Delta T and Earth-rotation uncertainty
Why historical calculations distinguish uniform terrestrial time from irregular Earth rotation.
Calculation contract · released 2026-08-18
Definition
Delta T is TT minus UT1. It links a uniform dynamical timescale to the observed rotation angle of Earth.
Calculation procedure
Use measured Earth-orientation values where available and a declared historical model outside that range; retain the model, release, and uncertainty envelope.
Required inputs
- Epoch
- TT or equivalent ephemeris time
- UT1 observation or Delta T model
Maha convention
Maha requires Earth-rotation provenance when a calculation depends on local sidereal time or horizon geometry.
Uncertainty and edge cases
Historical Delta T uncertainty propagates strongly into local horizon angles and eclipse paths, while geocentric zodiac longitude may be less sensitive.
What this does not establish
A preferred Delta T reconstruction cannot validate an astrological interpretation.
Authoritative references
These sources establish the calculation vocabulary, data product, or transformation convention. They do not establish astrological interpretation.
- IERS Bulletins
International Earth Rotation and Reference Systems Service · live bulletin index
Bulletin A and B Earth-orientation parameters, Bulletin C leap-second announcements, and Bulletin D DUT1 announcements.
Boundary: The bulletin number or data-product version used in a computation must be retained; “latest” is not a reproducible version identifier.
- 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.