Why your Western sign differs from your rasi

A Western sun sign and a rasi divide the same circle. They do not start counting from the same place. The Western zodiac measures from the March equinox. The Indian sidereal zodiac measures from a point fixed against the stars. The angle between the two zero points is the ayanamsa, and it is about 24 degrees at present.

Twenty-four degrees is most of a sign, so for most birthdays the rasi is the sign before the Western one. Someone born a Leo is usually Kadagam. That is not an error in either system. It is the whole of the difference between them.

The tropical zodiac ties 0° Aries to the March equinox, the instant the Sun crosses the celestial equator going north. It is a zodiac of seasons, and its first point is wherever the equinox falls this year.

The sidereal zodiac ties its zero point to the fixed stars. It is a zodiac of constellations, and its first point stays where it is.

The equinox does not stay where it is. The Earth’s axis wheels slowly, so the equinox creeps backwards along the ecliptic at about 50 arc-seconds a year. The figure this engine uses is the IAU 2006 general precession in longitude, 5,028.8 arc-seconds a century, so a whole degree takes about seventy-two years.

Run that backwards and the two zero points meet. On the Lahiri definition they coincided in the late third century of the common era, which is why the two zodiacs still carry the same twelve names, and why they no longer point at the same stretch of sky.

On 26 September 2026 the Lahiri ayanamsa is 24.23 degrees, which is 24°14′. Every tropical longitude on this site has that subtracted from it to give the sidereal one, and that single subtraction is the only thing standing between a sun sign and a rasi.

What it does to a birthday is easy to state. If the Sun stood in the first 24 degrees of its tropical sign, the rasi is the sign before. If it stood in the last six degrees or so, which is the final five or six days of a Western sign period, the two name the same sign.

Counted across a year of birthdays that is about one in five keeping the same name and four in five moving back one. So this is not a near-boundary effect. It is the ordinary case, and a reader who finds the two agree is the unusual one.

The ayanamsa is a definition rather than a measurement, and there is more than one of them. This site offers nine and uses Lahiri unless you change it on the Settings page.

Lahiri is the Chitrapaksha ayanamsa adopted by the Indian Calendar Reform Committee and carried by the Indian Astronomical Ephemeris. It is the default here because it is the one nearly every Indian almanac and nearly every piece of Indian astrological software already uses, so a chart cast with it can be set beside the one an astrologer has without a conversion step.

The near neighbours differ by small constants. Raman runs 1 degree 26 arc-minutes 47 arc-seconds behind Lahiri. Krishnamurti, the KP ayanamsa, runs 5 arc-minutes 49 arc-seconds behind it. True Chitrapaksha and Pushya-paksha are recomputed at every instant so that a named star sits exactly at its nominal sidereal degree. Fagan–Bradley, Sri Yukteshwar and Sassanian are fixed at one epoch and carried forward by precession, and a custom value can be typed in.

All of them follow the Swiss Ephemeris definitions to within 0.001 arc-seconds over 1800 to 2200, so the sidereal zero point here is theirs rather than an approximation of theirs. Which of the nine to use is a choice the traditions make differently, and the app names the one in force rather than hiding it.

The Sun covers about a degree a day, so a sign boundary is an instant, not a date. The printed ranges — about 23 July to 22 August, and so on — are approximate, and the true boundary moves by several hours from year to year, because the calendar year and the tropical year are not quite the same length.

This site computes the boundary from the real equinox rather than from a printed range. When the Sun was within one degree of a sign boundary at birth, the zodiac page says so and asks for the time of birth, because inside that degree the hour decides the sign.

The same caution runs through the sidereal side of the app. Four minutes of clock moves the rising sign by a degree, and a Moon half a degree from a nakshatra boundary turns on whether a remembered birth time is right. Where a position sits that close, the pages say so rather than printing a clean answer they cannot support.

A zodiac is a way of naming a division of the circle. Two traditions have named two different divisions for the better part of two thousand years, one framed on the seasons and one on the stars, and each is internally consistent. Nothing on this site adjudicates between them.

What does go wrong is mixing them. A rasi, a nakshatra, a dasha, a porutham and everything built on them are sidereal, and every one of them is computed here with the ayanamsa applied. A Western sun sign is tropical and computed without it. Reading a sidereal chart under a tropical sign name, or the reverse, is the one move in this area that is actually a mistake.

The zodiac page prints your position in both zodiacs at once, with the angle between them drawn, so you can see the single subtraction that separates the two answers.

The Western sun sign, settled by the hour.

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