Composite chart calculator: the chart of your relationship

A composite chart is one chart made from two. For each planet, it takes the point halfway between where that planet stood at one person's birth and where it stood at the other's, and draws a single chart from those midpoints. Astrologers read it as the relationship itself: a third character with its own Sun, Moon and rising sign, separate from either of you.

The technique became popular in the 1970s through the astrologer Robert Hand, and today it sits beside synastry as one of the two main ways to look at a couple, a friendship or a working partnership. It is a tradition of reflection, so read it as a portrait of the bond, never as a verdict on it.

Each composite planet stands at the midpoint of the shorter arc between the two birth positions. If one Sun is at 10° Aries and the other at 20° Gemini, the arc between them is 70 degrees and the composite Sun falls halfway, at 15° Taurus.

The shorter arc matters when two positions lie on either side of 0° Aries. A Venus at 20° Pisces and another at 10° Aries are 20 degrees apart across the start of the zodiac, so the composite Venus is at 0° Aries; the long way around would put it at 0° Libra. The calculator always takes the shorter arc, to the minute.

This calculator uses the midpoint method for the angles too. The composite Midheaven is the midpoint of the two Midheavens, and the composite Ascendant is the midpoint of the two Ascendants. When the two Ascendants lie far apart, the shorter-arc midpoint can land on the western side of the composite Midheaven, where an Ascendant never stands; in that case the opposite point of the axis is used, so the Ascendant always rises in the eastern half of the chart.

Houses are equal houses of 30 degrees each, counted from the composite Ascendant. Some astrologers derive the houses from the composite Midheaven and a chosen place instead; the planets' signs are the same in both methods, and only the houses can differ.

The angles and houses need both birth times. If either time is unknown, the calculator leaves them out and marks the composite Moon as approximate.

Synastry lays one birth chart over the other and looks at the aspects between them: how your Venus meets their Mars, how your Moon sits with their Sun. It describes what each of you stirs in the other.

The composite chart blends the two into a single chart and describes the relationship as a unit: what it is for, how it feels from the inside and where it is headed. Many astrologers read both. The synastry calculator compares the two charts aspect by aspect, and this page gives the composite.

Start with the composite Sun, the purpose of the bond, then the composite Moon, its emotional climate, then the composite Ascendant, the face the two of you show together. The house each one falls in shows the area of life where the relationship does most of its living.

Next look at Venus and Mars, affection and drive, then at the tightest aspects. A trine or sextile between two composite planets is read as an easy gift, a square or opposition as a tension the pair works with. Read each line as a prompt for conversation between the two of you.

The Ascendant crosses all twelve signs in a day, so each person's birth time moves the composite Ascendant and every house with it. The Moon moves about half a degree an hour, so a few hours' error can shift the composite Moon by a degree or two and, near a sign edge, change its sign.

The Sun and the other planets hardly move in a day, so their composite positions stay reliable even without the times. A birth certificate is the best source for the hour.

Both birth charts come from the same engine as the birth chart calculator: VSOP87 for the planets, the Meeus series for the Moon and Pluto, and sidereal time for the angles, checked against the Swiss Ephemeris. The midpoints and the aspects between the composite planets are then computed to the minute, with orbs of six degrees for conjunctions and oppositions, five for trines and squares and three for sextiles.

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