The Story of the Solar System

by George F. Chambers

When an inferior planet attains its greatest angular distance from the Sun, as we see it (which I have already stated to be about 27° in the case of Mercury and 47° in the case of Venus), such planet is said to be at its “greatest elongation,” “east” or “west,” as the case may be. At eastern elongation or indeed whenever the planet is east of the Sun, it is, to use a familiar phrase, an “evening star”; on the other hand, at western elongation, or whenever it is on the western side of the Sun, it is known as a “morning star.”

If the movements of an inferior planet are followed sufficiently long by the aid of a star map, it will be seen that sometimes it appears to be proceeding in a forward direction through the signs of the zodiac; then for a while it will seem to stand still; then at another time it will apparently go backwards, or possess a retrograde motion. All these peculiarities have their originating cause in the motion of the Earth itself, for the absolute movement of the planet never varies, being always in the same direction, that is, forwards in the order of the signs.

Turning now to the superior planets, we have to face an altogether different succession of circumstances. A superior planet is not, as it were, chained to the Sun so as to be unable to escape beyond the limits of morning or evening twilight; it may have any angular distance from the Sun up to 180°, reaching which point it approaches the Sun on the opposite side, step by step, until it again comes into conjunction with the Sun. As applied to a superior planet, the term “conjunction” means the absolute moment when the Earth and the Sun and the planet are in the same straight line, the Sun being in the middle. In such a case, to us on the Earth the planet is lost in the Sun’s rays, whilst to a spectator on the planet the Earth would appear similarly lost in the Sun’s rays, as the Earth would be at that stage of her orbit which we, speaking of our inferior planets, call superior conjunction.

For a clear comprehension of all the various matters which we have just been speaking of, a careful study of diagrams of a geometrical character, or better still, of models, would be necessary.

Something must now be said about the phases of the planets. Mercury and Venus, in regard to their orbital motions, stand very much on the same footing with respect to the inhabitants of the Earth as the Moon does, and accordingly both those planets in their periodical circuits round the Sun exhibit the same succession of phases as the Moon does. In the case, however, of the superior planets things are otherwise. Two only of them, Mars and Jupiter, are sufficiently near the Earth to exhibit any phase at all. When they are in quadrature (i. e., 90° from the Sun on either side) there is a slight loss of light to be noticed along one limb. In other words, the disc of each ceases for a short time, and to a slight extent, to be truly circular; it becomes what is known as “gibbous.” This occasional feature of Mars may be fairly conspicuous, or, at least, noticeable; but in the case of Jupiter it will be less obvious unless a telescope of some size is employed.

If the major planets are arbitrarily ranged in two groups, Mercury, Venus, the Earth and Mars being taken as an interior group, comparatively near the Sun; whilst Jupiter, Saturn, Uranus and Neptune are regarded as an exterior group, being at a great distance from the Sun, it will be found that some important physical differences exist between the two groups.

Fig. 4.—Comparative size of the Sun as seen from the Planets named.

Of the interior planets, the Earth and Mars alone have satellites, and between them make up a total of only three. The exterior planets, on the other hand, all have satellites, the total number being certainly seventeen, and possibly eighteen. In detail, Jupiter has four, Saturn eight, Uranus four, and Neptune one, and perhaps two. These facts may be regarded as an instance of the beneficence of the Creator of the Universe if we consider that the satellites of these remoter planets are so numerous, in order that by their numbers they may do something to make up for the small amount of light which, owing to their distance from the Sun, their primaries receive. Then again, the average density of the first group of planets greatly exceeds the average density of the second group in the approximate ratio of 5 to 1. Finally, there is reason to believe that a marked difference exists in the axial rotations of the planets forming the two groups. We do not know the precise figures for all the exterior planets, but the knowledge which we do possess seems to imply that the average length of the day in the case of the interior planets is about twenty-four hours, but that in the case of the exterior planets it is no more than about ten hours. These figures can, however, only be presented as possibly true, because observations on the rotation periods of Mercury and Venus on the one hand, and of Uranus and Neptune on the other, are attended with so much difficulty that the recorded results are of doubtful trustworthiness. It is, however, reasonable to presume that the actual size of the respective planets has more to do with the matter than their distances from the Sun.

I think that the foregoing summary respecting the planets collectively embraces as many points as are likely to be of interest to the generality of readers; we will therefore pass on to consider somewhat in detail the several constituent members of the solar system, beginning with the Sun.