Coordinate systems
Natal chart software stacks several astronomical coordinate frames on top of each other. The calculation layer in core.chart ultimately needs geocentric ecliptic longitude (and latitude where used), but understanding the stack prevents common mix-ups (especially equator vs ecliptic, or geographic lat/long vs sky coordinates).
Frames used in charting
| Frame | What it fixes | Role in core.chart |
|---|---|---|
| Geographic | Birth place on Earth (lat/long) | BirthInput.latitude, BirthInput.longitude — horizon and meridian for houses and angles |
| Equatorial | Celestial equator + hour angle / RA | Intermediate step for local sidereal time and horizon geometry |
| Ecliptic | Sun’s orbital plane, longitude 0° at vernal equinox (tropical) | Primary output: planetary ecliptic longitude and house cusps |
| Horizon / topocentric | Observer’s local sky | Ascendant, MC, house cusps as intersections of great circles |
Right ascension and declination
Equatorial coordinates measure position along the celestial equator (projection of Earth’s equator onto the sky):
- Right ascension (RA) — eastward angle, often in hours (0–24h)
- Declination (δ) — north/south of the equator, like terrestrial latitude
Ephemeris backends may compute in equatorial form internally, then rotate into the ecliptic for sign-based astrology. See ecliptic-and-equator and obliquity.
Ecliptic longitude and latitude
Ecliptic coordinates use the ecliptic as the fundamental plane:
- Ecliptic longitude (λ) — 0°–360° along the ecliptic from the tropical vernal equinox
- Ecliptic latitude (β) — perpendicular distance from the ecliptic; most planets stay near β ≈ 0°
Western tropical charts map λ into signs via tropical-zodiac. See ecliptic-longitude.
Geographic names are not sky longitudes
Terrestrial latitude and longitude (WGS84-style degrees on Earth) are unrelated to ecliptic longitude despite similar words. Only birth place + instant tie the local horizon to the ecliptic for chart-angles and houses.
References
- Equatorial coordinate system — Wikipedia
- Ecliptic coordinate system — Wikipedia
- Celestial coordinate system — overview
See also natal-chart, geocentric-vs-topocentric, julian-day-ephemeris.