Why is Vedic astrology different from Western astrology? What is ayanāṁśa?

Vedic astrology measures signs against the fixed stars; Western uses the seasons. The gap between them, about 24° today, is the ayanāṁśa.

Western astrology uses the tropical zodiac: 0° Aries is wherever the Sun stands at the spring equinox. Vedic astrology uses the sidereal (nirayaṇa) zodiac, tied to the actual star background. Because the Earth's axis slowly wobbles (precession), the equinox point drifts backwards against the stars by about 1° every 72 years.

The two zodiacs coincided roughly 1,500 years ago; today they differ by about 24°. That difference is called the ayanāṁśa. Most Indian software and the Government of India's calendar use the Lahiri (Citrāpakṣa) ayanāṁśa; a few schools use Raman, KP or others, which differ by small amounts.

So a planet at 10° Leo in a Western chart sits at about 16° Cancer in a Vedic chart. Both systems also differ in method — Vedic uses nakṣatras, daśās and divisional charts.

Source: Sūrya Siddhānta; Indian Calendar Reform Committee (1955) · Lahiri ayanāṁśa ≈ 24° in the 2020s

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