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.
The two scales share the circle, the twelve divisions and the 30° width of each. Only the starting point differs, and every position measured from it 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. It is not a different position but 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.
Which one is in use
A sidereal zodiac carries an ayanamsa with it, whether or not the choice is ever presented. Some software offers a menu of them; some fixes one and says which. Western sidereal work generally means Fagan/Bradley, and Jyotisha software almost always means Lahiri.
Which one it is accounts for the commonest disagreement between two sidereal sources: a gap of about a degree. Neither source is in error; the two schools disagree about where the stars' zero sits, as the table above shows.
What it is not
The ayanamsa is often misunderstood in two ways.
It is not a correction. Neither zodiac is the true one that the other approximates; they are two scales with two different zeros, and the ayanamsa is 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 that separates the two zodiacs has now been described. The last page of the category sets them side by side: what differs between them, and what stays the same.
Next: The Two Zodiacs Compared.