Composite Chart vs Synastry: What Each One Tells You

Synastry lays two birth charts over each other and reads how one person's planets touch the other's, so it shows how each of you experiences the other. A composite chart merges the two charts into a single new chart, built from the midpoint of each pair of planets, so it describes the relationship itself as a third thing with its own character. Synastry answers "what do we do to each other?". The composite answers "what are we, together?". Most astrologers read synastry first and use the composite to see the shared purpose and tone of the bond.

This guide explains both methods, the midpoint calculation and its "nearer midpoint" rule, the Davison chart (the composite's main alternative), composite houses and what to read first. It ends with a worked example of two fictional people, with every position computed from the same astronomical engine our tools use.

What is the difference between a composite chart and synastry?

The two methods are not rivals. Synastry is personal and directional: your Venus on their Mars is a different experience for each of you. A composite has no direction. Neither person "owns" the composite Venus. It belongs to the relationship.

If you want the synastry side in depth, our step-by-step guide to reading a synastry chart covers the aspects, the house overlays and how our tool scores them. This article stays on the composite and Davison side and on how the two views fit together.

What is a composite chart?

A composite chart takes the same point in both birth charts, finds the halfway point between them on the zodiac and plots that halfway point as a new planet. Do this for the Sun, the Moon, every planet and the angles. The result is a full chart that belongs to neither person. Astrologers read it like a birth chart for the relationship: the composite Sun is what the bond is for, the composite Moon is its emotional climate and the composite Ascendant is how the couple comes across to other people.

The midpoint composite was popularised in the 1970s, most influentially by Robert Hand's book Planets in Composite (1975). It is now the default "relationship chart" in most astrology software.

How do you calculate a composite chart?

You need both birth charts with exact degrees. Then, for each planet and angle:

1. Convert each position to a 0 to 360 degree longitude. Each sign covers 30 degrees, starting from Aries.
2. Find the shorter arc between the two positions. Two points on a circle always have a short way round and a long way round.
3. Take the halfway point of the shorter arc. That is the composite position. This is the nearer midpoint rule.

So 20 degrees 51 minutes of Capricorn is 270 + 20.85 = 290.85 degrees, and 19 degrees 10 minutes of Gemini is 60 + 19.17 = 79.17 degrees.

What is the nearer midpoint rule?

Every pair of points has two midpoints, exactly opposite each other. If you simply add the two longitudes and divide by two, you sometimes land on the far one. Take the two Moons above. The plain average is (290.85 + 79.17) / 2 = 185.01, which is 5 degrees 1 minute of Libra. But going forward from 290.85 through 0 to 79.17 is only 148.3 degrees, so the short arc runs through Aquarius, Pisces and Aries, not through Libra. Half of 148.3 is 74.16, and 290.85 + 74.16 = 365.01, which wraps round to 5.01: 5 degrees 1 minute of Aries.

The rule: whenever the two positions are more than 180 degrees apart by simple subtraction, the plain average gives the wrong answer. Add 180 degrees to fix it. Good software does this for you. A hand calculation that skips it puts the composite Moon in the opposite sign and, usually, the opposite house.

Two practical notes. Astrologers argue over what to do when the nearer midpoint rule puts composite Mercury or Venus implausibly far from the composite Sun, since in a real sky they never stray far from it. Most simply accept the result and read it as a feature of a calculated chart. The other note is the angles: the composite Ascendant and composite Midheaven are averaged separately, so they do not always sit the usual distance apart, which is one reason composite houses need care.

What is a Davison chart?

A Davison chart, introduced by the British astrologer Ronald Davison in his book Synastry (1977), skips the averaging of planets. It averages the two birth moments and the two birthplaces instead, then casts an ordinary chart for that midpoint moment at that midpoint place. Because it is a real sky, every planet sits where it actually was at some instant, Mercury and Venus stay close to the Sun and the houses work normally.

