Zodiac
The zodiac is the celestial band centered on the ecliptic, in which the Sun, Moon and planets are seen to move from Earth. It is divided into twelve sectors of 30 degrees, corresponding to the twelve signs of the zodiac.
Origin and etymology
The word comes from the Greek zodiakos kuklos, circle of small animals, referring to the animal figures that populate most of the zodiacal constellations. The division of the sky into twelve equal sectors is attributed to the Babylonian astronomers of the 5th century BCE, who systematized older Mesopotamian celestial observations. This division was transmitted to the Greek world and codified by Claudius Ptolemy in the Almagest and the Tetrabiblos in the 2nd century. The tropical zodiac, anchored on the spring equinox, is distinguished from the sidereal zodiac, anchored on the actual constellations. The precession of the equinoxes gradually separates the two.
Evolution and tradition
Two traditions still coexist today. Western astrology mostly uses the tropical zodiac, where Aries begins at the spring equinox. Indian Vedic astrology uses the sidereal zodiac, offset by about 24 degrees from the tropical one. The precession of the equinoxes, discovered by Hipparchus in the 2nd century BCE, explains this shift: the vernal point slowly retreats by about one degree every 72 years. The twelve signs (Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, Pisces) are distributed across four elements and three modes (cardinal, fixed, mutable).
Practical use
The zodiac serves as the reference frame for every astrological calculation: planetary positions, aspects, transits. Each planet in a natal chart is located at a precise degree in a sign of the zodiac, which determines the coloration of its function. On Tarotoui, the tropical zodiac is used by default in natal chart and transit calculations. The solar sign, the best known to the general public, is only one piece of data among many: lunar sign, rising sign and planetary placements complete the reading. Modern ephemerides locate the planets on the zodiac to the second of arc.
Going further
Confusing tropical zodiac with actual constellations is a frequent mistake. Because of precession, the Sun at the beginning of tropical Aries (March 21) is today in the constellation of Pisces. The twelve signs are symbolic 30-degree sections, not physical constellations. Note also that a thirteenth sign, Ophiuchus, is sometimes mentioned: it is a constellation crossed by the Sun but traditionally not retained in the zodiac.