Since the orbits of the inferior planets lie within the orbit of the Earth, the angular distances of these bodies from the Sun, as seen from the Earth, must always be included within fixed limits; and these planets must seem to oscillate from the east to the west, and from the west to the east of the Sun during their sidereal revolution. In this process of oscillation these planets sometimes pass between the Earth and the Sun, and sometimes behind the Sun. When they pass between us and the Sun they are said to be in "inferior conjunction," while, when they pass behind the Sun, they are said to be in "superior conjunction." When such a planet reaches its greatest distance, either east or west, it is said to be at its greatest elongation east or west, as the case may be, or in quadrature. The superior planets, whose orbits lie beyond that of the Earth and enclose it, present a different appearance. A superior planet never passes between the Earth and the Sun, since its orbit lies beyond that of our globe, and, therefore, no inferior conjunction of such a planet can ever occur. When one of these planets passes beyond the Sun, just opposite to the place occupied by the Earth, the planet is said to be in "conjunction;" while, when it is on the same side of the Sun with our globe, it is said to be in "opposition." While occupying this last position, the planet is most advantageously situated for observation, since it is then nearer to the Earth. The period comprised between two successive conjunctions, or two successive oppositions of a planet, is called its "synodical period." This period differs for every planet. It is supposed that all the planets rotate from west to east, like our globe; although no direct evidence of the rotation of Mercury Uranus, and Neptune has yet been obtained, it is probable that these planets rotate like the others. It results from the rotation of the planets that they have their days and nights, like our Earth, but differing in duration for every planet. The axes of rotation of the planets are more or less inclined to their respective orbits, and this inclination varies but little in the course of time. From the inclination of the axes of rotation of the planets to their orbits, it results that these bodies have seasons like those of the Earth; but, of course, they differ from our seasons in duration and intensity, according to the period of revolution and the inclination of the axis of each separate planet. THE PLANET MARS PLATE VIII Mars is the fourth of the planets in order of distance from the sun; Mercury, Venus and the Earth being respectively the first, second and third.