Tropical zodiac longitude
The equinox-anchored 0–360° coordinate used before any sidereal ayanāṁśa is applied.
Calculation contract · released 2026-08-18
Definition
Tropical zodiac longitude divides an ecliptic-of-date coordinate into twelve 30° sectors beginning at the adopted March equinox direction.
Calculation procedure
Compute ecliptic longitude in the declared of-date frame, normalize it to [0°, 360°), then derive sign index and degree within sign.
Required inputs
- Ecliptic longitude of date
- Equinox and frame convention
- Normalization policy
Maha convention
Maha retains tropical longitude as the common calculated value and derives a separate Lahiri sidereal value rather than overwriting it.
Uncertainty and edge cases
A result exactly on a 30° boundary has no robust sign label unless the continuous-value uncertainty lies wholly on one side.
What this does not establish
Calling a sector Aries or Cancer does not scientifically establish traits or future events.
Authoritative references
These sources establish the calculation vocabulary, data product, or transformation convention. They do not establish astrological interpretation.
- Horizons System Manual
NASA/JPL Solar System Dynamics · 4.98d (2025-11-21)
Solar-system object identifiers, observing centers, time inputs, reference frames, corrections, output quantities, and stated ephemeris limitations.
Boundary: Horizons output is authoritative only with the complete query contract preserved; transformed or rounded values require their own provenance record.
- Standards of Fundamental Astronomy
International Astronomical Union SOFA Board · 2023-10-11
IAU-approved algorithms for astrometry, time scales, Earth rotation, precession, nutation, coordinate transformations, and related fundamental astronomy.
Boundary: SOFA supplies standard algorithms and validation programs; an implementation must still record its exact release and input data products.