Hence, with the exception of Mercury, all the planets and the sun and the moon travel in a path six degrees wide, which is only one degree wider than the distance between the pointers as we see them in the Great Dipper. The fact that the zodiac is sixteen degrees wide, or eight degrees on each side of the ecliptic, is due only to a very generous allowance for the vagaries of Mercury, which he really does not quite need. For Mercury is always as much as twice the breadth of the moon, or one degree, inside of the zodiac, and usually more than that. Because the earth is tilted on its axis twenty-three and a half degrees from the perpendicular, the ecliptic runs through the heavens in an oblique circle, crossing the line of the equator at two points called the vernal and autumnal equinoxes. The equator in the heavens is the great circle extending around the celestial sphere half-way between the north and south poles. It is always practically ninety degrees from the north star, and the points at which the ecliptic intersects it are called the equinoxes. These are the only two points on the ecliptic that are just ninety degrees from the pole. The word equinox is derived from _equus_ (equal) and _nox_ (night), and when the sun is at the equinoxes the days and nights are of equal length. From the vernal to the autumnal equinox the line of the ecliptic is north of the equator, and hence high in the sky, reaching its highest point midway between the equinoxes. It then crosses the equator again and runs obliquely south to the lowest point in its path, and then curves northerly back to the vernal equinox. The vernal equinox is the point at which the sun arrives when spring begins. This results in the sun’s being north of the equator from spring until autumn, and south of it from autumn to spring. As the part of the zodiac that we can see best at night is that opposite where the sun is, so in summer, when the sun is high, we see best the part of the zodiac which is low in the southern skies in the evening; and in the winter, when the sun is in the southern half of his journey, the part of the zodiac best seen by us is high in the heavens. No part of it, however, is ever as high as the zenith, or directly overhead, and no planet is ever seen as far north as the zenith in any place whose latitude is more than twenty-three and one-half degrees from the equator.