Sidereal vs. Tropical

One of the most frequent questions I am asked, especially in the ‘West’ is the difference between the ‘Western’ and ‘Eastern’ schools of astrology or in other words Tropical vs. Sidereal Astrology?

 

To clarify all the discussions here on this website are based on Vedic Astrology which utilizes a Sidereal zodiac as opposed to the Tropical zodiac. Even the “astrology illiterate” probably understands the outcome of the difference between Sidereal vs Tropical. The entire zodiac shifts which changes the signs/ houses and hence planetary positions. As an example, as per the Western – Tropical zodiac, a person born on September 7 is a Virgo. As per Vedic astrology – Sidereal zodiac, the individual is a Leo.

Another important point is – 3,000 yrs. from now, the individual born on Sep 7 would definitely be Virgo as per the Tropical Zodiac. However this might have shifted as per the Sidereal Zodiac!

The reason can get very technical in terms of physics/ astronomy. The difference is due to a frame of reference with respect to an observer on earth, in a simplified way – where exactly does zero degrees of Aries start and hence starts the zodiac with 12 signs (30 degrees each).

Without getting into any technical details or complications, the simple explanation is that the tropical zodiac does not incorporate the fact that the earth not only has a rotational axis, but the axis itself is slowly rotating. A phenomenon described in Physics as ‘Precession of the Equinoxes’.  

One rotation around the earth’s axis obviously takes about 24 hrs., One rotation of the axis itself takes about 26,000 years.

The Tropical year will be approximately 20 minutes shorter than the Sidereal year.

The two systems will be the same every 26,000 years approximately (25,772 years to be precise). However in between these years, the systems will continue to have a divergence. This divergence will cause the relative start and end points of the zodiac signs to be different.

The astrological scriptures described this Precession by a Sanskrit word called as ‘Ayanamsa’ (translated as AYAN = movement, AMSA = component) or Movement Component. Vedic astrology takes into account this difference in its calculations and Ayanamsa is the longitudinal difference.

In the Vedic astrology scriptures these difference in calculations is explained in detail and the Tropical system is called ‘Sayana’ & Sidereal system is called ‘Nirayana’.

One very important point to note is that the sages who wrote the earliest known vedic astrology texts (Bhrigu Parasara Hora Shastra) mathematically quantified the Ayanamsa (Precession of the equinoxes) at least 2500 yrs before Hipparchus (the Greek astronomer) studied and quantified it in astronomy/ physics by observing the relatively ‘fixed’ star SPICA in the constellation Virgo around 190 BC.

The above is simplified to a large extent for the casual reader to understand. There is a lot of research in this field and to be noted is the fact that lately the Western school of thought has many modified versions too which incorporate this movement in one form or another.

 

3 Responses to Sidereal vs. Tropical

  1. And yet when it comes down to a specific or individual comparison between the two or even multible systems certain grains of truth emerge. Maybe the truth of everything is reflected in its seeming otherness, one mind one god one eternal truth expressing itself in infinite variety……

  2. sambit says:

    What setting do you use in Jhora for bhava chalit chart?
    Is lagna in bhaav chalit middle of first house or lagna at beginning of 1st house?

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