We now take up the various planets singly. The nearest to the sun, as far as known, is Mercury, its average distance being only 36,000,000 miles. But its orbit is so eccentric that the distance varies from 28,500,000 miles at perihelion to 43,500,000 at aphelion. In consequence its speed in its orbit is very variable, and likewise the amount of heat and light received by it from the sun. On the average it gets more than 6½ times as much solar light and heat as the earth gets. But at perihelion it gets 2½ times as much as at aphelion, and the time which it occupies in passing from perihelion to aphelion is only six weeks, its entire year being equal to 88 of our days. Being situated so much nearer the sun than the earth is, Mercury is never visible to us except in the morning or the evening sky, and then not very far from the sun. Its diameter is about 3000 miles, but its mass is not certainly known from lack of knowledge of its mean density. This lack of knowledge is due to the fact that Mercury has no satellite. When a planet has a satellite it is easy to calculate its density from its measured diameter combined with the orbital speed of its satellite. Certain considerations have led some to believe that the mean density of Mercury may be very great, perhaps as great as that of lead, or of the metal mercury itself. Not knowing the mass, we cannot say exactly what the weight of bodies on Mercury is. We are also virtually ignorant of the condition of the surface of this planet, the telescope revealing very little detail, but it is generally thought that it bears a considerable resemblance to the surface of the moon. There is another way in which Mercury is remarkably like the moon. The latter, as we have seen, always keeps the same side turned toward the earth, which is the same thing as saying that it turns once on its axis, while going once around the earth. So Mercury keeps always the same side toward the sun, making one rotation on its axis in the course of one revolution in its orbit. Consequently, one side of Mercury is continually in the sunlight, while the opposite side is continually buried in night. There must, however, be regions along the border between these two sides, where the sun does rise and set once in the course of one of Mercury's years. This arises from the eccentricity of the orbit, and the consequent variations in the orbital velocity of the planet, which cause now a little of one edge and now a little of the other edge of the dark hemisphere to come within the line of sunlight. (The same thing occurs with the moon, though to a less degree owing to the smaller eccentricity of the moon's orbit, which, however, is sufficient to enable us to see at one time a short distance round one side of the moon and at another time a short distance round the opposite side.) This phenomenon is known as libration. Mercury apparently possesses an atmosphere, but we know nothing certain concerning its density.
The next planet, in the order of distance from the sun, is Venus, whose average distance is 67,200,000 miles. The orbit of Venus is remarkable for its small eccentricity, so that the difference between its greatest and least distances from the sun is less than a million miles. The period, or year, of Venus is 225 of our days. Owing to her situation closer to the sun, she gets nearly twice as much light and heat as the earth gets. In size Venus is remarkably like the earth, her diameter being 7713 miles, which differs by only 205 miles from the mean diameter of the earth. Her axis is nearly perpendicular to the plane of her orbit. Her globe is a more perfect sphere than that of the earth, being very little flattened at the poles or swollen at the equator. Although Venus, like Mercury, has no satellite, her mean density has been calculated by other means, and is found to be 0.89 that of the earth. From this, in connection with her measured diameter, it is easy to deduce her mass, and the force of gravity on her surface. The latter comes out at about 0.85 that of the earth, i. e. a body weighing 100 pounds on the earth would weigh 85 pounds if removed to Venus. She possesses an atmosphere denser and more extensive than would theoretically have been expected—indicating, perhaps, a difference of constitution. Her atmosphere has been estimated to be twice as dense as ours, a great advantage, it may be remarked, from the point of view of aëronautics. But this dense and abundant atmosphere renders Venus a very difficult object for the telescope on account of the brilliance of its reflection. In consequence, we know but little of the surface of the planet.
One important result of this is that the question remains undecided whether Venus rotates on her axis at a rate closely corresponding with that of the earth, as some observers think, or whether, as others think, she, like Mercury, turns only once on her axis in going once round the sun. The importance of the question in its bearing on the habitability of Venus is apparent, for if she keeps one face always sunward, then on one side there is perpetual day and on the other perpetual night. On the other hand, if she has days and nights approximately equal in length to those of the earth, it may well be thought that she is habitable by beings not altogether unlike ourselves, because the force of gravity on her surface is not much less than on the earth, and her dense atmosphere, filled with clouds, might tend to shield her inhabitants from the effects of the greater amount of heat poured upon her by the sun. As her orbit is inside that of the earth, Venus, like Mercury, is only visible either in the evening or in the morning sky, but owing to her greater actual distance from the sun, her apparent distance from it in the sky is greater than that of Mercury.
The Great Nebula in Orion
Photographed at the Lick Observatory by J. E. Keeler, with the Crossley reflector. Exposure one hour.
Both of these planets, in consequence of passing alternately between the sun and the earth and round the opposite side of the sun, present phases resembling those of the moon. The reader can explain these to himself by means of the experiment, before mentioned, with a billiard ball and a lamp. In this case let the observer remain seated in his chair while another person carries the ball round the lamp in such a manner that it shall alternately pass between the lamp and the observer and round the other side of the lamp. When Venus comes nearly in line between the earth and the sun, she becomes an exceedingly brilliant object in either the evening or the morning sky, although at such times we see, in the form of a crescent, only a part of that half of her surface which is illuminated. Her increase of brightness at such times is due to her greater nearness to the earth. When between the earth and the sun she may be only about 26,000,000 miles away, while when she is on the other side of the sun she may be over 160,000,000 miles away. Both Venus and Mercury when passing exactly between the sun and the earth are seen, in the form of small black circles, moving slowly across the sun's disk. These occurrences are called transits, and in the case of Venus have been before referred to. They are more frequent with Mercury than with Venus, but Mercury's transits are not utilisable for parallax observations. The latest transit of Venus occurred in 1882, and there will not be another until 2004. The latest transit of Mercury occurred in 1907, and there will be another in 1914.
The earth is the third planet in order of distance, and then comes Mars, whose average distance from the sun is 141,500,000 miles. The orbit of Mars is so eccentric that the distance varies between 148,000,000 and 135,000,000 miles. Its period or year is about 687 of our days. In consequence of its distance, Mars gets, on the average, a little less than half as much light and heat as the earth gets. When it is on the same side of the sun with the earth, and nearly in line with them, it is said to be in opposition. At such times it is manifestly as near the earth as it can come, and thus an opposition of Mars offers a good opportunity for the telescopic study of its surface. These oppositions occur once in about 780 days, but they are not all of equal importance, because the distance between the two planets is not the same at different oppositions. The cause of the difference of distance is the eccentricity of the orbit. If an opposition occurs when Mars is in aphelion its distance from the earth will be about 61,000,000 miles, but if the opposition occurs when Mars is in perihelion the distance will be only about 35,000,000 miles. The average distance at an opposition is about 48,500,000 miles. The most favourable oppositions always occur in August or September, and are repeated at an interval of from fifteen to seventeen years. But at some of the intervening oppositions the distance of the planet is not too great to afford good views of its surface. The diameter of Mars is about 4330 miles, with a similar polar flattening to that of the earth. Its density is 0.71 that of the earth, and the force of gravity on its surface 0.38. A body weighing 100 pounds on the earth would weigh 38 pounds on Mars. The evidence in regard to its atmosphere is conflicting, but the probability is that it has an atmosphere not denser than that existing on our highest mountain peaks. Opinions concerning the existence of water vapour on Mars are also conflicting. One fact tending to show that its atmosphere must be very rare and cloudless is that its surface features are very plainly discernible with telescopes.
