Retrograde, station, and direct-motion labels
How daily finite differences and instantaneous speed can disagree near a planetary station.
Calculation contract · released 2026-08-18
Definition
Retrograde and direct describe the sign of apparent longitudinal motion; stationary is a thresholded label around zero speed.
Calculation procedure
Compute a signed longitude difference over a declared interval or request instantaneous speed, normalize wraparound, and apply a published stationary threshold.
Required inputs
- Apparent longitude at one or more instants
- Sampling interval or speed quantity
- Stationary threshold
Maha convention
Maha uses a forward 24-hour longitude difference and labels absolute motion below 0.01° per day stationary.
Uncertainty and edge cases
An independent engine using instantaneous speed can assign a different label close to the zero crossing without either longitude being defective.
What this does not establish
Retrograde motion is apparent geometry and does not by itself imply delays, reversals, or outcomes.
Authoritative references
These sources establish the calculation vocabulary, data product, or transformation convention. They do not establish astrological interpretation.
- Astronomy Engine
Don Cross (cosinekitty) · 2.1.19
Locally computed Sun and Moon positions derived from the VSOP87 and JPL-derived lunar theories, together with rise and set times, at the accuracy stated in the library documentation.
Boundary: Positions are computed in process rather than fetched from an authority, so a fact citing this source carries a digest over the computed values and not over a provider response body. Its accuracy is adequate for calendrical work and is not a substitute for a Horizons query where sub-arcsecond agreement matters.
- 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.
- Swiss Ephemeris general documentation
Astrodienst AG · 2.10 documentation, checked 2026-08-18
An independent ephemeris implementation, supported reference choices, house calculations, and its dual-license boundary.
Boundary: Maha does not bundle or call Swiss Ephemeris in production. Frozen conformance results are validation evidence, not permission to redistribute its software or data.