Synastry: how astrologers compare two birth charts

Synastry is the technique of comparing two birth charts to see how the planets of one person relate to the planets and houses of another. The word comes from Greek: syn, together, and astron, star. It is used most often for couples, but the same method works for parent and child, business partners or close friends, because it describes contact between two charts rather than romance as such.

Synastry is not sign compatibility

Sign compatibility, the familiar "Aries with Taurus" kind, compares two Sun signs and nothing else. It is a quick way to talk about temperament, and it says the same thing about every Aries and every Taurus. Synastry works with the full charts: the exact degrees of ten planets, the angles and the houses of both people. Two couples with the same pair of Sun signs can have almost nothing in common in synastry.

The difference is also one of scale. A Sun sign covers a twelfth of the zodiac and about a month of births. A synastry contact is defined to within a few degrees and depends on the birth date, often on the birth time as well. That is why synastry needs a chart calculation, and why it cannot be reduced to a table of sign pairs.

The layers of a chart comparison

A full comparison has four layers. They answer different questions, and an astrologer usually moves from the first to the last.

Inter-aspects

Inter-aspects are aspects between a planet in one chart and a planet in the other: A's Venus to B's Mars, B's Moon to A's Saturn. The angles are the same as in a single chart, from the conjunction to the opposition, and they are read with the same logic: which two functions meet and how easily they work together.

What changes is that each planet now belongs to a different person. In a natal square both sides of the tension live inside one character. In a synastry square the friction runs between two people, and each of them tends to feel one side of it. A contact that appears in both directions, say A's Venus to B's Mars and B's Venus to A's Mars, is noted separately, since the theme is then mutual.

House overlays

House overlays place one person's planets into the other person's houses. If B's Sun at 20°00′ Scorpio falls between A's 7th cusp at 9°40′ Scorpio and 8th cusp at 8°15′ Sagittarius, B's Sun lands in A's 7th house. Inter-aspects describe how two people act on each other; overlays describe in which area of life each one feels the other's presence. The overlay is read both ways, so one comparison gives two sets of placements.

The composite chart

The composite chart is a single chart built from midpoints. For each pair of planets the midpoint between A's position and B's position is taken: A's Sun at 10°00′ Aries and B's Sun at 20°00′ Gemini give a composite Sun at 15°00′ Taurus. No one was born with this chart. It is a construction that astrologers read as the chart of the relationship itself, apart from the two people in it.

Every pair of positions has two midpoints 180° apart, here 15° Taurus and 15° Scorpio. The usual convention takes the one on the shorter arc, and this choice is a known weak spot of the method, discussed on the composite page.

The Davison chart

The Davison chart, named after the British astrologer Ronald Davison, also aims to describe the relationship as one chart, but it is built differently. It is a real chart cast for the moment halfway in time between the two births and for the point halfway between the two birthplaces. Because it is a real sky, its planets keep their actual relationships, and it can be progressed and transited like any birth chart.

The order of reading

Each layer only makes sense against the previous one, so the order matters.

Both natal charts first. A contact between two charts cannot be read without knowing what each planet means in its own chart. Mars that is already strained by a natal square will respond to a partner's Saturn differently from a Mars that is well placed.
Inter-aspects, tightest first. The aspects with the smallest orbs and those involving the Sun, Moon, Ascendant and personal planets set the main themes.
House overlays. They show where those themes play out: home, work, money, daily routine.
Composite or Davison. These describe the relationship as a whole and are read last, once the two-way contacts are clear. Many astrologers use one of the two rather than both.

Patterns that repeat across layers carry the most weight. If a theme shows up as an inter-aspect, again as an overlay and once more in the composite, it is central to the pair. A single wide aspect that nothing else confirms is a detail.

Orbs between two charts

Two full charts produce dozens of potential contacts, so most astrologers use narrower orbs in synastry than in a natal chart. With natal-sized orbs nearly every planet in one chart aspects something in the other, and the comparison stops telling you anything.

Take A's Venus at 12°10′ Leo and B's Mars at 14°40′ Leo: a conjunction with an orb of 2°30′, which almost any school would count. Now B's Moon at 3°20′ Aquarius against the same Venus: the angle is 171°10′, an opposition with an orb of 8°50′. Some astrologers would accept that in a natal chart with the Moon involved. In synastry most would leave it out. Priority goes to contacts of the Sun, Moon, Ascendant and personal planets; aspects between slow planets of two people born a few years apart are shared by a whole generation and say little about the pair.

What you need for the calculation

Synastry needs the date, time and place of birth for both people. Each item has its own job.

Date. Gives the planetary positions. For the slow planets the date alone is enough.
Time. Fixes the Ascendant, the Midheaven and the houses, and refines the Moon, which moves about 12-15° a day.
Place. Needed for the houses and, in the Davison chart, for the midpoint location.

When one partner's birth time is unknown, the comparison loses that person's houses and angles. You can still place their planets in the houses of the partner whose time is known, but not the other way round. Their Moon is uncertain by several degrees in either direction, so tight Moon contacts become guesswork. A composite without both times has no reliable Ascendant or houses, and a Davison chart without both times has an uncertain midpoint moment and therefore an uncertain Moon and no usable houses. Inter-aspects between the planets other than the Moon survive best.

What synastry does not tell you

Synastry describes how two people tend to affect each other: which contacts come easily, where friction is built in, which themes keep returning. It does not decide whether a couple will last, and it cannot be turned into a compatibility score. Plenty of long relationships show hard squares between the charts, and plenty of easy contacts never turn into anything.

The method also says nothing about circumstances: age, culture, timing in each person's life. Those are read separately, through transits and progressions in both charts.

What's in this section

Inter-aspects: aspects between two charts, which ones to look at first, orbs and mutual contacts.
House overlays: how one person's planets fall into the other's houses and how to read them in both directions.
Composite chart: the midpoint method, how composite houses are calculated, and the 180° midpoint problem.
Davison chart: a real chart for the midpoint in time and space, and how it differs from the composite.

Compare two charts