The history of sidereal and tropical systems
The sidereal (nirayana) and tropical (sayana) zodiacs were both in use in India over history, and the choice between them has been debated since the classical period.
What it is
The tropical zodiac is tied to the seasons, and the sidereal to the stars. The difference grew over the centuries because of precession, which is why older texts that used a sidereal reference can give different signs from modern tropical readings.
How it is worked out or read
The shift to a standard sidereal reference in modern India came through the work of calendar reformers and astronomers in the 20th century, including the Lahiri ayanamsa.
Where it appears on this site
The concept is in the sidereal vs tropical article, and the ayanamsa history is in the Lahiri article.
Discovering precession
The slow drift of the equinoxes against the stars was first described in the Greek world by Hipparchus in the second century BCE, who compared his star positions with older records and found that they had shifted. The Greek astronomers who followed, including Ptolemy, defined the zodiac from the equinox, so that the first point of Aries was the March equinox itself. This is the tropical, or sayana, zodiac still used in Western astrology.
The Indian choice
Indian astronomy kept the zodiac fixed to the stars. The nakshatras were star-marked divisions from the beginning, and the signs were laid over them in a fixed relation. In the siddhanta period, the zodiac's starting point was tied to a reference near the star Revati, and planetary positions were computed relative to that fixed frame. This is the nirayana, or sidereal, zodiac.
Around the early centuries of the Common Era the two zodiacs nearly coincided. Different authorities place the year of coincidence differently; the Lahiri ayanamsa, for example, implies a date in the late third century CE. Since then the two have drifted apart by about one degree every 72 years.
The trepidation theory
Some Indian texts, including the Surya Siddhanta, describe the motion of the equinox not as a continuous drift but as an oscillation back and forth across a range of 27 degrees, a model known in the history of astronomy as trepidation. Similar ideas appeared in medieval Islamic and European astronomy. Modern astronomy has shown that precession is a continuous circuit, taking about 25,800 years, and modern ayanamsas are built on that understanding.
Modern choices
Today, Indian practitioners almost all use a sidereal zodiac, but they differ on the exact ayanamsa. The Lahiri ayanamsa, adopted by the Calendar Reform Committee, is the most widely used. Others include the Raman ayanamsa and the Krishnamurti, or KP, ayanamsa. In the West, a sidereal school founded by Cyril Fagan and Donald Bradley in the twentieth century uses its own ayanamsa, while most Western astrologers continue to use the tropical zodiac.
Why the history matters
Knowing this history explains why a Western sun sign and an Indian sun sign usually differ by one sign for the same birth: the two systems measure from different starting points that are now about 24 degrees apart. Neither is a mistake; they answer different questions, one about the seasons and the other about the stars. This site uses the sidereal zodiac and lets the reader choose the ayanamsa.