House systems comparison
BirthInput.house_system selects how the twelve house cusps are constructed from birth time and place. core.chart defaults to Placidus when houses are computed; the core enum also includes Whole Sign, Equal, and many others used in SharpAstrology-aligned backends.
Planetary signs do not change with house system — only house placement and sometimes which planets are angular near cusps.
Placidus (default)
- Idea — Time-based quadrant system: divide diurnal/nocturnal semi-arcs so ecliptic degrees rise in equal time steps between horizon and meridian.
- House sizes — Unequal; can be very skewed at high latitudes; some births produce intercepted signs.
- MC — 10th house cusp equals the Midheaven (ecliptic–meridian intersection above the horizon).
- Western modern — Dominant in 20th–21st century psychological/timing-oriented practice.
Whole Sign
- Idea — Hellenistic style: the entire sign containing the Ascendant is the 1st house; each following sign is the next house (30° each).
- House sizes — Always equal (one sign = one house); no intercepted signs.
- Ascendant — Rising degree marks the start of the sign, but the cusp is the sign boundary (0° of that sign in many conventions).
- MC — May fall in the 9th, 10th, or 11th sign/house, not necessarily on the 10th cusp — changes “angular” emphasis vs Placidus.
Equal House
- Idea — First cusp at the Ascendant degree; each subsequent cusp is exactly +30° ecliptic longitude.
- House sizes — Equal 30° bands; works at all latitudes.
- MC — Not forced onto the 10th cusp; Midheaven is a separate angle that may fall inside the 9th–11th equal slice.
Quick comparison
| System | Cusp spacing | MC = 10th cusp? | High-latitude behavior |
|---|---|---|---|
| Placidus | Unequal (time) | Yes | Can fail or distort; very unequal houses |
| Whole Sign | 30° per sign | No | Stable |
| Equal | 30° from ASC | No | Stable |
Choosing a system
There is no single “correct” division — systems answer slightly different interpretive questions. Many astrologers compare Placidus and Whole Sign for the same chart when a planet sits near a sign or house boundary.
In AstroRust, pass house_system on BirthInput (e.g. "WholeSign", "Equal", "Placidus") when calling core.chart calculate or calculate_sidereal.
References
- House (astrology) — Wikipedia overview
- Equal house system — Wikipedia
- Whole sign houses — Wikipedia
See also houses, chart-points, birth-time-accuracy, planet-positions.