Index › S › Sidereal and tropical zodiacs

Sidereal and tropical zodiacs

Revised 2025-12-16 · Contributors · How entries are written

Both zodiacs divide the ecliptic into twelve equal thirty-degree signs. They differ in where zero degrees of Aries is placed.

Tropical zodiac

Zero Aries is fixed to the March equinox point. The signs are tied to the seasons and not to the constellations.

Standard in Western practice.

Sidereal zodiac

Zero Aries is fixed to a reference point among the stars. The signs remain aligned with the constellations of the same name.

Standard in Indian practice and used by a minority of Western practitioners.

Precession

The equinox point drifts westward against the stars at approximately one degree every seventy-two years, a motion called the precession of the equinoxes. The two zodiacs coincided at some point in the early centuries of the common era and have been separating ever since. The current difference, called the ayanamsa, is roughly 24 degrees, which is enough to place the Sun in the previous sign for most birth dates.

Ayanamsa

Lahiri

The most widely used value, adopted as the Indian civil standard.

Krishnamurti

A variant differing from Lahiri by a small fraction of a degree.

Fagan-Bradley

The value most used by Western siderealists.

The choice of ayanamsa shifts every position in a sidereal chart by the same amount, so it rarely changes sign placements between the competing values, which differ by well under a degree. It can matter for a position near a sign boundary.

The thirteenth sign

Periodic news reports announce that the zodiac has changed and that Ophiuchus is a thirteenth sign. This confuses the constellations, which are irregular star groups of unequal extent defined by modern astronomical boundaries, with the signs, which are twelve equal divisions of the ecliptic. The tropical zodiac has never claimed alignment with the constellations. Nothing changes when the story recurs, which it does at intervals of a few years.