Sidereal and Tropical

Two ways to draw the same circle

The zodiac is a circle of twelve equal signs laid around the path the Sun appears to travel through the year. Everyone agrees on that. The disagreement is about where to put the starting line.

Western astrology starts the circle at the point where the Sun crosses the equator heading north in March, the equinox. Wherever the Sun is at that moment is called 0 degrees Aries, and the other eleven signs follow on from there in thirty degree steps. This is the tropical zodiac, and it is tied to the seasons. Aries always begins at the March equinox, Cancer at the June solstice, Libra at the September equinox and Capricorn at the December solstice, whatever the stars are doing behind them.

Jyotish starts the circle against the stars. The twelve signs are laid over the actual constellations that gave them their names, so that when the chart says the Moon is in Taurus, the Moon is sitting among the stars of Taurus, close to the Pleiades and Aldebaran. This is the sidereal zodiac, from the Latin word for star, and it stays fixed to the sky.

One zodiac is anchored to the Earth's year. The other is anchored to the stars. Each is a perfectly good circle, and for one brief period in history they were the same circle.

Why they drifted apart

The Earth wobbles. Very slowly, like a spinning top running down, its axis traces a circle in the sky, and one full turn takes about 26,000 years. Astronomers call it the precession of the equinoxes, and what it means in practice is that the equinox point, the tropical starting line, slides backwards against the stars by about 50 seconds of arc a year. That is one degree every seventy two years or so, and one whole sign every two thousand one hundred and fifty years.

Around seventeen centuries ago the two starting lines sat on top of each other. Since then the tropical zodiac has slipped backwards against the stars, and the gap has grown steadily. The gap has a name, the ayanamsa, and a Vedic astrologer has to choose which value of it to use, because there has never been complete agreement on exactly where the sidereal zero point sits.

The one I use, and the one my chart generator is built on, is the Lahiri ayanamsa, named for the Indian astronomer who defined it and adopted as the official standard by the Indian government in the 1950s. It is the most widely used value in Jyotish today. By Lahiri the gap was about 23.85 degrees in the year 2000 and sits at about 24 degrees 12 minutes in 2026, growing by about 50 seconds a year. On these pages I write "about 24 degrees" and leave it there.

The one sign back rule

This is how the 24 degrees works out for you.

Take any planet in your Western chart. Subtract about 24 degrees from its position and you have its position in the Vedic chart. That is the whole of the arithmetic.

If your tropical Sun sits anywhere in the first 24 degrees of its sign, taking away 24 degrees carries it back across the boundary into the sign before. A Sun at 10 degrees of Leo becomes a Sun at about 16 degrees of Cancer. A Sun at 2 degrees of Leo becomes a Sun at about 8 degrees of Cancer. Only a Sun in the last six degrees of its sign, from 24 to 30, stays where it is. A Sun at 27 degrees of Leo becomes a Sun at about 3 degrees of Leo, still a Leo, but only just.

Twenty four of the thirty degrees move back and six stay. So about four people in five find their sidereal Sun one sign earlier than their Western Sun, and the same rule applies to the Moon, to the Rising sign and to every other planet in the chart. If you were born in the last few days of a Western sign, you are among the one in five who keep the same sign in both systems. Everyone else moves back one.

The sign boundaries move with the years too, so if your Western Sun is anywhere near the 24 degree line, have the position checked. My readings do this, and the Find Your Numbers and Signs page will add the Vedic Moon sign shortly.

So am I a Leo or not

You are a tropical Leo and, most likely, a sidereal Cancer. Both are true at once, and neither one cancels the other.

I have watched a lot of people meet their sidereal chart for the first time, and the reaction is usually a flicker of loss, as if someone had come for a part of their name. Let me put that to rest. The Western signs were never really about the constellations, whatever the names suggest. They are a way of dividing the year, and the Aries energy of the tropical zodiac is the energy of the season when the Sun comes north and the light returns, which is why it reads as fresh starts and courage and impatience. That is the Leo you know. It has not gone anywhere.

The sidereal sign is a different question asked of the same birth. It is the Sun against the backdrop of the stars, and Jyotish reads it as the deeper purpose underneath the personality, the fire the person was built around rather than the way they come into a room. A tropical Leo with a sidereal Cancer Sun is somebody who presents warmth and shine and whose deeper purpose is to shelter and to belong. In my experience the two descriptions rarely contradict each other. They sit at different depths.

Neither zodiac is wrong. One measures the Earth's year and the other measures the sky, and a good reader uses the one that answers the question in front of them.

Side by side in a reading

When I write a full portrait, the Western chart comes first, because it is the one most people know their way around. Then I give the sidereal Sun and Moon beside the tropical ones, and I say plainly which is which. I never swap one for the other and I never tell someone they have been reading the wrong sign all their life. They have been reading one sign. Now they have two.

The Moon is where the sidereal chart earns its keep. Jyotish reads the Moon above the Sun, and it goes finer than the sign, into the twenty seven nakshatras, the lunar mansions that each cover 13 degrees 20 minutes of the sidereal circle. The nakshatra your Moon sat in at birth is the single most telling Vedic detail I know, and The Nakshatras index will take you through them. The Moon page and the Moon in the Signs page give you the Western view of the same Moon.

The pages that pair up

Every sign in this library now has two pages. The Signs, in the Western section, reads each sign through the tropical zodiac, as Sun, as Moon and as Rising sign. The Rashis, here in the Vedic section, reads the same sign through the sidereal zodiac, as your Moon first and as your Sun second. Each Rashi page names its Western pair and each Western sign page points back here.

Read your tropical Sun sign, then your sidereal Sun sign, and notice that one of them describes how you look from the outside and the other describes something quieter underneath. Then do the same with the Moon.

Which one feels more like the room you live in, and which one feels more like the house?

If you would like this read against your own chart, my readings are here.