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History of Astronomy — The Zodiac

The Zodiac. The word Zodiac comes to us from the Greek work zoidiakos, meaning little animal sign. In Latin, this became zodiacus. Ancient star gazers imagined outlines of stars forming constellations (stars grouped together with each other). They divided the ecliptic (Earth's orbit around the Sun) into 12 zodiacal signs. This made their job of noting positions of the Sun, Moon, and planets easier by giving them reference points of measure: the stars.

We still use these 12 signs today. They are Aries the Ram, Taurus the Bull, Gemini the Twins, Cancer the Crab, Leo the Lion, Virgo the Virgin, Libra the Scales, Scorpio the Scorpion (now called Scorpius by astronomers), Sagittarius the Archer, Capricorn the Goat, Aquarius the Water Bearer, and Pisces the Fish. Constellation boundaries have changed over the centuries. There is now a thirteenth constellation, Ophiucus (the Serpent Holder), that occupies part of what was Sagittarius.

Why were 12 signs chosen? Ancient astronomers knew that the Lunar Month lasted approximately 30 days. Dividing the ecliptic into 12 signs, each of approximately 30 days, gave a 360-day Lunar year made up of 12 thirty-day Lunar Months. The positions of the stars moved approximately one degree per day — the difference between the sidereal day (based on the position of the stars) and the tropical day (based on the position of the Sun). The tropical day is longer than the sidereal day by approximately four minutes.

Where does the Zodiac begin? It begins at the "First Point of Aries" (that is, the beginning of the Aries constellation). Where is this First Point? Its location has changed over the course of history. To make matters more complicated, there is the Tropcial Zodiac (determined by the position of the Sun) and the Sidereal Zodiac (determined by the position of the Stars).

The Tropical Zodiac is usually accepted as beginning with the Spring Equinox. The everyday Western world no longer uses a Sidereal Zodiac, though that was the Zodiac of the Ancients, as they measured all planet positions relative to surrounding stars. However, modern astronomers have made precise definitions of the boundaries of each constellation. Yet some cultures (notably Hindus in India) still use their own definition of the First Point of Aries for religious festivals.

Ancient Babylonian and Hindu astronomers placed Spica at or near the middle of the Zodiac, so that Aries began opposite Spica. Many Hindus still take a point at or 10' distant from the star Revati (ζ Piscium) as marking the First Point of Aries. In India, many religious festivals are tied to the Sidereal Zodiac and corresponding phase of the Moon. This lack of complete agreement means that religious festivals can be celebrated at different times when using slightly different Zodiacs (somewhat like the difference in the date of Easter between the Eastern Orthodox and Roman Catholic churches).

Hindus call the difference between the Tropical and Sidereal Zodiacs ayanamsa. The Indian Government adopted a recommendation of N.C. Lahiri and defined this zodiacal difference as 23°10' on 1 January 1950, based on the position of Spica at that time. This value increases by approximately 50 arcseconds per year, owing to precession of the equinoxes.