Birth time accuracy
House cusps, the Ascendant, and Midheaven depend on the rotation of Earth at the birth instant. A small clock error can move angles by a full sign at extreme latitudes or fast-rising signs.
What still works without a precise time
With date only (midnight placeholder or noon convention):
- Sun sign is usually stable for that calendar day.
- Moon may change sign within the day (≈12–15° per day).
- Outer planets move slowly; signs are stable for months or years.
core.chart can still return ecliptic longitudes and tropical or sidereal signs if you supply an instant — but treat houses and angles as unreliable without a documented birth time.
Sensitivity rules of thumb
| Factor | Typical sensitivity |
|---|---|
| Ascendant | 1° of ecliptic per **4 minutes** of clock error (varies by latitude and sign) |
| Moon | ~0.5° per hour; sign change possible within hours |
| MC / house cusps (Placidus) | Coupled to ASC; similar minute-level sensitivity |
| Sun, outer planets | Often robust to hours of error for sign only |
Recording and rectification
- Prefer official records (birth certificate, hospital log) over family memory when possible.
- Note whether the recorded time is standard or daylight-saving civil time and convert via time-zones before passing UTC to the API.
- Chart rectification (adjusting time to match life events) is an interpretive practice outside the provider; the engine assumes the instant you supply is exact.
Provider implication
BirthInput.instant must be UTC (RFC 3339). Garbage time → garbage angles, even when VSOP planetary longitudes are astronomically fine. See data-quality, birth-input.
References
- Ascendant — Wikipedia (rising sign and time dependence)
See also chart-angles, houses, time-zones.