Tropical vs sidereal zodiac: what the ayanamsa actually changes, and what it does not

Two zero points for the same sky

The planets do not care which zodiac you use. The difference is entirely in where you put the zero, and the zero moves.

The tropical zodiac puts 0 degrees of Aries at the vernal equinox, the moment the Sun crosses the celestial equator going north, around 21 March. That keeps the signs lined up with the seasons, which is why it works for planting, and it is the default in Western astrology.

The sidereal zodiac puts 0 degrees of Aries at a fixed point in the background stars. It stays with the constellations, and it is what Jyotish, the Indian tradition of astrology, uses.

The distance between the two is the ayanamsa, and it is not a rounding error. It is currently about 24 degrees, which is a little under a whole sign.

What the ayanamsa is

Precession is the slow drift of the equinoxes along the ecliptic, caused by the Earth's axis precessing like a spinning top. The whole cycle takes about 25,700 years, and the equinox currently sits about 24 degrees behind where it sat when the zodiac was defined. That difference, measured from a chosen reference star, is the ayanamsa.

Two things follow from this that are worth holding onto:

• The ayanamsa is a choice of reference point, not a measurement of the sky. Different traditions anchor it to different stars or epochs, and the values differ by a degree or more.
• It changes by about 50 seconds of arc per year. A value quoted for 2020 is out of date for 2026 by roughly three minutes of arc, which is enough to move a planet across a house cusp.

How far it has moved by 2026

The Lahiri ayanamsa, also called Chitrapaksha, was about 23 degrees 51 minutes at the start of 2000. Adding the precession rate of roughly 50.3 seconds a year for the 26 years since gives something close to 24.2 degrees in 2026.

That figure is worth quoting, and worth checking against the source rather than taking from an article, including this one: the values published by the Indian government and by the Swiss Ephemeris documentation are the ones to compare against, and they move every year.

At 24 degrees, and with each sign spanning 30 degrees, a planet can be in a different sign in the two zodiacs, and for a period of about 24 days per year it can be in a different sign in the two systems even when the ayanamsa is computed exactly. That is the everyday source of the arguments: the same birth chart, two sign labels.

What changes and what does not

This is the part that most explanations of the two zodiacs get wrong, and getting it wrong leads to a lot of wasted argument.

So if two charts disagree about whether there is a square between Venus and Mars, the ayanamsa is not the reason. A square is a square in both systems, and the disagreement comes from a different time, a different latitude, or a different house system.

The opposite mistake is just as common: treating a sign label as if it carried meaning on its own. A planet at 4 degrees of Scorpio in the tropical zodiac is at about 10 degrees of Scorpio in the sidereal one, and that is all that changed. The conjunction, the opposition, the house it falls in: all still there, and if the house changed, the house is the thing to check.

Which ayanamsa: Lahiri, Raman and the rest

There is no single sidereal zodiac. Each tradition anchors the zero differently, and the results differ by a degree or so, which is enough to change a sign label in the marginal cases.

When software lists an "ayanamsa" option, the question to ask is not which one is right but which one the comparison chart was drawn with. Two charts can use two different ayanamsas and two different house systems and still be presented as a disagreement about the sky.

House systems, and where they break

The house system decides where the sky is divided into twelve, and this is where most of the confusion in a comparison comes from, because the two things are usually discussed together and are entirely separate.

• Whole sign gives each sign its own house, starting from the sign containing the Ascendant. Simple, and the oldest of the four in regular use.
• Placidus divides by time: the cusp of the tenth house is placed by semi-arc division, the eleventh by the projection of the equator onto the ecliptic, and so on. It is the most common system in Western practice.
• Koch is a variant that places the Ascendant on the ecliptic and divides the rest from there. It differs from Placidus in the higher degrees and in the size of the intermediate houses.
• Equal divides the ecliptic from the Ascendant in equal steps. It always produces a house of a sensible size, which is its main practical advantage.
• Porphyry takes the quadrants of the ecliptic and divides each by time. It is the standard in Hellenistic work and it stays well behaved at high latitudes.

Every system except whole sign and equal breaks down as you approach the poles. Placidus houses become undefined above the Arctic Circle, and the divisions become strange well before that. If you were born above 66 degrees north or south, this is not a detail: it is the reason two programs will not give you the same chart, and the reason to prefer equal, whole sign or Porphyry for a chart you intend to keep.

What our app does, and why

Astral Lab uses the tropical zodiac, the default for Western practice, and the house systems it offers are listed in the app.

We are not going to argue that one is more correct. A zodiac is a convention for labelling a rotation of the sky, and the two conventions exist because different traditions want different things from them. If you work in Jyotish, you want a sidereal ayanamsa, usually Lahiri, and you want the houses calculated for the system your source uses, and no Western app is going to serve that well.

What a Western app can do well is the part that is not a convention: computing the positions accurately, handling the time zone and the daylight saving rule correctly, and letting you check the result. That is where we put the effort, and the ephemeris comparison sets out the series we use and how to verify any app's output, including ours, against a reference.

How to check it yourself

Two checks, both quick.

1. The ayanamsa. Take a position from your chart, subtract the ayanamsa for that date, and see whether the result is the sidereal position your sidereal source gives you. If your source uses a different ayanamsa than the one you subtracted, the difference you see is the difference between the two ayanamsas, not an error.
2. The house system. Compute the same chart in a second program set to a different house system, and compare the cusps rather than the planet positions. The planets will match exactly, because the planets do not depend on the house system. If the planets differ, you have a time problem, not a house problem.

FAQ

Is the sidereal zodiac more accurate than the tropical one?

That question has no answer inside astrology, because the two are measuring the same sky and labelling it differently. The accuracy question that does have an answer is whether the underlying positions are computed correctly, and that is about the ephemeris rather than the zero point.

How much does the ayanamsa change in a year?

About 50 seconds of arc, which is the general precession in longitude. Over a decade, roughly 8 minutes of arc, which is enough to move a slow planet across a house cusp. Always use the value for the year of the chart.

Do the aspects change between the two zodiacs?

No. Angular separation does not depend on where the zero is, so a conjunction, square or opposition is the same in both systems. Only the sign labels and the house placements move, and the house movement follows the ayanamsa.

Which ayanamsa should I use?

Whichever the source you are reading or the chart you are comparing against uses. If you are working from Indian sources, it is Lahiri. If you are reading a Western sidereal book, check which one it uses, because Fagan-Bradley and Raman give visibly different results.

Does the house system matter more than the zodiac?

For most charts, yes. The zodiac choice changes sign labels; the house system changes the houses themselves, and therefore changes which part of a chart a planet belongs to. A planet that is in the tenth house in Placidus can be in the ninth in Koch.