The “Inferior” planets it will be seen from the above table comprise Mercury and Venus, whilst the “Superior” planets are Mars and all those beyond.

Great differences exist in the inclinations of the orbits of the different planets to the plane of the ecliptic, a fact which is better shown by a diagram than by a table of mere figures. The orbit of Uranus is indeed so much inclined that its motion is really retrograde compared with the general run of the planets: and the same remark applies, though much more forcibly, to the case of Neptune.

The actual movements of the planets round the Sun are extremely simple, for they do nought else but go on, and on, and on, incessantly, always in the same direction, and almost, though not quite, at a uniform pace, though in orbits very variously inclined to the plane of the ecliptic. But an element of extreme complication is introduced into their apparent movements by reason of the fact that we are obliged to study the planets from one of their own number, which is itself always in motion.

If the Earth itself were a fixture, the study of the movements of the planets would be a comparatively easy matter, whilst to an observer on the Sun it would be a supremely easy matter.

Greatly as the planets differ among themselves in their sizes, distances from the Sun, and physical peculiarities, they have certain things in common, and it will be well to make this matter clear before we go into more recondite topics. For instance, not only do they move incessantly round the Sun in the same direction at a nearly uniform pace, but the planes of their orbits are very little inclined to the common plane of reference, the ecliptic, or to one another.[1] The direction of motion of the planets as viewed from the north side of the ecliptic is contrary to the motion of the hands of a watch. Their orbits, unlike the orbits of comets, are nearly circular, that is, they are only very slightly oval. Agreeably to the principles of what is known as the Law of Universal Gravitation, the speed with which they move in their orbits is greatest in those parts which lie nearest the Sun, and least in those parts which are most remote from the Sun; in other words, they move quickest in Perihelion and slowest in Aphelion.

The physical peculiarities which the planets have in common include the following points:—they are opaque bodies, and shine by reflecting light which they receive from the Sun. Probably all of them are endued with an axial rotation, hence their inhabitants, if there are any, have the alternation of day and night, like the inhabitants of the Earth, but the duration of their days, measured in absolute terrestrial hours, will in most cases differ materially from the days and nights with which we are familiar.

I stated on a previous page that, owing to the circumstances in which we find ourselves on the Earth, the apparent and real movements of the planets are widely different. It would be beyond the scope of this little work to go into these differences in any considerable detail; suffice it then to indicate only a few general points. In the first place, an important distinction exists between the visible movements of the inferior and superior planets. The inferior planets, Mercury and Venus, lying as they do within the orbit of the Earth, are much restricted in their movements, in the sky. We can never see them except when they are more or less near to the rising (or risen) or setting (or set) Sun. The extreme angular distance from the Sun in the sky to which Mercury can attain is but 27°, and therefore we can never observe it otherwise than in sunlight or twilight, for it never rises more than 1½ hours before sunrise nor sets later than 1½ hours after sunset. Of course between these limits the planet is above the horizon all the time that the Sun is above the horizon, but except in very large telescopes is not usually to be detected during the day-time. These remarks regarding Mercury apply likewise in principle to Venus; only the orbit of Venus being larger than the orbit of Mercury, and Venus itself being larger in size than Mercury, the application of these principles leads to somewhat different results. The greatest possible distance of Venus may be 47° instead of Mercury’s 27°. Venus is therefore somewhat more emancipated from the effects of twilight. The body of Venus being also very much larger and brighter than the body of Mercury, it may be more often and more easily detected in broad daylight.

It follows from the foregoing statement that the inferior planets can never be seen in those regions of the heavens which are, as it is technically called, in “Opposition” to the Sun; that is, which are on the meridian at midnight whilst the Sun is on the meridian in its midday splendour to places on the opposite side of the Earth. On the other hand, the two inferior planets on stated, though rare, occasions exhibit to a terrestrial spectator certain phenomena of great interest and importance in which no superior planet can ever take part. I am here referring to the “Transits” of Mercury and Venus across the Sun. If these planets and the Earth all revolved round the Sun exactly in the plane of the ecliptic, transits of these planets would be perpetually recurring after even intervals of only a few months; but the fact that the orbit of Mercury is inclined 7°, and that of Venus about 3½, to the ecliptic, involves such complications that transits of Mercury only occur at unequal intervals of several years, whilst, in extreme cases, more than a century may elapse between two successive transits of Venus. For a transit of an inferior planet over the Sun to take place, the Earth and the planet and the Sun must be exactly in the same straight line, reckoned both vertically and horizontally. Twice in every revolution round the Sun an inferior planet is vertically in the same straight line with the Earth and the Sun; and it is said to be in “inferior conjunction” when the planet comes between the Earth and the Sun; and in “superior conjunction” when the planet is on the further side of the Sun, the Sun intervening between the Earth and the planet. But for all three to be horizontally in the same straight line is quite another matter. It is the orbital inclinations of Mercury and Venus which enable them, so to speak, to dodge an observer who is on the lookout to see them pass exactly in front of the Sun, or to disappear behind the Sun; and so it comes about that a favourable combination of circumstances which is rare is needed before either of the aforesaid planets can be seen as round black spots passing in front of the Sun. A passage of either of these planets behind the Sun could never be seen by human eye, because of the overpowering brilliancy of the Sun’s rays, even though an Astronomer might know by his calculations the exact moment that the planet was going to pass behind the Sun.

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.