What is the difference between Western and Sidereal (Indian or Vedic) Zodiacs? There is so much confusion among astrologers and non-astrologers alike on this topic. And it is a very sensitive topic. I will try to shed some light on this in plain English.
Tropical or Western Zodiac – this zodiac is based on the seasons. It is based purely on relationship between Earth and the Sun. There is no role of Moon or any stars in the sky. If we extend the equator of the Earth into the skies, that plane is called Celestial Equator. Point in the sky where Sun crosses the Celestial Equator while moving ‘upwards’ is called Vernal Equinox, the first day of Spring. In Tropical zodiac, this point of vernal equinox is considered as the starting point of Aries. There is no connection with the actual stars in the Aries constellation though. The stars in the backdrop of this point of vernal equinox may belong to a completely different constellation. And they do right now.
Sidereal or Vedic or Indian Zodiac – this zodiac on the other hand is based on the actual position of stars in the skies. Hence it is called Sidereal (related to stars). When we say ‘0 degrees of Aries’ in Sidereal zodiac, it actually refers to the observable constellation of Aries in the skies. It just so happened that in 285 AD, the Vernal Equinox took place exactly at the beginning point of the constellation of Aries i.e. Sun was right on top of Aries at the time of Vernal Equinox in 285 AD. So the last time both the zodiacs coincided was in 285 AD. Since then, the two zodiacs have been drifting apart. Why?
What Caused the Difference? Why do we have a vernal equinox (and hence the seasons) in the first place? This is because of the tilt of the Earth’s axis. And this tilt itself is shifting by 0.014 degrees every year due to gravitational pull of Sun, Moon and other planets on the Earth. Hence, the two zodiacs have been drifting by this amount every year – a term known as Precession of the Equinoxes.
In summary, Tropical or Western zodiac is connected with the seasons (Sun’s position wrt Earth) whereas Sidereal or Vedic zodiac is based on actual position of stars in the skies. Due to the shift in the tilt of Earth’s axis, the two zodiacs drift by 0.014 degrees every year. Last time they coincided was in 285 AD. If you multiply 0.014 degrees by the number of years since 285 (2014 – 285), you will get approx 24 degrees. That is exactly how far apart the two zodiacs are right now.