Zodiac is a belt of the heavens extending about 8° on either side of the elliptic. This gives it a width of 16°, and it was so chosen by the ancients since it included the paths of the planets then known -- Mercury, Venus, Mars, Jupiter, and Saturn. But the discoveries of this century have shown that the planets are by no means confined to this zone, many of the asteroids traveling far beyond its limits. The zone was so named on account of certain resemblances to animals, which early astronomers imagined to be possessed by the constellations within it. The elliptic and equator bisect each other in a line, and the points in which this line cuts the sphere of the heavens are called the equinoctial points. That point which marks the passage of the sun from south to north across the equator is called the vernal equinox, and has always been considered a convenient starting-point for many astronomical measurements. When the zodiac was first used, this point was known as the first point of Aries, the first six signs being then to the north and the last six signs to the south of the elliptic. But, owing to the movement of the Earth's axis, the vernal equinox moves backward on the elliptic about 50 seconds in a year, so that now the constellation of Aries seems to have moved about 30° forward. For convenience, however, the vernal equinox is still called the first point of Aries, and 30° farther on will be the commencement of Taurus, while the autumnal equinox will occur at the first point of Libra. As time goes on these divisions will get farther and farther away from the constellations of the same name, but in about 25,800 years the whole circuit will be accomplished, so that sign and constellation will again be united.