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.
Venus. H. MacEwen. 5-inch Refractor.
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.