Sidereal & Tropical Zodiac

Definition

These are the two ways astrologers set the zodiac's starting point. The sidereal zodiac (Nirayana) ties itself to the fixed stars: its zero point of Aries is pinned to a fixed star in the constellation Aries, so it does not drift over the centuries. The tropical zodiac instead uses the equinoctial points, placing the first degree of Aries at the Vernal Equinox — and because of the precession of the equinoxes, that point shifts a little each year. Hindu (Vedic) astrology keeps the sidereal zodiac; Western astrology uses the tropical.

In Tradition

These sources agree that Vedic astrology works from the sidereal zodiac of the fixed stars, set apart from the tropical zodiac built on the equinoctial points. Frawley defines the sidereal zodiac as the zodiac of fixed stars and the tropical as the one defined by the equinoctial points. Ponde describes the Hindu zodiac as anchored to a fixed star so that it does not move, against the Western zodiac that shifts each year through precession.

In Practice

This choice decides which longitudes a jyotishi (Vedic astrologer) actually reads. The Vedic signs, the rasis, are the twelve sidereal signs, Aries through Pisces, each thirty degrees wide. The rasi a graha (planet) sits in is its sidereal sign, and per-planet predictions are laid out sign-by-sign on that sidereal coordinate. Ponde holds that only the fixed zodiac is logical, because the traits of each thirty-degree stretch belong to that fixed patch of the heavens — whereas in the tropical zodiac the first degree of Aries keeps shifting with precession.

Historical Origin

Modern Vedic textbooks lay the contrast out plainly. Frawley sets out both terms in the English Glossary of The Astrology of the Seers. Ponde treats the fixed (sidereal) zodiac at length in Hindu Astrology (Planets in Stars), and Rayudu frames every per-planet prediction in How to Read a Horoscope on the sidereal rasi the planet occupies.

Further Reading

• Frawley, The Astrology of the Seers
• Ponde, Hindu Astrology (Planets in Stars)
• Rayudu, How to Read a Horoscope