Synastry: Astrological Compatibility Between Charts

Synastry is the branch of astrology concerned with comparing two natal charts to assess the dynamics between the people they represent. Practiced across Western, Vedic, and Hellenistic traditions, it operates by overlaying one person's planetary positions onto another's chart and reading the geometric relationships that form. Those relationships — called aspects in astrology — determine whether two charts reinforce, challenge, or complicate each other. Understanding synastry means understanding that compatibility is rarely a verdict; it's a map of where connection flows easily and where friction tends to accumulate.

Definition and scope

Synastry examines the space between two charts rather than the content of either one in isolation. When an astrologer places Person A's natal Sun at 14° Scorpio next to Person B's natal Moon at 16° Scorpio, those two points are in a near-exact conjunction — a 2° orb — and that alignment becomes a data point in the relationship's astrological signature. The technique applies to any two people: romantic partners, business collaborators, parent and child, close friends.

The scope extends naturally from each person's natal chart basics. Every planet, luminary, and major point — the Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto, and typically the Ascendant — is carried into the comparison. Practitioners operating in traditional frameworks sometimes include Chiron in astrology and the lunar nodes (see North Node and South Node), which are thought to describe karmic or evolutionary dimensions of a pairing. The result is not a single number or a thumbs-up but a layered picture, sometimes running to dozens of individual aspect contacts.

How it works

The mechanical process is straightforward: both charts are calculated for the exact birth date, time, and location of each person — a requirement that makes accurate birth data consequential. Charts calculated to the wrong rising sign, for instance, shift house cusps and can misplace the Ascendant entirely. (The rising sign is particularly important in synastry because it represents the face each person presents to the world and to a partner.)

Once both charts are in hand, the comparison proceeds through three layers:

Planet-to-planet aspects — The geometric angles between one person's planets and the other's. A trine (120°) between Person A's Venus and Person B's Jupiter is generally read as ease and generosity in affection; a square (90°) between Person A's Mars and Person B's Saturn often signals recurring friction around ambition or initiative.
Planet-to-house overlays — Where one person's planets fall in the other person's chart houses. If Person A's Sun lands in Person B's 7th house (the house of partnership), that placement tends to orient Person A toward Person B as a primary relational figure.
Angle contacts — Aspects involving one person's Ascendant, Descendant, Midheaven, or IC to the other person's planets. These are considered among the most personally felt contacts in synastry, since the angles represent identity, public life, and private foundation.

The astrological houses framework matters significantly here. A Venus landing in someone's 2nd house (resources, values) reads differently than the same Venus in their 12th (hidden life, solitude). The house destination gives the planet's energy a context — a room to operate in.

Common scenarios

Synastry shows up most recognizably in romantic compatibility analysis, but its applications are broader than that cultural shorthand suggests.

Romantic synastry focuses heavily on Venus-Mars contacts (a dedicated treatment lives at Venus-Mars compatibility), the Sun-Moon dynamic, and Ascendant interactions. A Sun-Moon conjunction between two people — where one person's Sun falls on or near the other's Moon — appears with notable frequency in long-term partnerships across historical astrological literature, including observations documented by astrologers like John Addey and Lois Rodden in their chart research.

Family synastry tends to surface strong Saturn and nodal contacts. Saturn's presence between parent and child charts often describes a relationship defined by structure, responsibility, and sometimes felt constraint — not a judgment about the quality of the relationship, just its gravitational character.

Professional synastry pays attention to Mercury contacts (communication style), Saturn contacts (who holds authority), and 10th house overlays (shared ambition and public reputation).

One useful contrast: synastry differs meaningfully from the composite chart method. Synastry shows how two existing people interact; the composite chart creates a third chart representing the relationship as its own entity. Both tools are valid and often used together.

Decision boundaries

Synastry does not produce a binary compatibility score. Astrologers apply judgment across the full picture, and that picture almost always contains contradictions — a harmonious Venus-Jupiter trine sitting alongside a tense Mars-Pluto square is entirely common. The question is not whether difficult aspects exist but how they're distributed and what context surrounds them.

A few structural thresholds guide interpretation:

Orb tolerance — Most practitioners apply orbs of 6° to 8° for major planet-to-planet aspects, tightening to 3° to 4° for minor aspects. Contacts outside the orb are typically set aside.
Mutual reception vs. one-way contacts — If Person A's Mars aspects Person B's Venus and Person B's Mars aspects Person A's Venus, the dynamic operates in both directions. One-way contacts indicate that one person feels the connection more acutely.
Outer planet contacts — Aspects from Uranus, Neptune, or Pluto to a personal planet (Sun, Moon, Mercury, Venus, Mars) carry generational weight. An entire peer group may share a Neptune placement, meaning that one person's Neptune conjuncting another's Venus describes a cultural-era dynamic as much as an individual one.

The interpretive framework matters too. Western and Vedic astrology approach synastry through different house systems, different planet weightings, and different aspect sets. Vedic synastry, for instance, places significant emphasis on the Moon sign comparison through a system called kuta matching, which assigns numerical scores to 8 categories of compatibility — a more structured framework than most Western approaches employ.