Chapter I: General Characteristics of the Groups

213. _The Inner Group._--The _inner group_ of planets is composed of _Mercury_, _Venus_, the _Earth_, and _Mars_; that is, of all the planets which lie between the asteroids and the sun. The planets of this group are comparatively small and dense. So far as known, they rotate on their axes in about twenty-four hours, and they are either entirely without moons, or are attended by comparatively few.

The comparative sizes and eccentricities of the orbits of this group are shown in Fig. 245. The dots round the orbits show the position of the planets at intervals of ten days.

214. _The Outer Group._--The _outer group_ of planets is composed of _Jupiter_, _Saturn_, _Uranus_, and _Neptune_. These planets are all very large and of slight density. So far as known, they rotate on their axes in about ten hours, and are accompanied with complicated systems of moons. Fig. 246, which represents the comparative sizes of the planets, shows at a glance the immense difference between those of the inner and outer group. Fig. 247 shows the comparative sizes and eccentricities of the orbits of the outer planets. The dots round the orbits show the position of the planets at intervals of a thousand days.

215. _The Asteroids._--Between the inner and outer groups of planets there is a great number of very small planets known as the _minor planets_, or _asteroids_. Over two hundred planets belonging to this group have already been discovered. Their orbits are shown by the dotted lines in Fig. 247. The sizes of the four largest of these planets, compared with the earth, are shown in Fig. 248.

The asteroids of this group are distinguished from the other planets, not only by their small size, but by the great eccentricities and inclinations of their orbits. If we except Mercury, none of the larger planets has an eccentricity amounting to one-tenth the diameter of its orbit (43), nor is any orbit inclined more than two or three degrees to the ecliptic; but the inclinations of many of the minor planets exceed ten degrees, and the eccentricities frequently amount to an eighth of the orbital diameter. The orbit of Pallas is inclined thirty-four degrees to the ecliptic, while there are some planets of this group whose orbits nearly coincide with the plane of the ecliptic.

Fig. 249 shows one of the most and one of the least eccentric of the orbits of this group as compared with that of the earth.

The intricate complexity of the orbits of the asteroids is shown in Fig. 250.