34. _The Planets._--On watching the stars attentively night after night, it will be found, that while the majority of them appear _fixed_ on the surface of the celestial sphere, so as to maintain their relative positions, there are a few that _wander_ about among the stars, alternately advancing towards the east, halting, and retrograding towards the west. These wandering stars are called _planets_. Their motions appear quite irregular; but, on the whole, their eastward motion is in excess of their westward, and in a longer or shorter time they all complete the circuit of the heavens. In almost every instance, their paths are found to lie wholly in the belt of the zodiac. [Illustration: Fig. 48.] Fig. 48 shows a portion of the apparent path of one of the planets. II. THE SOLAR SYSTEM. I. THEORY OF THE SOLAR SYSTEM. 35. _Members of the Solar System._--The solar system is composed of the _sun_, _planets_, _moons_, _comets_, and _meteors_. Five planets, besides the earth, are readily distinguished by the naked eye, and were known to the ancients: these are _Mercury_, _Venus_, _Mars_, _Jupiter_, and _Saturn_. These, with the _sun_ and _moon_, made up the _seven planets_ of the ancients, from which the seven days of the week were named. The Ptolemaic System. 36. _The Crystalline Spheres._--We have seen that all the heavenly bodies appear to be situated on the surface of the celestial sphere. The ancients assumed that the stars were really fixed on the surface of a crystalline sphere, and that they were carried around the earth daily by the rotation of this sphere. They had, however, learned to distinguish the planets from the stars, and they had come to the conclusion that some of the planets were nearer the earth than others, and that all of them were nearer the earth than the stars are. This led them to imagine that the heavens were composed of a number of crystalline spheres, one above another, each carrying one of the planets, and all revolving around the earth from east to west, but at different rates. This structure of the heavens is shown in section in Fig. 49. [Illustration: Fig. 49.] 37. _Cycles and Epicycles._--The ancients had also noticed that, while all the planets move around the heavens from west to east, their motion is not one of uniform advancement. Mercury and Venus appear to oscillate to and fro across the sun, while Jupiter and Saturn appear to oscillate to and fro across a centre which is moving around the earth, so as to describe a series of loops, as shown in Fig. 50. [Illustration: Fig. 50.]