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.
CHAPTER VII
THE MOON
Our attention is next engaged by the body which is our nearest neighbour in space and our most faithful attendant and useful servant. The moon is an orb of 2,163 miles in diameter, which revolves round our earth in a slightly elliptical orbit, at a mean distance of about 240,000 miles. The face which she turns to us is a trifle greater in area than the Russian Empire, while her total surface is almost exactly equal to the areas of North and South America, islands excluded. Her volume is about 2⁄99 of that of the earth; her materials are, however, much less dense than those of which our world is composed, so that it would take about eighty-one moons to balance the earth. One result of these relations is that the force of gravity at the lunar surface is only about one-sixth of that at the surface of the earth, so that a twelve-stone man, if transported to the moon, would weigh only two stone, and would be capable of gigantic feats in the way of leaping and lifting weights. The fact of the diminished force of gravity is of importance in the consideration of the question of lunar surfacing.