← Encyclopedia of Astrology · lesson 221/235
Synastry
What it is
Synastry is the comparison of two natal charts to see how two people meet. The word comes from the Greek syn, "together", and astron, "star": two skies laid side by side. It is the technique behind the question "are we compatible?", and it serves just as well for a friendship, a family tie or a working partnership as for a romance.
The idea is old. Ptolemy, in the fourth book of the Tetrabiblos, already judges marriage and friendship by comparing the two nativities — whether the Sun and the Moon of one agree with those of the other, and whether the rising degrees are in harmony. Modern synastry keeps the same move and widens it to every planet and point.
How it is worked out
Synastry has three main layers.
- Inter-aspects. The aspects between one person's planets and the other's: "my Venus trine your Mars" rather than "my Venus trine my Mars". These are the same conjunctions, squares, trines and oppositions found inside a single chart, now reaching across from one life to the other, usually with tighter orbs than in a natal reading.
- House overlays. Where one person's planets fall in the other's houses. A partner's Saturn in someone's seventh house is read differently from the same Saturn in their fifth.
- Contacts to the angles and nodes. A planet on the other person's Ascendant, Descendant or Midheaven, or on their North Node, is read as a strong, immediate link — often the sense of recognition people describe on first meeting.
How it is read
A few contacts are traditionally given most weight:
- Sun–Moon — the classic marriage contact in the older literature: one person's sense of self meeting the other's feelings.
- Venus–Mars — affection meeting desire; read as attraction and spark.
- Moon–Moon and Moon–Venus — emotional ease, feeling at home together.
- Saturn contacts — commitment, duty and staying power, and sometimes a weight or a lesson.
- Outer planets (Uranus, Neptune, Pluto) on personal planets — excitement and disruption, idealisation, or intensity.
Easy and hard aspects both matter. Pure ease between two charts can feel pleasant but flat; squares and oppositions are where friction, heat and growth tend to live. Most lasting bonds are read as having both.
Synastry describes a dynamic, not a destiny. It points to where two people tend to harmonise and where they will have to work; it cannot say whether a relationship will last, and what each person does with the pattern is their own. Astrologers also differ on orbs, on whether to include minor aspects and asteroids, and on how much weight house overlays deserve — so two readings of the same pair can stress different things.
Beside the relationship charts
Synastry compares two charts. The composite chart and the Davison chart take the other road and build one chart for the relationship itself. Many astrologers read them together: synastry for the chemistry between two people, the relationship chart for the character of the bond they share.
In a reading here
Relationship readings here compute the aspects between the two charts and read them — in a synastry reading itself and in the friendship, business-partnership and other two-person readings built on it — alongside composite and Davison readings for those who want the relationship's own chart.