Synastry: How Astrologers Actually Compare Two Charts

Sun-sign compatibility columns compare one variable per person. Synastry is what Western astrologers actually do: take two complete birth charts, lay one over the other, and read the connections between them, whose Venus touches whose Mars, whose Saturn sits on whose Moon, whose planets land in whose houses. The claim of the method is simple: the relationship is not in either chart alone; it is in the geometry between them.

This page explains how a synastry chart works, which contacts the tradition weighs most heavily, how synastry differs from a composite chart, and what an honest reading needs from you before it can say anything worth hearing.

What a synastry chart actually is

A synastry chart is two birth charts overlaid on one wheel. Nothing new is calculated; the astrologer reads inter-chart aspects, the angular relationships between one person's planets and the other's. A conjunction merges two planets' agendas, trines and sextiles let them cooperate easily, squares and oppositions create friction and charge. Both readings matter: a synastry with no hard aspects at all is traditionally read as pleasant but low-traction, while hard contacts are where the heat and the growth are.

The second layer is house overlays: where one person's planets fall in the other person's houses (the houses guide explains what each house governs). Your planet in my 7th house activates my partnership sector; the same planet in my 12th works on me in a much less visible way.

The contacts the tradition weighs most

Every aspect between two charts says something, but the conventional short list looks like this.

Sun-Moon contacts. The classic marker. One person's Sun aspecting the other's Moon is read as a natural sense of recognition and fit, identity meeting emotional need. Older astrologers treated it as a primary marriage indicator, and it remains the first thing many readers check.

Moon-Moon contacts. The livability test. Two Moons in harmonious aspect are read as emotional instincts that understand each other: the same things feel like comfort. Hard Moon contacts are read as home-life friction, two different definitions of safety under one roof.

Venus-Mars contacts. The chemistry axis. Venus is attraction and affection, Mars is desire and pursuit, and contacts between one person's Venus and the other's Mars are the tradition's signature marker of romantic and physical spark, in hard aspect as much as soft.

Saturn contacts: the glue or the weight. Saturn aspecting a partner's Sun, Moon, or Venus is the tradition's commitment signature, and it is honestly double-edged. Read well, Saturn is the glue: seriousness, loyalty, the intention to stay, and it turns up prominently in long marriages. Read badly, it is the weight: criticism, restriction, one person feeling judged by the other. Most long-term synastries carry significant Saturn; the question a reader asks is which face of it the couple is living. If Saturn's role in a chart interests you, the Saturn return guide covers the same planet's most famous solo act.

7th-house overlays. Planets falling in a partner's 7th house, the partnership sector, pull the relationship toward "we": the house person tends to see the planet person as partner material, whatever else the charts say. Sun, Moon, or Venus in the 7th are among the tradition's strongest markers of commitment potential.

Synastry vs composite: overlay vs merger

The two-person toolkit has a second instrument, and the difference is clean. Synastry overlays two charts and reads the contacts between them: how you affect each other, where the spark and the friction live. A composite chart mathematically merges the two charts into one new chart, built from the midpoints of each pair of planets, and reads that single chart as the relationship itself, a third entity with its own character, strengths, and blind spots. Synastry answers "what happens between us"; the composite answers "what are we, together." Practitioners commonly read synastry first and the composite second, and the two can disagree, an easy synastry inside a heavy composite is a known pattern.

What a good synastry reading needs

Two complete birth charts, honestly gated: date, place, and birth time for both people. Without a time, the Moon's position can be off by up to about 13 degrees, the ascendant and houses are unknown, and the entire overlay layer, including those 7th-house placements, is unavailable. A synastry run on two dates alone is planet-to-planet only, a real but reduced reading, and any service that returns house overlays without asking for birth times is inventing them. Birth chart basics covers what goes into a full chart and why the time matters so much.

One honest limit, stated plainly: synastry describes dynamics between two charts; it does not issue verdicts on relationships, and no chart has ever ended a good one.

The same question in the Chinese system

Western astrology is not the only tradition that compares two births. The Chinese system's folk layer reads pairs by year-branch relations, the trine harmonies and clash pairs of the zodiac animals, mapped in full on the Chinese zodiac compatibility page. Its deep layer, like synastry, wants both complete charts: two full BaZi charts compared pillar to pillar, part of the cross-system picture our love astrology page lays out side by side with the Western and Vedic methods.

Quick answers

What is a synastry chart?

A synastry chart is two people's complete birth charts overlaid on one wheel. Astrologers read the aspects between one person's planets and the other's, plus where each person's planets fall in the other's houses, as a map of the relationship's dynamics.

What are the best aspects to have in synastry?

By convention: Sun-Moon contacts for a core sense of fit, Moon-Moon harmony for day-to-day emotional ease, Venus-Mars contacts for attraction, and supportive Saturn contacts for staying power. Practitioners also note that some friction aspects are desirable; charts with no hard contacts at all are traditionally read as low-charge.

Do you need exact birth times for synastry?

For the full method, yes, both of them. Planet-to-planet aspects survive a missing time, but Moon positions lose precision and the entire house-overlay layer, including 7th-house placements, requires both ascendants, which only birth times can provide.

What is the difference between synastry and a composite chart?

Synastry overlays two charts and reads the contacts between them: how two people affect each other. A composite chart merges the two charts into a single new one, calculated from planetary midpoints, and reads it as the relationship's own chart. More two-chart questions are answered in the FAQ.

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Sources and standard: synastry conventions (inter-chart aspects, the primacy of Sun-Moon, Moon-Moon, Venus-Mars, and Saturn contacts, and 7th-house overlays as commitment markers) cross-checked 11 Jul 2026 against Cafe Astrology's synastry and inter-chart aspect references, Celesian's and Astronidan's synastry guides, and Sasstrology's and The Inner Wheel's house-overlay material; the synastry-vs-composite distinction cross-checked against Astro.com's composite chart reference and The Astrology Place. Everything on this page is presented as the tradition's method, not as prediction or a verdict on any real relationship.