What Is Synastry?

Synastry is the branch of astrology that compares two birth charts and reads the angles between their planets; the contacts it weights most for love are Sun to Moon and Venus to Mars, then Moon to Moon and Venus to Venus.

On this site, the birth chart compatibility calculator does exactly this for any two birth dates. It is free, needs no account, and uses the order set out below.

Which synastry aspects matter most in love?

The tradition weights the contacts between the two lights, the Sun and the Moon, and between the two love planets, Venus and Mars, above everything else. This site's calculator puts every contact between two charts into one of five classes, heaviest first:

1. Sun to Moon, and Venus to Mars. Sun to Moon is the tradition's classic marriage contact, one person's sense of self meeting the other's feelings. Venus to Mars is its attraction contact, read in full in Venus and Mars sign compatibility.
2. Moon to Moon, and Venus to Venus. Moon to Moon is emotional habit meeting emotional habit, the core of moon sign compatibility. Venus to Venus is what each person values in love.
3. Sun to Venus. Identity meeting affection.
4. Any contact with Saturn. Read as stabilising rather than warm.
5. Everything else. Mercury, Jupiter, the rising sign, Moon to Venus, Mars to Mars and the rest count as supporting contacts.

The order is this site's scoring of how astrologers read these contacts. The planet positions are measured: they come from the astronomy-engine library and agreed with NASA's JPL Horizons ephemeris to within 0.003 degrees when checked. The weights are a convention, published on the methodology page, and the astrologer Jahben R. reviewed and approved them, unchanged, on 2026-09-23. Searched as a soulmate synastry calculator, it still gives the same thing: a score built from these weighted contacts, not a soulmate verdict.

How do you read a synastry aspect?

An aspect in synastry is the angle between a planet in one chart and a planet in the other, measured in degrees rather than counted in signs apart. The calculator keeps five: the conjunction, the sextile, the square, the trine and the opposition, each within a set tolerance, and a sextile counts as warm like a trine. The calculator page lists the most common synastry aspects with their tolerances.

On a result, each row names the two planets, the aspect between them, how far it sits from exact and the tolerance it was allowed. A contact that lands close to exact counts for more than one near the edge of its tolerance. A heavy contact counts for more than a supporting one, so two results with the same aspects can still differ. Every row is a description of two charts, which is how the four aspects below should be read.

What does a conjunction mean in synastry?

A conjunction is two planets at the same point, 0 degrees apart. The tradition reads the two as blended, each colouring the other, and the calculator counts a conjunction as warm.

What does a trine mean in synastry?

A trine is two planets 120 degrees apart. The tradition reads it as easy flow, a contact that needs no effort, and the calculator counts it as warm.

What does a square mean in synastry?

A square is two planets 90 degrees apart, read as friction. The calculator counts a square against the score only when one of the two is a personal planet: the Sun, the Moon, Mercury, Venus or Mars. Between two slower planets it counts as neutral.

What does an opposition mean in synastry?

An opposition is two planets 180 degrees apart, read as attraction with tension, and the calculator scores it by the same rule as the square. Aspects are angles between planets. Houses are divisions of the sky, and they need more than a birth date.

What is a house overlay in synastry?

A house overlay lays one person's planets over the other person's twelve houses. The tradition reads the 5th house as romance, pleasure and creativity and the 7th as partnership, marriage and contracts. So a partner's Venus in your 7th house is read differently from the same Venus in your 5th.

Houses need an exact birth time, date and place for both people, astrologers use several house systems that give different answers, and this site's calculator does not draw houses. Without a birth time, any astrologer can still place the planets, but not the rising sign or the houses. A composite chart is a different method again: it averages two charts into a single third chart, where synastry keeps both charts whole. The first house begins at the rising sign, which is explained in rising sign compatibility (sun, moon and rising).