What is synastry in astrology?
Synastry is the comparison of two birth charts to describe a relationship. An astrologer lays one chart over the other and looks at the aspects between them: whose Venus touches whose Mars, whose Moon sits on whose Sun. Sun-sign compatibility is the shortcut version; synastry uses every planet and point.
Two charts, one table
The word joins the Greek syn, together, and astron, star. The method uses the same rules as a single chart: aspects show connection, and the condition of the planets shows the quality of it. The difference is that every aspect runs between your planets and the other person's. 1
Astrologers pay most attention to the Sun, Moon, Venus, Mars and the Ascendant of each chart. A trine from one person's Moon to the other's Sun is the classic sign of ease; a square between Mars and Venus the classic sign of spark and friction.
A related technique, the composite chart, averages the two charts into a single one for the relationship itself. It is a 20th-century invention.
Beyond aspects, astrologers look at house overlays: where one person's planets fall in the other person's houses. A partner's Venus landing in your seventh house, the house of partners, is the textbook example. Overlays need both birth times, because houses do.
Synastry is not only for couples. Ptolemy used the same comparison for friends and enemies, and astrologers apply it to parents and children, business partners and teams. 2
As old as the horoscope
The modern name is recent; the practice is not. Ptolemy's chapter on marriage says marriages last when the Sun and Moon of the two charts are in trine or sextile, especially if the husband's Moon aspects the wife's Sun, and that squares and oppositions bring separation. His chapter on friends and enemies compares the two charts' Sun, Moon, rising degree and Lot of Fortune. 1,2
How the machine compares two charts
Calculated facts only, worked out the same way the machine does it.
- It compares the Sun, Moon, Venus, Mars and Ascendant of each person, every point of one chart against every point of the other, in both directions.
- Orbs are tighter than in a single chart: 8° for conjunctions and oppositions, 6° for trines and squares, 4° for sextiles.
- Without a birth time it leaves out the Ascendant, and the Moon if it changed sign that day.
Where the idea comes from
Era by era, with sources. More in the Reading Room.
- Classical2nd century CEPtolemy compares the Suns and Moons of two charts in his chapter on marriage. 2
- 1970s1978Linda Goodman's Love Signs brings sun-sign compatibility, the shortcut to synastry, to a mass audience. 3
- 1980s–90s1980–1996Chart services by computer make two-chart comparisons quick; Astrodienst goes online in 1996. 4
Common misconceptions
“Sun-sign compatibility is synastry.” It is one small slice of it: the angle between two Suns, out of dozens of contacts. 1
“Synastry can say whether a relationship will last.” Astrologers read it as a description of dynamics, not a verdict. People with "difficult" synastry stay together all the time.
Questions people ask
Do both people need birth times?
For the Ascendant and houses, yes. The Sun, Venus and Mars can be compared from dates alone.
What is the difference between synastry and a composite chart?
Synastry compares two charts; a composite blends them into one.
Which contacts do astrologers look for first?
Sun–Moon, Venus–Mars, and anything touching the Ascendant or Descendant. 2
Sources
- Synastry, Skyscript, Glossary of Astrological Terms (Deborah Houlding). Read on .
- Ptolemy, Tetrabiblos, Book IV, chapters 5–7 (Of Marriage; Of Children; Of Friends and Enemies), LacusCurtius, University of Chicago (Loeb Classical Library translation by F. E. Robbins, 1940). Read on .
- Linda Goodman's Love Signs (first published 1978, Harper & Row), Open Library (Internet Archive) catalogue record. Read on .
- About Astrodienst, Astrodienst AG (company history). Read on .