Nutation and true versus mean coordinates
Why “true of date” and “mean of date” use different treatments of short-period Earth-orientation motion.
Calculation contract · released 2026-08-18
Definition
Mean coordinates include precession without short-period nutation; true coordinates include the selected nutation model.
Calculation procedure
Declare mean or true equator/equinox, apply the matching precession-nutation transformation, and do not combine longitudes from unlike frames.
Required inputs
- Input frame and epoch
- Precession model
- Nutation model
- Desired true or mean output
Maha convention
Maha labels planetary production positions as apparent geocentric ecliptic longitudes of date and records frame language in methodology.
Uncertainty and edge cases
Libraries can use different IAU model generations; small differences matter most at discrete boundaries.
What this does not establish
True coordinates are not “truer astrology”; the word identifies an astronomical correction convention.
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.