All the above are major planets and also primary planets. In between Nos. 4 and 5 circulate the “Minor” planets, an ever-increasing body, now more than 400 in number, but all, except one or perhaps two, invisible to the naked eye. 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. [Illustration: Fig. 2.—Inclination of Planetary Orbits.] 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. [Illustration: Fig. 3.—Comparative Sizes of the Major Planets.]