Composite chart: building a relationship chart from midpoints

A composite chart is a single chart built out of two birth charts. Each of its planets is the midpoint between the matching planets of the two partners: the composite Sun sits halfway between A's Sun and B's Sun, the composite Moon halfway between the two Moons, and so on. Where synastry compares two charts with each other, the composite describes the couple as a single whole.

How a midpoint is calculated

Two points on a circle split it into two arcs, a short one and a long one. The composite uses the middle of the short arc. A plain average of the two longitudes does not always give the right answer, so it is safer to count from one of the points.

A made-up example. Partner A's Sun is at 8°14′ Aries, which is 8°14′ of absolute longitude. Partner B's Sun is at 21°40′ Capricorn, or 291°40′. Going forward in zodiac order from B's Sun to A's Sun (through 0° Aries) covers 76°34′, and that is the short arc. Half of it is 38°17′. Adding that to 291°40′ gives 329°57′, which is 29°57′ Aquarius. That is the composite Sun.

The plain average, (8°14′ + 291°40′) / 2, would give 149°57′, or 29°57′ Leo. That point lies exactly opposite, in the middle of the long arc. This is why every composite calculation has to check which of the two arcs is shorter.

The two-midpoint problem

Every pair of points has two midpoints, and they are always 180° apart. While the planets are far from an opposition, the choice is obvious. When the angle between them approaches 180°, the two arcs are almost equal, and a small change in the input flips the midpoint to the other side of the wheel.

Take A's Moon at 12°20′ Taurus (42°20′) and B's Moon at 11°50′ Scorpio (221°50′). They are 179°30′ apart, the short arc runs from Taurus through Leo, and the composite Moon lands at 12°05′ Leo. Now move B's Moon to 13°10′ Scorpio, a difference of about two hours of birth time for the Moon. The angle becomes 180°50′, the other arc is now the shorter one, and the composite Moon jumps to 12°45′ Aquarius.

A shift of 1°20′ in the input produced a shift of almost 180° in the output. If either partner's birth time is uncertain, the composite Moon and especially the house cusps should be treated as unreliable. Some astrologers handle such cases by reading both midpoints as an axis rather than as a single planet.

Composite houses

There is no single rule for the houses, and software handles them in different ways.

• Midpoint of the MCs. The MC of the composite is the midpoint of the two MCs, and the Ascendant and other cusps are derived from it with a chosen house system (Placidus most often) at the latitude where the couple lives. This is Robert Hand's approach and a common default.
• Midpoints of the cusps. Each composite cusp is the midpoint of the partners' matching cusps. The method is simple, but the houses can come out very uneven, and the Ascendant loses the geometric relationship to the MC that every real chart has.

Both methods depend on accurate birth times for both people. If even one time is unknown, it is better to skip composite houses altogether and work with planets and aspects.

Why a composite has no birth moment

A composite does not correspond to any moment in the actual sky. It is a geometric construction rather than a chart of a real time. One consequence surprises newcomers: composites can contain positions that are impossible in a birth chart.

In the real sky Mercury never gets further than about 28° from the Sun, and Venus about 47°. The composite has no such limits. Take A's Sun at 8°14′ Aries and B's Sun at 2°40′ Libra: the composite Sun falls at 5°27′ Cancer. A's Venus is at 25°00′ Pisces and B's Venus at 20°00′ Libra, each a normal distance from its own Sun. Yet the composite Venus lands at 7°30′ Capricorn, opposite the composite Sun. This tends to happen when the partners' Suns are close to opposing each other.

The missing birth moment has another consequence: a composite cannot be progressed like an ordinary chart. Astrologers do read transits to composite points, but for progressions they progress both natal charts and recalculate the midpoints.

Reading the composite

A composite is read like a birth chart whose subject is the relationship rather than a person. The composite Sun shows what the couple forms around and what they do together, the Moon the emotional tone of shared daily life, Venus the style of affection, Saturn the duties and boundaries the couple takes on.

Most astrologers start with the lights. The sign and house of the composite Sun show what holds the couple together, and the aspects to it show what supports that and what gets in the way. The Moon comes next: its sign and aspects describe how much at ease the two people feel with each other in everyday life. Third come the composite Ascendant and any planets near the angles, if the houses are reliable; the Ascendant describes how other people see the couple.

After that come the aspects between composite planets, with the same orbs as in a natal chart or slightly tighter, and any concentrations of planets in signs and houses, which show where the couple's energy goes. Aspects to the lights and angles carry more weight than aspects between the slow planets, which are nearly identical in the composites of people of the same age.

A composite does not replace inter-aspects or house overlays: it does not show how each partner experiences the other. It is usually examined after the synastry, to see what shared structure the two people form.

Transits to the composite

Although a composite has no date, its points can serve as targets for transits. When a slow planet in today's sky crosses the composite Sun, Moon or angles, or forms an opposition or square to them, astrologers read it as a period that affects the couple as a whole rather than either partner alone. The orb is kept tight, 1-2°, just as for transits to a birth chart.

Transits of Saturn, Uranus, Neptune and Pluto to the composite lights and angles get the most attention. A transit to an angle only makes sense when both partners' birth times are known precisely. A transit to the composite Moon is worth checking against the two-midpoint problem: if the Moon sits near the tipping point, the real target may be the opposite point.

It helps to compare transits to the composite with transits to the natal charts. When the same slow planet touches the composite and an important point in one partner's chart at the same time, the theme of that period for the couple stands out more clearly.

When you need a relationship chart with real planetary positions that can be progressed, the tool is the Davison chart.

Calculate a composite chart