Geocentric vs Topocentric: Origins, Parallax and Comparison
Compare Earth-centered and observer-centered positions, the role of parallax, and the shared frames and corrections needed for a meaningful comparison.
Calculation contract · released 2026-08-18
Definition
Geocentric coordinates use Earth’s center as origin; topocentric coordinates use a specified observing site on or near Earth’s surface.
Calculation procedure
Declare the origin, compute the target state relative to it, apply the same frame and corrections, and retain observer coordinates for topocentric output.
Required inputs
- Target body
- Instant and timescale
- Geocenter or topocentric observer
- Reference frame and correction model
Maha convention
Maha natal planetary placements are apparent geocentric ecliptic longitudes of date; horizon and rise/set calculations are observer-specific.
Uncertainty and edge cases
The Moon shows much larger topocentric parallax than distant planets. Mixing origins can change boundary classifications.
What this does not establish
Topocentric output is not intrinsically more appropriate for every astrological tradition.
Authoritative references
These sources establish the calculation vocabulary, data product, or transformation convention. They do not establish astrological interpretation.
- 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.
- 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.
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