high in the South at midnight, and far above the mists of the horizon. Were it possible to see her under such conditions, she would indeed be a most glorious object. Even as it is, with all the disadvantages of a comparatively low position and a denser stratum of atmosphere, her brilliancy is extremely striking, having been estimated, when at its greatest, at about nine times that of Sirius, which is the brightest of all the fixed stars, and five times that of Jupiter when the giant planet is seen to the best advantage. It is, in fact, so great that, when approaching its maximum, the shadows cast by the planet's light are readily seen, more especially if the object casting the shadow have a sharply defined edge, and the shadow be received upon a white surface—of snow, for instance. This extreme brilliance points to the fact that the surface of Venus reflects a very large proportion of the sunlight which falls upon it—a proportion estimated as being at least 65 per cent., or not very much less than that reflected by newly fallen snow. Such reflective power at once suggests an atmosphere very dense and heavily cloud-laden; and other observations point in the same direction. So that in the very first two planets of the system we are at once confronted with that diversity in details which coexists throughout with a broad general likeness as to figure, shape of orbit, and other matters. Mercury's reflective power is very small, that of Venus is exceedingly great; Mercury's atmosphere seems to be very attenuated, that of Venus, to all appearance, is much denser than that of our own earth.

Periodically, when Venus appears in all her splendour in the Western sky, one meets with the suggestion that we are having a re-appearance of the Star of Bethlehem; and it seems to be a perpetual puzzle to some people to understand how the same body can be both the Morning and the Evening Star. Those who have paid even the smallest attention to the starry heavens are not, however, in the least likely to make any mistake about the sparkling silver radiance of Venus; and it would seem as though the smallest application of common-sense to the question of the apparent motion of a body travelling round an almost circular orbit which is viewed practically edgewise would solve for ever the question of the planet's alternate appearance on either side of the sun. Such an orbit must appear practically as a straight line, with the sun at its middle point, and along this line the planet will appear to travel like a bead on a wire, appearing now on one side of the sun, now on another. If the reader will draw for himself a diagram of a circle (sufficiently accurate in the circumstances), with the sun in the centre, and divide it into two halves by a line supposed to pass from his eye through the sun, he will see at once that when this circle is viewed edgewise, and so becomes a straight line, a planet travelling round it is bound to appear to move back and forward along one half of it, and then to repeat the same movement along the other half, passing the sun in the process.

Like Mercury, and for the same reason of a position interior to our orbit, Venus exhibits phases to us, appearing as a fully illuminated disc when she is furthest from the earth, as a half-moon at the two intermediate points of her orbit, and as a new moon when she is nearest to us. The actual proof of the existence of these phases was one of the first-fruits which Galileo gathered by means of his newly invented telescope. It is said that Copernicus predicted their discovery, and they certainly formed one of the conclusive proofs of the correctness of his theory of the celestial system. It was the somewhat childish custom of the day for men of science to put forth the statement of their discoveries in the form of an anagram, over which their fellow-workers might rack their brains; probably this was done somewhat for the same reason which nowadays makes an inventor take out a patent, lest someone should rob the discoverer of the credit of his discovery before he might find it convenient to make it definitely public. Galileo's anagram, somewhat more poetically conceived than the barbarous alphabetic jumble in which Huygens announced his discovery of the nature of Saturn's ring, read as follows: 'HÆc immatura a me jam frustra leguntur o. y.' This, when transposed into its proper order, conveyed in poetic form the substance of the discovery: 'CynthiÆ figuras Æmulatur Mater Amorum' (The Mother of the Loves [Venus] imitates the phases of Cynthia). It is true that two letters hang over the end of the original sentence, but too much is not to be expected of an anagram.

As a telescopic object, Venus is apt to be a little disappointing. Not that her main features are difficult to see, or are not beautiful. A 2-inch telescope will reveal her phases with the greatest ease, and there are few more exquisite sights than that presented by the silvery crescent as she approaches inferior conjunction. It is a picture which in its way is quite unique, and always attractive even to the most hardened telescopist.

Still, what the observer wants is not merely confirmation of the statement that Venus exhibits phases. The physical features of a planet are always the most interesting, and here Venus disappoints. That very brilliant lustre which makes her so beautiful an object to the naked eye, and which is even so exquisite in the telescopic view, is a bar to any great progress in the detection of the planet's actual features. For it means that what we are seeing is not really the surface of Venus, but only the sunward side of a dense atmosphere—the 'silver lining' of heavy clouds which interpose between us and the true surface of the planet, and render it highly improbable that anything like satisfactory knowledge of her features will ever be attained. Newcomb, indeed, roundly asserts that all markings hitherto seen have been only temporary clouds and not genuine surface markings at all; though this seems a somewhat absolute verdict in view of the number of skilled observers who have specially studied the planet and assert the objective reality of the markings they have detected. The blunting of the South horn of the planet, visible in Mr. MacEwen's fine drawing (Plate X.), is a feature which has been noted by so many observers that its reality must be conceded. On the other hand, some of the earlier observations recording considerable irregularities of the terminator (margin of the planet between light and darkness), and detached points of light at one of the horns, must seemingly be given up. Denning, one of the most careful of observers, gives the following opinion: 'There is strong negative evidence among modern observations as to the existence of abnormal features, so that the presence of very elevated mountains must be regarded as extremely doubtful.... The detached point at the South horn shown in SchrÖter's telescope was probably a false appearance due to atmospheric disturbances or instrumental defects.' It will be seen, therefore, that the observer should be very cautious in inferring the actual existence of any abnormal features which may be shown by a small telescope; and the more remarkable the features shown, the more sceptical he may reasonably be as to their reality. The chances are somewhat heavily in favour of their disappearance under more favourable conditions of seeing.

