Computer graphic illustration of Scorpius, one of twelve astrological signs of the zodiac. The zodiac is a belt around the sky extending nine degrees on either side of the ecliptic, the plane of Earth's orbit and of the sun's apparent annual path. The twelve astrological signs of the zodiac are each roughly considered to occupy 1/12 (30 degrees) of its circle. The size and number of zodiac constellations varied in antiquity and became fixed only with the development of mathematical astronomy.