The Ayanamsa
The gap between the two zeros has a name.
The ayanamsa is the angle between the tropical zero point — the vernal equinox — and the sidereal zero point.
It is currently a little over 25° — 25°07′ as this is written — and it grows by one degree every seventy-two years, because precession is its cause.
Same circle, same twelve divisions, same 30° each. Only the starting point differs — and everything downstream of the starting point differs with it.
Converting between the zodiacs is a subtraction
Because the two scales are identical apart from where they start, moving a position from one to the other is arithmetic of the simplest kind. Subtract the ayanamsa and you have the sidereal longitude; add it and you are back.
| Tropical | Sidereal | |
|---|---|---|
| Longitude along the ecliptic | 45° | 20° |
| Written as | 15° Taurus | 20° Aries |
A body at 15° Taurus tropically is at 20° Aries sidereally. Not a different position — the same point in the sky, measured from a different zero.
Because the gap is currently about five-sixths of a sign, almost every body lands in the previous sign when converted. That single fact is why the two traditions produce charts that look so different from identical data, and why a position quoted without its zodiac is close to useless.
There is more than one ayanamsa
The tropical zero is a construction and needs no choosing. The sidereal zero has to be nominated, so different schools have nominated different ones, and each choice gives a slightly different ayanamsa.
| Ayanamsa | Where it is used | Zero year |
|---|---|---|
| Fagan/Bradley | Western sidereal astrology | 221 |
| Lahiri | Indian astrology; the Indian national calendar | 285 |
| Krishnamurti | A branch of Indian astrology | — |
The "zero year" is the date at which that school's sidereal zero coincided with the equinox — the crossing point from the previous page. They differ by about a degree, which is around seventy years of precession, and that degree is the whole of the disagreement. Everything else about their zodiacs is identical.
What the applications use
Aquila and Compass use the Fagan/Bradley ayanamsa.
It is not a setting. The Zodiac dropdown offers Tropical and Sidereal, and selecting Sidereal applies Fagan/Bradley — the Western sidereal standard. There is no ayanamsa menu, and no alternative is reachable from the interface.
This is worth knowing if you are comparing output against Jyotisha software, which will almost always be using Lahiri. Positions will differ by roughly a degree: not an error in either program, just the two schools disagreeing about where the stars' zero sits, exactly as the table above says they do.
What it is not
Two clarifications, because the ayanamsa attracts more mysticism than a subtraction deserves.
It is not a correction. Nothing is being fixed. Neither zodiac is the true one that the other approximates; they are two scales with two different zeros, and the ayanamsa is simply the distance between the marks.
It is not the same as the drift from the constellations. The ayanamsa measures the gap between two zodiacs. The mismatch between any zodiac and the ragged constellations is a separate matter, built in from the beginning, and no ayanamsa addresses it.
What comes next
Everything needed to choose between the two settings is now on the table. The last page of the category is about what actually changes on screen when you do — and the answer includes one genuine surprise.
Next: Choosing a Zodiac.