Use this page when converting between radians, arcseconds, parsecs, years, transverse velocity, and proper motion, or when auditing the convention behind a stored factor.
Representation contract¶
| Contract field | Current representation |
|---|---|
| Mathematical object | Scalar conversion constants connecting angular, distance, time, and transverse-velocity representations. |
| Physical convention | IAU exact astronomical-unit and parsec definitions, radians as the angular base, Julian-year-derived velocity conversion, and the RA-star proper-motion convention. |
| Runtime owner | jaxstro.astrometry owns KM_PER_PC, MAS_PER_RAD, ARCSEC_PER_RAD, DEG_PER_RAD, YR_PER_MYR, and K_PROPER_MOTION. |
| Shape and unit policy | Values are scalar Python floats with units named in symbols and docstrings; array broadcasting is caller-owned. |
| Transform boundary | Constants can participate in transformed JAX expressions but are static coefficients; frame-dependent astrometric maps are owned by jaxstro.coords. |
| Evidence | Unit tests check stored conversion values and coordinate tests check parallax and proper-motion use on regular geometries. |
| Downstream interpretation boundary | These factors do not define epochs, catalogs, covariance models, solar-system corrections, or survey calibration. |
Proper-motion scale¶
The conventional relation used by Jaxstro is
K_PROPER_MOTION retains the long-standing rounded compatibility literal. The exact
conventional value derived from one astronomical unit per Julian year has more
digits. Code requiring a different precision or epoch convention must make that
choice explicit.
For angular conversions, MAS_PER_RAD, ARCSEC_PER_RAD, and DEG_PER_RAD all
represent the same angle scale in different units. KM_PER_PC is derived from the
Jaxstro parsec and kilometer definitions, while YR_PER_MYR is exactly one million.
From constants to coordinate maps¶
compute_parallax and compute_proper_motions live in jaxstro.coords because their
outputs depend on spatial and tangent-frame geometry, not only scalar conversion.
The conversion in (1) is meaningful only after the
transverse basis and observer-star distance are defined.
The current evidence verifies constants and regular-domain coordinate calculations. It does not provide a complete astrometric model with epoch propagation, aberration, light-time, covariance, or survey systematics.