Precession and longitude of date
How long-term motion of Earth’s orientation changes equinox-based coordinates and the tropical–sidereal offset.
Calculation contract · released 2026-08-18
Definition
Precession is the modeled long-period change in the orientation of Earth’s rotation axis and equatorial reference system.
Calculation procedure
Transform coordinates between epochs with a declared IAU precession model and retain whether output is fixed-epoch or of-date.
Required inputs
- Source and target epochs
- Reference frames
- Precession model and release
- Timescale
Maha convention
Maha’s Lahiri calculation advances its J2000 anchor with an IAU 2006 general-precession polynomial and exposes the resulting offset.
Uncertainty and edge cases
Different historical precession models diverge over long spans; model error must not be hidden behind modern decimal precision.
What this does not establish
Precession explains coordinate drift, not whether tropical or sidereal interpretation predicts events.
Authoritative references
These sources establish the calculation vocabulary, data product, or transformation convention. They do not establish astrological interpretation.
- 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.
- SPICE Reference Frames
NASA/JPL Navigation and Ancillary Information Facility · 2021-12-31 revision
Frame identifiers, frame centers, transformations, orientation relationships, and the distinction between frames and coordinate representations.
Boundary: A frame name alone does not establish the observing origin, epoch, equinox, time scale, or correction model.