The trade-off shows in the planets that move unevenly. The Sun moves at a nearly steady pace, so the Davison Sun always lands close to one of the two midpoints of the Suns, the same place the composite Sun lands. Venus and Mars speed up, slow down and turn retrograde, so their Davison positions can be far from their composite positions. The worked example below shows exactly that.

Two smaller quirks: the midpoint place is often in the middle of an ocean when the two people were born far apart, and some astrologers average the locations along the shortest path on the globe rather than averaging latitude and longitude separately. Both are conventions, not errors.

How do composite houses work?

There are three common ways to give a composite chart houses:

• Average the house cusps. Each composite cusp is the midpoint of the two matching cusps. This is the default in a lot of software.
• Derive the houses from the composite Midheaven. Take the composite Midheaven, then calculate an Ascendant and the other cusps for the latitude where the couple lives. This keeps the angles a realistic distance apart.
• Whole-sign houses from the composite Ascendant. The sign holding the composite Ascendant is the 1st house, the next sign the 2nd and so on. This is the system our engine uses for birth charts, so we use it in the example.

The house meanings are the same as in a birth chart (the 12 houses explains each one), but they describe where the relationship puts its energy. A composite Sun in the 10th is a couple that is visible or works together. A composite Moon in the 4th is a couple whose emotional life centres on home. The composite 7th house describes how the pair relates to other people as a unit, and the 7th house guide covers its meaning in a single chart.

What should you read first in a composite chart?

Read it in this order and stop when the picture is clear:

1. The composite Sun, by sign and house. What the relationship is for and where it shines.
2. The composite Moon. The emotional weather: what makes the pair feel safe and what unsettles it.
3. Aspects between the composite Sun and Moon. Whether the purpose and the feelings pull together.
4. Composite Venus and Mars. How affection is shown and how the couple acts, argues and desires.
5. The composite Ascendant and planets on the angles. How the couple comes across. A planet within a few degrees of the composite Ascendant or Midheaven tends to dominate.
6. Saturn's aspects to the Sun, Moon and Venus. Where the relationship asks for effort, structure and patience.

One trap to avoid: if the two people were born within a few years of each other, their Uranus, Neptune and Pluto sit in nearly the same place, so the composite outer planets are almost identical to each person's own. They describe the generation, not the couple. Read them only when they sit on an angle or tightly aspect the composite Sun, Moon or Venus.

A worked example: Nina and Theo (fictional)

Nina and Theo are fictional people. Their birth details are invented for this example and every position below comes from our chart engine.

• Nina: born July 8, 1990, at 7:30 am in Toronto (Eastern Daylight Time). Sun in Cancer, Moon in Capricorn, Venus in Gemini, Mars in Aries, rising sign Leo.
• Theo: born November 24, 1988, at 10:10 pm in London (Greenwich Mean Time). Sun in Sagittarius, Moon in Gemini, Venus in Scorpio, Mars in Aries, rising sign Leo.

Step 1: the composite midpoints

The Moon is the case where the nearer midpoint rule matters. A plain average would have put the composite Moon at 5°01' Libra, in the composite 3rd house. The correct composite Moon is in Aries, in the 9th. The composite houses here are whole-sign houses counted from the composite Ascendant in Leo.

Step 2: read it in order

Composite Sun in Virgo in the 2nd house, conjunct composite Mercury (1.6 degrees). This is a relationship built on practical care and talking things through: lists, plans, shared money and the small daily acts that say "I've got this". The 2nd house puts resources and shared values at the centre. The composite Sun also squares composite Jupiter (1.4 degrees), a classic signature of a pair that encourages each other to say yes to too much.

Composite Moon in Aries in the 9th house. The emotional climate is quick, restless and hungry for new horizons: travel, study, big conversations about what you both believe. The Moon squares composite Uranus within 1.6 degrees, so the relationship feels safest when it has room to change and suffers when it goes stale. The composite Sun and Moon make no major aspect, which suggests the practical purpose (Virgo) and the restless feelings (Aries) need conscious linking.

