House Systems

House systems divide the celestial sphere into twelve segments, called houses, starting from the Ascendant (the eastern horizon at the moment of birth). Each system uses a different mathematical projection to do this division, which is why the same chart can look markedly different depending on which system you choose. Ketu ships six systems and lets you register additional ones.

Supported Systems

The six built-in systems are exposed in the SYSTEMS dict:

Key	Name	Description
`placidus`	Placidus	Time-based; most common in modern western practice
`koch`	Koch	Birthplace system; similar to Placidus at mid-latitudes
`porphyry`	Porphyry	Divides quadrants into thirds; works at high latitudes
`whole_sign`	Whole Sign	Each house = one full sign; oldest recorded system
`equal`	Equal	Houses of 30° each, starting from the Ascendant
`regiomontanus`	Regiomontanus	Space-based; common in horary astrology

from ketu.houses import SYSTEMS

print(list(SYSTEMS.keys()))
# ['placidus', 'koch', 'porphyry', 'whole_sign', 'equal', 'regiomontanus']

calculate_houses

calculate_houses(jd, lat, lon, system="placidus", polar_fallback="raise")

Computes the twelve house cusps and the angles (Ascendant, Midheaven, ARMC, Vertex) for a given moment and location. Returns a NumPy structured array of HOUSES_DTYPE.

Parameters

jd — Julian Day (float, UTC)
lat — geographic latitude in degrees (+N / -S)
lon — geographic longitude in degrees (+E / -W)
system — one of the six keys above (default "placidus")
polar_fallback — behaviour above the Arctic/Antarctic circles (see below)

Example — Paris chart at J2000

from ketu.houses import calculate_houses, house_of, SYSTEMS, HOUSES_DTYPE, HighLatitudeError

jd = 2451545.0       # J2000: 2000-01-01 12:00 UTC
lat = 48.8566        # Paris latitude
lon = 2.3522         # Paris longitude

h = calculate_houses(jd, lat, lon, system="placidus")

print(h["asc"])      # Ascendant longitude (degrees)
print(h["mc"])       # Midheaven longitude (degrees)
print(h["cusps"])    # Array of 12 cusp longitudes

Switching systems

for system in SYSTEMS:
    h = calculate_houses(jd, lat, lon, system=system)
    print(f"{system:16s}  ASC={h['asc']:.2f}  MC={h['mc']:.2f}")

house_of

house_of(planet_lon, cusps)

Places one or more planet longitudes into a house number (1–12), vectorised over an array of longitudes.

from ketu.houses import calculate_houses, house_of

jd = 2451545.0
h = calculate_houses(jd, 48.8566, 2.3522)
cusps = h["cusps"]

# Single planet
sun_lon = 280.5
print(house_of(sun_lon, cusps))   # e.g. 10

# Multiple planets at once
import numpy as np
planet_lons = np.array([280.5, 45.3, 123.7, 198.2])
houses = house_of(planet_lons, cusps)
print(houses)   # array of house numbers 1–12

HOUSES_DTYPE

The structured array returned by calculate_houses has the following fields:

Field	Type	Description
`jd`	float64	Julian Day of the calculation
`lat`	float64	Geographic latitude
`lon`	float64	Geographic longitude
`system`	U20	House system name
`cusps`	float64[12]	Twelve house cusp longitudes (degrees)
`asc`	float64	Ascendant longitude (degrees)
`mc`	float64	Midheaven longitude (degrees)
`armc`	float64	ARMC (sidereal time angle)
`vertex`	float64	Vertex longitude (degrees)

High-Latitude Handling

Projection-based systems (Placidus, Koch) break down at extreme latitudes (roughly above 66° N or below 66° S) because the ecliptic can fail to intersect the prime vertical.

polar_fallback="raise" (default) — raises HighLatitudeError when the calculation is geometrically impossible.

Alternative fallback — pass the name of another system to use as a fallback:

from ketu.houses import calculate_houses, HighLatitudeError

# Attempt Placidus; fall back to Porphyry at high latitudes
try:
    h = calculate_houses(jd, 70.0, 25.0, system="placidus", polar_fallback="porphyry")
except HighLatitudeError:
    # polar_fallback="raise" and the location is beyond the polar circle
    pass

Whole Sign and Equal never raise HighLatitudeError — they work everywhere.

Registering Custom Systems

from ketu.houses import register

@register("my_system")
def my_system_calculator(jd, lat, lon):
    # Returns (cusps_array[12], asc, mc, armc, vertex)
    ...

After registration, "my_system" is available as a system= argument to calculate_houses.

Next Steps

Relational charts — synastry and composite charts using house cusps
API reference — full ketu.houses signature list