Tropical vs Sidereal-Lahiri! What’s the different exactly?
Many people who get into the astrology or starting to see natal charts, Always having at least two choice in their chart options. Tropical or Sidereal!
The difference may be a question for any beginner or advanced astrologer. Because even advanced astrologers in each Vedic or Western types are not aware of the reason of the calculation differences between these two.
First of all let’s cover about the calculation systems and see what makes them different from each other.
0° Point!
So, When does exactly circle starts? or when it gets back to the same point after 360° journey?
Tropical says that he put the start of circle to the 0° of Aries. So when the Sun is returning to this point, It has done a full solar year!
Actually this is base of Jalali/Shamsi calendar which is used in Iran and some other countries and Nowruz (New day) is the day that Sun is coming to 0° of Aries.
But Sidereal works with fixed stars. For example in the most popular calculation system called Lahiri, they start the round from the fixed star of Spica/Chitra.
On September 2, 285, Spica was on 0° of Libra. This day is the only day which Lahiri and Tropical was the same!
But are fixed stars always fixed? No!
Spica/Chitra moves 0° 0' 4" forward every tropical solar month. So we have about 50 seconds move for Spica each year. It’s about 1° 23‘ 48.83" in each century!
How to calculate your Sidereal-Lahiri chart based on Tropical?
Here are some formula that is going to help you for calculating your Lahiri chart based on Tropical chart.
But the most easy way is to find the position of Spica in your Tropical chart and then subtract each element like Asc, Sun, Moon, Nodes and etc from that.
For example if you have Spica on 23°44’ of your natal chart and Sun on 12° 24' of Aries, Then your Sun in Lahiri is on 11° 20' of Pisces.
How to find Spica position by formula?
Write down these numbers,
- Y = Birth Year in Gregorian
- M = Birth Month in Gregorian
- D =Birth Day in Gregorian
A = ( 16.90709 * (Y/1000) )- ( 0.757371* (Y/1000)* (Y/1000) ) - 6.92416
B = (M-1+(D/30)) * (1.1574074/1000)
Spica = A+B
