What is the ayanamsa?
The ayanamsa is the angle between the tropical and sidereal zodiacs at a given date. It is subtracted from a tropical longitude to give a sidereal one. Different schools fix the starting point differently, so the value depends on the ayanamsa chosen.
Definition
At any moment the ayanamsa is a single number of degrees. It grows by roughly 50.3 arcseconds a year, because of precession. Tools calculate it from a base value at the year 2000 (J2000) and that yearly rate.
Base values used on this site
| Ayanamsa | Value at J2000 |
|---|---|
| Lahiri (Chitrapaksha) | 23.8571 degrees |
| Raman | 22.4108 degrees |
| KP (Krishnamurti) | 23.7602 degrees |
The yearly drift is the same for all three. The differences between them are fixed offsets.
Why a choice is needed
No single ayanamsa is defined by a physical law. Each is a convention, set by an astronomer or school to start the zodiac at a chosen star or point. For that reason two correct calculators can give different signs for the same planet. The tools on this site show the ayanamsa they used, so you can compare.
What to check
If your result differs from another calculator, first compare the ayanamsa value. A difference of about 0.5 degree is usually the choice of ayanamsa, not an error in the calculation.
How it is used in a calculation
In practice the ayanamsa is applied last. First the tool computes the tropical longitude of each graha from the ephemeris, which gives its angle from the spring equinox. Then it subtracts the ayanamsa for the date, modulo 360 degrees. The result is the sidereal longitude, which determines the sign, the degree within the sign, the nakshatra and the house. Because the ayanamsa is applied to every graha alike, the relative positions of the planets do not change. Only the absolute reference shifts. This is why an ayanamsa error moves everything together, and why comparing two charts is easiest when both use the same value.
Questions readers ask
Does the ayanamsa change every year? Yes, by about 50 arcseconds a year, so a chart computed today differs from one computed a hundred years ago by roughly one degree and a half. Is one ayanamsa the true one? No. Each is a convention. Lahiri is the most common in India and is the default here. Raman and KP are used by some practitioners. The site shows the chosen value on each tool so the reader can compare with another source. If your answer is different by about half a degree from a printed almanac, the ayanamsa is the first thing to check.
Summary for a quick check
If you remember only one thing: the ayanamsa is a number of degrees that changes slowly with the year, and every graha's sidereal longitude equals its tropical longitude minus that number. The choice of ayanamsa is a convention that each tradition makes. Before you compare two charts or two almanacs, confirm that both use the same value, and confirm the date at which the value was computed. With those two checks, most apparent disagreements disappear, and the remaining differences are small enough to explain.