148. Appearance.—Omitting the earth, Venus is by far the most conspicuous member of this group, and when at its brightest is, with exception of the sun and moon, the most brilliant object in the sky, and may be seen with the naked eye in broad daylight if the observer knows just where to look for it. But its brilliancy is subject to considerable variations on account of its changing distance from the earth, and the apparent size of its disk varies for the same reason, as may be seen from [Fig. 94]. These drawings bring out well the phases of the planet, and the student should determine from [Fig. 17] what are the relative positions in their orbits of the earth and Venus at which the planet would present each of these phases. As a guide to this, observe that the dark part of Venus's earthward side is always proportional in area to the angle at Venus between the earth and sun. In the first picture of [Fig. 94] about two thirds of the surface corresponding to the full hemisphere of the planet is dark, and the angle at Venus between earth and sun is therefore two thirds of 180°—i. e., 120°. In [Fig. 17] find a place on the orbit of Venus from which if lines be drawn to the sun and earth, as there shown, the angle between them will be 120°. Make a similar construction for the fourth picture in [Fig. 94]. Which of these two positions is farther from the earth? How do the distances compare with the apparent size of Venus in the two pictures? What is the phase of Venus to-day?
The irregularities in the shading of the illuminated parts of the disk are too conspicuous in [Fig. 94], on account of difficulties of reproduction; these shadings are at the best hard to see in the telescope, and distinct permanent markings upon the planet are wholly lacking. This absence of markings makes almost impossible a determination of the planet's time of rotation about its axis, and astronomers are divided in this respect into two parties, one of which maintains that Venus, like the earth, turns upon its axis in some period not very different from 24 hours, while the other contends that, like the moon, it turns always the same face toward the center of its orbit, making a rotation upon its axis in the same period in which it makes a revolution about the sun. The reason why no permanent markings are to be seen on this planet is easily found. Like Jupiter and Saturn, its atmosphere is at all times heavily cloud-laden, so that we seldom, if ever, see down to the level of its solid parts. There is, however, no reason here to suppose the interior parts hot and gaseous. It is much more probable that Venus, like the earth, possesses a solid crust whose temperature we should expect to be considerably higher than that of the earth, because Venus is nearer the sun. But the cloud layer in its atmosphere must modify the temperature in some degree, and we have practically no knowledge of the real temperature conditions at the surface of the planet.
It is the clouds of Venus which in great measure are responsible for its marked brilliancy, since they are an excellent medium for reflecting the sunlight, and give to its surface an albedo greater than that of any other planet, although Saturn is nearly equal to it.
Of course, the presence of such cloud formations indicates that Venus is surrounded by a dense atmosphere, and we have independent evidence of this in the shape of its disk when the planet is very nearly between the earth and sun. The illuminated part, from tip to tip of the horns, then stretches more than halfway around the planet's circumference, and shows that a certain amount of light must have been refracted through its atmosphere, thus making the horns of the crescent appear unduly prolonged. This atmosphere is shown by the spectroscope to be not unlike that of the earth, although, possibly, more dense.
Mercury
149. Chief characteristics.—Mercury, on account of its nearness to the sun, is at all times a difficult object to observe, and Copernicus, who spent most of his life in Poland, is said, despite all his efforts, to have gone to his grave without ever seeing it. In our more southern latitude it can usually be seen for about a fortnight at the time of each elongation—i. e., when at its greatest angular distance from the sun—and the student should find from [Fig. 16] the time at which the next elongation occurs and look for the planet, shining like a star of the first magnitude, low down in the sky just after sunset or before sunrise, according as the elongation is to the east or west of the sun. When seen in the morning sky the planet grows brighter day after day until it disappears in the sun's rays, while in the evening sky its brilliancy as steadily diminishes until the planet is lost. It should therefore be looked for in the evening as soon as possible after it emerges from the sun's rays.
Mercury, as the smallest of the planets, is best compared with the moon, which it does not greatly surpass in size and which it strongly resembles in other respects. Careful comparisons of the amount of light reflected by the planet in different parts of its orbit show not only that its albedo agrees very closely with that of the moon, but also that its light changes with the varying phase of the planet in almost exactly the same way as the amount of moonlight changes. We may therefore infer that its surface is like that of the moon, a rough and solid one, with few or no clouds hanging over it, and most probably covered with very little or no atmosphere. Like Venus, its rotation period is uncertain, with the balance of probability favoring the view that it rotates upon its axis once in 88 days, and therefore always turns the same face toward the sun.
If such is the case, its climate must be very peculiar: one side roasted in a perpetual day, where the direct heating power of the sun's rays, when the planet is at perihelion, is ten times as great as on the moon, and which six weeks later, when the planet is at its farthest from the sun, has fallen off to less than half of this. On the opposite side of the planet there must reign perpetual night and perpetual cold, mitigated by some slight access of warmth from the day side, and perhaps feebly imitating the rapid change of season which takes place on the day side of the planet. This view, however, takes no account of a possible deviation of the planet's axis from being perpendicular to the plane of its orbit, or of the librations which must be produced by the great eccentricity of the orbit, either of which would complicate without entirely destroying the ideal conditions outlined above.