Sidereal vs tropical
Both frames use the same ecliptic longitude arithmetic; they differ in where 0° Aries is anchored on the ecliptic.
Tropical (Western default)
- 0° Aries = vernal equinox direction (season-based).
- Signs drift relative to constellations over centuries because of precession (~50″/year).
core.chartoperationcalculatereturns tropical longitudes in body snapshots.
See tropical-zodiac.
Sidereal (many Jyotish systems)
- Longitudes are shifted by an ayanamsa Δ so 0° aligns more closely with fixed star divisions (definition varies by school).
- Sidereal longitude ≈ tropical longitude − Δ (mod 360°).
core.chartoperationcalculate_siderealbuilds the same chart structure, then replaces displayed longitudes with sidereal values whenBirthInput.ayanamsais set (default Fagan–Bradley in the ephemeris config).
Same VSOP engine
Planetary theory does not “know” tropical vs sidereal — the ephemeris yields tropical λ; sidereal is a subtraction of ayanamsa (julian-day-ephemeris). House cusps in sidereal mode are likewise shifted when the backend runs in sidereal calculation mode.
Obliquity is not ayanamsa
Obliquity ε tilts equator vs ecliptic (obliquity). Ayanamsa is a longitude offset along the ecliptic. Do not conflate them.
Choosing an operation
| Goal | Operation | Reading notes |
|---|---|---|
| Western tropical wheel | calculate |
natal-wheel |
| Sidereal signs / Vedic-style wheel | calculate_sidereal + ayanamsa |
jyotish-overview, nakshatra-wheel |
References
- Zodiac — tropical vs sidereal overview
- Ayanamsa — Wikipedia
- Axial precession — why tropical and stars diverge
See also ecliptic-longitude, planet-positions, natal-chart.