An illustration of the constellation Aquarius (water bearer), one of the twelve astrological signs of the zodiac. The zodiac is a belt around the sky extending 9 degrees on either side of the ecliptic, which is the plane of the Earth's orbit and of the Sun's apparent annual path. The 12 astrological signs of the zodiac are each roughly considered to occupy one twelfth (30 degrees) of its circle. The size and number of zodiacal constellations varied in antiquity and became fixed only with the development of mathematical astronomy. Astrological dates: January 20 - February 18.