Composite Chart #

A composite chart is a single chart standing for the relationship itself, rather than a comparison of two charts the way synastry is. CompositeSubjectFactory builds one two ways from the same pair of subjects.

Midpoint composite #

from pathlib import Path

from kerykeion import (
    AstrologicalSubjectFactory,
    ChartDataFactory,
    ChartDrawer,
    CompositeSubjectFactory,
)

angelina = AstrologicalSubjectFactory.from_birth_data(
    "Angelina Jolie", 1975, 6, 4, 9, 9,
    city="Los Angeles", nation="US",
    lng=-118.15, lat=34.03, tz_str="America/Los_Angeles",
    online=False,
)
brad = AstrologicalSubjectFactory.from_birth_data(
    "Brad Pitt", 1963, 12, 18, 6, 31,
    city="Shawnee", nation="US",
    lng=-96.56, lat=35.20, tz_str="America/Chicago",
    online=False,
)

factory = CompositeSubjectFactory(angelina, brad)
composite = factory.get_midpoint_composite_subject_model()

print(composite.name)
print(composite.composite_chart_type)
print(f"Sun: {composite.sun.sign} {composite.sun.abs_pos:.2f}°")
print(f"Moon: {composite.moon.sign} {composite.moon.abs_pos:.2f}°")
print(f"Ascendant: {composite.first_house.sign} {composite.first_house.abs_pos:.2f}°")

Output:

Angelina Jolie and Brad Pitt Composite Chart
Midpoint
Sun: Pis 349.64°
Moon: Pis 332.96°
Ascendant: Lib 185.62°

The result is a CompositeSubjectModel, which ChartDataFactory draws like any other subject:

chart_data = ChartDataFactory.create_composite_chart_data(composite)

output_dir = Path("charts_output")
output_dir.mkdir(exist_ok=True)
ChartDrawer(chart_data).save_svg(
    output_path=output_dir,
    filename="jolie-pitt-composite",
)

Davison composite #

The Davison chart is a real chart: its positions actually occurred, at a moment and a place halfway between the two births.

davison = factory.get_davison_composite_subject_model()

print(davison.composite_chart_type)
print(davison.iso_formatted_utc_datetime)
print(f"{davison.lat:.3f}, {davison.lng:.3f}")
print(f"Sun: {davison.sun.sign} {davison.sun.abs_pos:.2f}°")

Output:

Davison
1969-09-10T14:20:00+00:00
34.615, -107.355
Sun: Vir 167.69°

The two methods give different charts on purpose: the midpoint composite is an average of two skies, the Davison is one sky that existed.

house_anchor #

Between two points on a circle there are two midpoints, half a turn apart. Taking the nearer one for each of the twelve cusps independently breaks down when the two charts’ angles are nearly opposed — the choice flips partway round the ring, and the twelve arcs come to 1080° instead of 360°. About one pair in sixteen is affected.

house_anchor decides which angle is held at its near midpoint while the others move, which is how the field repairs it:

Anything else raises KerykeionException.

anchored = CompositeSubjectFactory(
    angelina, brad, house_anchor="midheaven"
).get_midpoint_composite_subject_model()

print(anchored.house_anchor)  # what was asked for
print(anchored.house_frame)   # what was actually built

house_frame reports the outcome rather than the request: "anchored" (a frame was hung from the angle and the twelve cover the circle exactly once), "midpoints" (no frame spans the two charts, so every cusp is its own near midpoint and the twelve are still a house division), or "gapped" (as "midpoints", but the twelve leave gaps). A Davison chart averages no cusps, so both fields are None on it.

What the two subjects must agree on #

The midpoint method averages cusps, so both parents’ cusps have to come from the same house division — and a subject’s requested and effective systems can differ, since a quadrant system is substituted inside the polar circle. The factory compares effective_houses_system_identifier and refuses with a message naming both. Zodiac type, sidereal mode, custom ayanamsa values, house system name and perspective must match too.

A Davison composite is free of all of it: it recasts a whole new chart rather than averaging cusps.

See Composite Subject Factory for the full methodology, and Relationship Score for the other way of measuring a pair.

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