The same remark applies, with some modifications, to the dark markings which have been detected on the planet by all sorts of observers with all sorts of telescopes. There is no doubt that faint grey markings, such as those shown in Plate X., are to be seen; the observations of many skilled observers put this beyond all question. Even Denning, who says that personally he has sometimes regarded the very existence of these markings as doubtful, admits that 'the evidence affirming their reality is too weighty and too numerously attested to allow them to be set aside'; and Barnard, observing with the Lick telescope, says that he has repeatedly seen markings, but always so 'vague and ill-defined that nothing definite could be made of them.'

The observations of Lowell and Douglass at Flagstaff, Arizona, record quite a different class of markings, consisting of straight, dark, well-defined lines; as yet, however, confirmation of these remarkable features is scanty, and it will be well for the beginner who, with a small telescope and in ordinary conditions of observing, imagines he has detected such markings to be rather more than less doubtful about their reality. The faint grey areas, which are real features, at least of the atmospheric envelope, if not of the actual surface, are beyond the reach of small instruments. Mr. MacEwen's drawings, which accompany this chapter, were made with a 5-inch Wray refractor, and represent very well the extreme delicacy of these markings. I have suspected their existence when observing with an 8½-inch With reflector in good air, but could never satisfy myself that they were really seen.

Up till the year 1890 the rotation period of Venus was usually stated at twenty-three hours twenty-one minutes, or thereby, though this figure was only accepted with some hesitation, as in order to arrive at it there had to be some gentle squeezing of inconvenient observations. But in that year Schiaparelli announced that his observations were only consistent with a long period of rotation, which could not be less than six months, and was not greater than nine. The announcement naturally excited much discussion. Schiaparelli's views were strongly controverted, and for a time the astronomical world seemed to be almost equally divided in opinion. Gradually, however, the conclusion has come to be more and more accepted that Venus, like Mercury, rotates upon her axis in the same time as she takes to make her journey round the sun—in other words, that her day and her year are of the same length, amounting to about 225 of our days. In 1900 the controversy was to some extent reopened by the statement of the Russian astronomer BÉlopolsky that his spectroscopic investigations pointed to a much more rapid rotation—to a period, indeed, considerably shorter than twenty-four hours. It is difficult, however, to reconcile this with the absence of polar flattening in the globe of Venus. Lowell's spectroscopic observations are stated by him to point to a period in accordance with his telescopic results—namely, 225 days. The matter can scarcely be regarded as settled in the meantime, but the balance of evidence seems in favour of the longer period.

Another curious and unexplained feature in connection with the planet is what is frequently termed the 'phosphorescence' of the dark side. This is an appearance precisely similar to that seen in the case of the moon, and known as 'the old moon in the young moon's arms.' The rest of the disc appears within the bright crescent, shining with a dull rusty light. In the case of Venus, however, an explanation is not so easily arrived at as in that of the moon, where, of course, earth-light accounts for the visibility of the dark portion. Had the planet been possessed of a satellite, the explanation might have lain there; but Venus has no moon, and therefore no moonlight to brighten her unilluminated portion; and our world is too far distant for earth-shine to afford an explanation. It has been suggested that electrical discharges similar to the aurora may be at the bottom of the mystery; but this seems a little far-fetched, as does also the attribution of the phenomenon to real phosphorescence of the oceans of Venus. Professor Newcomb cuts the Gordian knot by observing: 'It is more likely due to an optical illusion.... To whatever we might attribute the light, it ought to be seen far better after the end of twilight in the evening than during the daytime. The fact that it is not seen then seems to be conclusive against its reality.' But the appearance cannot be disposed of quite so easily as this, for it is not accurate to say that it is only seen in the daytime, and against Professor Newcomb's dictum may be set the judgment of the great majority of the observers who have made a special study of the planet.

We may, however, safely assign to the limbo of exploded ideas that of the existence of a satellite of Venus. For long this object was one of the most persistent of astronomical ghosts, and refused to be laid. Observations of a companion to the planet, much smaller, and exhibiting a similar phase, were frequent during the eighteenth century; but no such object has presented itself to the far finer instruments of modern times, and it may be concluded that the moon of Venus has no real existence.

Venus, like Mercury, transits the sun's disc, but at much longer intervals which render her transits among the rarest of astronomical events. Formerly they were also among the most important, as they were believed to furnish the most reliable means for determining the sun's distance; and most of the estimates of that quantity, up to within the last twenty-five years, were based on transit of Venus observations. Now, however, other methods, more reliable and more readily applicable, are coming into use, and the transit has lost somewhat of its former importance. The interest and beauty of the spectacle still remain; but it is a spectacle not likely to be seen by any reader of these pages, for the next transit of Venus will not take place until June, 2004.

As already indicated, Venus presents few opportunities for useful observation to the amateur. The best time for observing, as in the case of Mercury, is in broad daylight; and for this, unless in exceptional circumstances, graduated circles and a fairly powerful telescope are required. Practically the most that can be done by the possessor of a small instrument is to convince himself of the reality of the phases, and of the non-existence of a satellite of any size, and to enjoy the exquisite and varying beauty of the spectacle which the planet presents. Should his telescope be one of the small instruments which show hard and definite markings on the surface, he may also consider that he has learned a useful lesson as to the possibility of optical illusion, and, incidentally, that he may be well advised to procure a better glass when the opportunity of doing so presents itself. The 'phosphorescence' of the dark side may be looked for, and it may be noted whether it is not seen after dark, or whether it persists and grows stronger. Generally speaking, observations should be made as early in the evening as the planet can be seen in order that the light of the sky may diminish as much as possible the glare which is so evident when Venus is viewed against a dark background.