True Sidereal vs. Lahiri: Which Sidereal Chart Is Yours?

Two calculators can assign different sidereal signs to the same birth moment because sidereal is not one calculation method. The offset, the sign intervals and the reference frame all matter.

This guide compares Lahiri's equal-sign zodiac with Synthesis True Sidereal, our custom 13-sign approximation. It also explains what our calculation does and what it does not do.

Method clarification, October 3, 2026: Earlier versions of this page described Synthesis as using actual IAU constellation boundaries and contrasted that with applying an offset. That description was inaccurate. Our website and composition plugin both apply a year-based offset and a custom table of fixed, unequal longitude intervals. They do not perform a full IAU sky-polygon calculation.

Lahiri and Synthesis Use Different Conventions#

Lahiri is an ayanamsa convention used in sidereal astrology. An ayanamsa specifies the angular separation between tropical and sidereal reference points. With a Lahiri zodiac, signs remain twelve equal intervals of 30 degrees. The offset changes with time; it should not be described as one permanently fixed number.

Synthesis uses a different convention. Its True Sidereal column subtracts a year-based offset, then maps the resulting longitude to thirteen fixed intervals of unequal width, including Ophiuchus. This is a chosen approximation for our astrological framework.

A thirteen-sign result alone does not prove that a calculator uses full astronomical constellation boundaries. Likewise, twelve signs alone do not tell you which ayanamsa it selected. Read the method rather than inferring it from the label.

The Synthesis Offset and Interval Table#

For a Tropical longitude in degrees, our current transform is:

offset = 31.38 + (UTC birth year - 2000) / 72
Synthesis Sidereal longitude = (Tropical longitude - offset) modulo 360

This formula uses the UTC birth year, not a continuously varying date-based precession model. The resulting longitude is assigned to this custom table, identified as synthesis-custom-13-v1:

These boundaries are in the transformed Synthesis longitude frame. They are not a table of IAU sky-polygon boundaries, and a sign assignment from this table should not be presented as a verified astronomical constellation identification.

For an arithmetic example, take a hypothetical Tropical longitude of 260 degrees in UTC year 2000. Subtracting 31.38 gives 228.62 degrees. That falls in the Synthesis Ophiuchus interval. This demonstrates the rule; it does not establish which astronomical constellation a real planet occupies.

The composition plugin's method tool returns this table, its fingerprint and the conventions. Each calculated result includes the longitudes, weights and normalization needed to reconstruct its percentages. Ask it to explain the calculation or show the audit.

What the Website and Plugin Share#

Both use the Sidereal offset and interval table above. The website displays Tropical, Synthesis True Sidereal and Draconic perspectives; Draconic rotates Tropical longitudes relative to the North Node and uses twelve equal signs.

The plugin calculates a three-system composition, weighted 45% Tropical, 35% Synthesis True Sidereal and 20% Draconic. Chinese astrology/BaZi and numerology belong to the broader website reading and do not enter those composition percentages. See how Synthesis works for the wider reading approach.

A shared Sidereal convention is not a promise of identical totals across versions. Plugin calculation version 1.1.0 corrected Vertex and minor-point handling in its composition adapter. The website's older calculation/export pathway still needs a separate parity review. Compare calculation versions and included placements before comparing totals.

Comparing Your Results#

When two charts disagree, check:

1. The same birth date, local time, birthplace and time zone were used.
2. The selected ayanamsa or custom offset is named.
3. The sign intervals are documented, including whether Ophiuchus is included.
4. The same planets and points are being compared.
5. The tools explain unknown birth times and boundary cases.

Synthesis's True Sidereal column is not a Lahiri calculation. To compare a personal Lahiri chart, obtain one from a calculator that explicitly supports that convention; do not simply relabel a Synthesis result. Chat explanations should distinguish supplied chart data from calculations they have not performed.

Choosing a Framework to Explore#

Use a Lahiri chart when you want that equal-sign sidereal convention and its associated interpretive tradition. Use Synthesis when you want to explore our custom thirteen-sign perspective alongside Tropical and Draconic.

Neither choice establishes personality facts or predictive accuracy. Precise astronomical inputs and an internally consistent calculation are different from evidence that an astrological interpretation describes a person. Treat the themes as prompts and compare them with your experience.

You can open the Synthesis calculator to compare the three perspectives. For a broader interpretation, see the website's reading options. If discussing your chart in Synthesis AI Chat, ask which supplied placements support the interpretation and which convention it is using.

Related explanations: Tropical vs. Sidereal, the Draconic chart, and precession of the equinoxes.