Reference frame, epoch, and equinox
The three labels required to understand what a celestial coordinate is measured against.
Calculation contract · released 2026-08-18
Definition
A frame defines axes, an epoch identifies a reference time, and an equinox can identify the origin and orientation used for equatorial or ecliptic coordinates.
Calculation procedure
Record origin, frame identifier, epoch, equinox where applicable, timescale, representation, and correction model as one reference contract.
Required inputs
- Coordinate origin
- Frame identifier
- Epoch and equinox
- Timescale and representation
Maha convention
Maha rejects provenance records that provide a frame name without the other reference-contract fields.
Uncertainty and edge cases
Informal labels such as “of date” or “J2000 positions” can hide materially different conventions.
What this does not establish
A standardized reference frame is a measurement convention, not an interpretive tradition.
Authoritative references
These sources establish the calculation vocabulary, data product, or transformation convention. They do not establish astrological interpretation.
- 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.
- 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.
- 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.