Composite Venus in Leo in the 1st house. Affection is visible. Friends would describe this couple as warm, demonstrative and a little theatrical. Venus sextiles composite Jupiter (2.0 degrees), adding generosity.

Composite Mars in Aries in the 9th. The pair acts fast when a shared cause or adventure is in play. Mars squares composite Saturn (3.9 degrees), so they will also hit moments where one wants to charge ahead and the structure of their life says wait.

The angles. Composite Pluto in Scorpio squares the composite Ascendant almost exactly (less than 0.1 degrees). Pluto is generational, but an exact contact to an angle is one of the cases worth reading: this relationship changes both people and does not stay light for long.

What to ignore. Composite Saturn, Uranus and Neptune all sit in Capricorn in the 6th house, clustered within nine degrees. That is because Nina and Theo were born about a year and a half apart and both have Saturn, Uranus and Neptune in or right next to Capricorn. It is their generation, not their relationship.

Step 3: the Davison chart for comparison

Nina and Theo's midpoint moment is September 16, 1989 at 04:50 UTC. The simple average of Toronto and London is 47.58 degrees north, 39.75 degrees west, a point in the North Atlantic. Cast for that moment and place:

The Sun, Moon and angles agree within two degrees. Venus and Mars do not: the Davison chart puts Venus in Scorpio in the 4th house and Mars in Virgo in the 2nd, so it describes a more private, intense way of loving and a more practical way of acting than the composite does. Neither is wrong. They are two different models of the same relationship, and astrologers who use both tend to trust what they agree on.

Step 4: the synastry side

For the person-to-person view, we ran Nina and Theo through our compatibility tool, which reads synastry. Their score is 76 out of 100, "Strong spark, steady ground": Heart 74, Spark 78, Mind 87, Rhythm 76. The strongest single contact is Nina's Mars in Aries opposite Theo's Venus in Scorpio, 4.5 degrees from exact, read as "heat and friction in equal measure". Nina's Venus also sits 3.2 degrees from Theo's Moon in Gemini, a tender contact that makes each feel liked.

Their Sun signs alone would undersell them. Cancer and Sagittarius averages 77.8 in our 60,000-pair study, joint 67th of 78 pairings. Put the views together and the story is consistent: real chemistry with some friction person to person (synastry), and a relationship that wants to be useful, warm in public and always going somewhere (composite).

What are the marriage indicators in a composite chart?

People search for this more than almost anything else about composites, so here is the honest answer: there is no single marriage indicator. No published research shows that any composite placement predicts whether a couple marries or stays together. What astrologers look for is a pattern that suggests durability and a shared home life:

Each of these is a tendency, not a promise. A composite full of commitment signs can belong to a couple who never marry, and plenty of long marriages have composites that show none of them. Treat the composite as a description of the relationship's style, then let the two people's choices do the predicting.

Should you use synastry or a composite chart?

Use synastry when the question is personal: why do I feel so calm around them, why do we fight about the same thing, is the attraction mutual? Use the composite when the question is about the relationship: what is it for, what does it need, how do other people see us? Use the Davison chart if you want a relationship chart that behaves like a real birth chart for transits and progressions.

If you only have time for one, start with synastry. It describes the experience each person actually has, and it needs no averaging conventions.

What composite and synastry cannot tell you

Both methods describe chart geometry. Neither can tell you whether two people are kind to each other, want the same life or will put in the work. A beautiful composite does not make a healthy relationship, and a difficult one does not doom it. Read both as a starting point for a conversation, never as a verdict.

Try it with your own charts

Our free compatibility tool reads full-chart synastry: enter two birth dates, times and places and you get a score, the Heart, Spark, Mind and Rhythm breakdown and the single strongest contact between you. Composite charts are not in the tool yet. To build one by hand, get each person's exact degrees from a free birth chart reading, then use the nearer midpoint method above. If you want to know which placements to compare first, start with what your birth chart says about your love life. For why some sign pairings flow more easily than others, read the elements and modalities explained.