Julian day and ephemeris
Julian Day (JD) is a continuous day count astronomers use to label an instant unambiguously (often JD at UT). BirthInput.instant in UTC is converted to JD, then planetary positions are evaluated for that JD.
Ephemeris role
An ephemeris is a model or table of celestial positions over time. AstroRust evaluates VSOP87 series at request time rather than shipping DE files — see vsop87.
At each birth JD, for each body:
- Evaluate VSOP (heliocentric) or the crate’s Moon model
- Transform to geocentric ecliptic λ, β (geocentric-vs-topocentric, ecliptic-longitude)
- Frame as tropical or apply ayanamsa for sidereal (sidereal-vs-tropical)
Providers
| Provider | Role |
|---|---|
vsop.ephemerides |
Raw planet_position, house_cusps |
core.chart |
Full NatalChartOutput via calculate / calculate_sidereal |
Same VsopEphemeris backend under both.
Time and precision
UTC → JD is standard for natal APIs. Sub-arcminute differences between UT and TT are usually smaller than birth-time error (ephemeris-uncertainty, time-zones).
References
- Julian day — Wikipedia
- Ephemeris — Wikipedia
See also natal-chart, planet-positions, data-quality.