Sidereal zodiac (Nirayana)
The sidereal (nirayana) zodiac measures the twelve signs against the fixed stars. Vedic astrology uses it; Western astrology uses the tropical zodiac, tied to the seasons. The two now differ by about 24°.
Also known as: fixed zodiac, vedic zodiac
Nirayana means “without ayana”, that is, without the precessional shift: its signs stay fixed relative to the stars and the 27 nakshatras. Sayana, the tropical zodiac, begins at the spring equinox, which drifts slowly against the stars. So the two zodiacs move apart by about a degree every 72 years, and the gap between them is the ayanamsa.
By the Lahiri reckoning, the two coincided around 285 CE. Today a planet’s sidereal position is about 24° less than its tropical one, so many planets, and often the Sun sign, fall one sign earlier in a Kundli than in a Western chart.
Neither zodiac is “wrong”; they simply measure from different starting points. Jyotish keeps the sidereal zodiac because its signs and nakshatras are defined by the stars themselves, and its calendar and timing tools, from the Sankrantis to the dashas, depend on them.