Sidereal vs tropical: what the Lahiri ayanamsha changes in your chart

The ayanamsha is the distance between the two zero points of the zodiac: the tropical zodiac starts at the vernal equinox, the sidereal zodiac at a point fixed against the stars, and precession pulls the equinox backwards along the ecliptic by about 50″ a year, so the gap keeps growing. In the Lahiri convention, the Indian standard since 1955 and the one our calculator uses, the ayanamsha is 24°14′ on 17 September 2026. To turn a tropical chart into a sidereal one, subtract that value from every longitude. About four placements in five then move back one sign; only planets in the last six degrees of a sign keep it. Every point shifts by the same amount, so aspects stay the same, and the houses change only because Jyotish counts them by whole signs. Which zodiac to read is a question for the tradition you follow, and the same birth data feed both. For the sidereal side, the nakshatra calculator takes your birth date, time and city and shows the nakshatras of your Moon, Ascendant and Sun.

Two zero points

In the tropical zodiac, 0° Aries is wherever the Sun stands at the March equinox. The sidereal zodiac is pinned to the stars instead, and the equinox drifts: precession drags it backwards through the constellations by one degree in roughly 72 years. Around the year 285 the two origins coincided. Today they are some 24° apart.

The Lahiri number

Several conventions exist for pinning the sidereal origin, and they differ by a degree or so. Jyotish, Indian astrology, mostly uses the Lahiri ayanamsha, also called Chitrapaksha: it is defined so that the star Spica, Chitra in Sanskrit, is at exactly 180° of the sidereal zodiac. The Indian Calendar Reform Committee made it the national standard in 1955 and fixed its value at 23°15′00″ for 21 March 1956.

Our calculator uses the Swiss Ephemeris and reports the Lahiri value for whatever date you enter: 22°57′ for 9 November 1934, the birth date in the table below, and 24°14′ for 17 September 2026. Between those two dates the offset grew by about 0.84′ a year, which is the precession rate in reverse.

Who set the zero point

Two sites can show the same birth data with a planet in different sidereal signs for one reason: they use different zero points for the stars. Each zero point has a person behind it.

The Lahiri value takes its name from Nirmala Chandra Lahiri (1906–1980), a Calcutta astronomer and astrologer who was on India's Calendar Reform Committee; the committee, set up in 1952 under the astrophysicist Meghnad Saha, reported in 1955 and adopted the Spica benchmark described above. It was then adopted by the Indian Ephemeris, the national almanac, and most Jyotish software.

The best-known dissent is B. V. Raman's. Editor of The Astrological Magazine for six decades after restarting it in 1936, he worked from a remark by the twelfth-century astronomer Bhaskara II, and his zero point is a degree and a half behind Lahiri's: enough to move every planet in the first degree and a half of a sign back into the previous one.

Western sidereal astrology has an origin story of its own, and it starts in cuneiform. Cyril Fagan's Zodiacs Old and New (1950) tried to recover the Babylonian zodiac from the tablets; with Donald Bradley he established a value, 24°02′31″ for 1 January 1950, and Fagan–Bradley has been the Western sidereal standard since.

All are in the Swiss Ephemeris. On 17 September 2026:

Raman and Fagan–Bradley differ by 2°20′: enough to move a planet in the first two degrees of a sign back into the previous one under one convention and leave it where it is under another. We compute Lahiri only; the other rows are here to let you compare charts from different sites.

Most Western readers first heard about this through the news. In January 2011 a statement by Parke Kunkle, an astronomer at the Minnesota Planetarium Society, that the signs had drifted and that there was a thirteenth constellation, Ophiuchus, made headlines around the world. In September 2016 the same story went round again, this time linked to NASA, and on 20 September NASA answered on its own blog: "Did you recently hear that NASA changed the zodiac signs? Nope, we definitely didn't… We didn't change any zodiac signs, we just did the math." The math was precession. Tropical signs are tied to the equinox and never moved, while sidereal astrology has been correcting for the drift with the ayanamsha all along. As for Ophiuchus, it is a constellation, and neither zodiac has ever used constellation boundaries.

One chart, two zodiacs

Carl Sagan was born on 9 November 1934 at 17:05 in New York; the Astro-Databank record is rated Rodden AA, meaning the time comes from a document. His chart was computed by our calculator in both zodiacs, with the Lahiri ayanamsha of 22°57′ for that date.

Eleven of the fourteen points change sign. The three that stay — the Ascendant, Uranus and Pluto — are late in their tropical signs, at 25°19′, 28°45′ and 26°02′, with more than 22°57′ of room behind them. Every value differs by the same amount: the Moon is 35°02′ ahead of the Sun in either zodiac, and the same is true for every pair.

Which placements move and which stay

The rule for your own chart: compare the ayanamsha of your birth year with each planet's degree inside its tropical sign. Smaller moves back, larger stays, and the last six degrees of every sign are the only safe zone.

Nakshatras, the 27 lunar mansions of 13°20′ each, exist only in the sidereal zodiac, which is why a Western chart has none. Rahu and Ketu, the lunar nodes, shift by the same amount as everything else.

Houses in the sidereal chart

Jyotish counts houses by whole signs: the Ascendant's sign is the first house, each following sign is the next. In Sagan's sidereal chart that puts the Sun, in Libra, in the sixth house, counting from Taurus rising. The tropical chart uses Placidus instead, with house boundaries computed from the birth time and the latitude of the place.

In whole-sign houses a planet's house is its sign, so a planet that shifts back a sign also shifts back a house. Whichever zodiac you use, the Ascendant stays where it is: the degree rising over the eastern horizon at the moment of birth is the same whichever label the zodiac gives it, and its sign is the first house by definition.

How to check your own chart

Two charts of the same person can disagree for three reasons: the ayanamsha, the house system, and what a site treats as exact without a birth time. Ours are written down in one place, the calculation passport on the methodology page: Swiss Ephemeris, Placidus for the tropical chart, whole-sign for the sidereal one, Lahiri for the ayanamsha. The sidereal placements themselves, with nakshatras for the Ascendant, the Sun, the Moon, Mercury, Venus, Mars, Jupiter, Saturn, Rahu and Ketu, come from the nakshatra calculator; it needs a birth time, because the Ascendant does. And if it is specifically the rising sign that changed between two sites, the zodiac is one of three usual causes, and the other two are covered in why your rising sign differs between websites.

Where the method ends

The subtraction is exact and checked against a published standard. Which zodiac to read is a question for the tradition you follow: Jyotish reads the sidereal chart and its nakshatras; Western astrology reads the tropical one.