Synastry Calculator

Overview

Synastry comparison is a mechanical procedure once two charts exist: build a grid of angular separations between every pair of planets, then determine house overlays. This page documents the computation and what data it needs.

• The inter-aspect grid is a matrix of angular separations — pure arithmetic on two sets of longitudes.
• Orb choice determines which cells count as aspects, and orbs are not standardised.
• House overlays require accurate birth times for both people.
• A composite chart is midpoint arithmetic; a Davison chart is a real chart for a midpoint moment.

Building the inter-aspect grid

With two charts computed, the comparison is a matrix. For each planet in chart A and each in chart B, take the difference in ecliptic longitude, normalise it to the range 0–180 degrees, and check whether it falls within an allowed orb of a recognised aspect angle.

For the classical seven bodies this is a 7-by-7 grid of 49 comparisons; including the outer planets and chart angles expands it considerably. Every cell is a straightforward subtraction — the computation involves no judgement at all.

Orb choice decides the output

Which cells count as aspects depends entirely on the orbs used, and orbs are not standardised. A practitioner allowing 8 degrees for major aspects will find substantially more inter-chart aspects than one allowing 4.

Traditions also vary orbs by body — commonly wider for the Sun and Moon — and by aspect type. Two tools applying different orb schemes to identical charts produce different aspect lists, so an aspect grid is only interpretable alongside the orb table used to generate it.

House overlays need both birth times

The overlay component places each person's planets into the other's house structure. Because houses derive from the Ascendant and Midheaven, which depend on birth time and place, this cannot be done without accurate times for both people.

Inter-chart aspects between slow-moving planets can be computed approximately from dates alone, since those bodies move little in a day. Aspects involving the Moon cannot, as it moves about 13 degrees daily. Any tool producing a full synastry reading from two dates alone is computing far less than it appears to.

Composite and Davison constructions

A composite chart takes the midpoint of each pair of corresponding planetary longitudes. The arithmetic has a subtlety: on a circle there are two midpoints between any two points, and which one to use is a convention that different software resolves differently — a real source of divergence between tools.

A Davison chart avoids this by taking the midpoint in time and in geographic space between the two births and casting a normal chart for that moment and place. Unlike a composite it corresponds to an actual instant, so its positions are genuine ephemeris values rather than averages.

Continue in the data

Catalogues, hubs, and reference pages that hold the underlying records for this topic.

• SynastryWhat the technique means in the tradition.
• AspectsAspect angles and the orb problem.
• Natal chart methodHow each chart is computed first.

Frequently asked

What does a synastry calculation involve?

Building a grid of angular separations between every planet in one chart and every planet in the other, then checking which separations fall within an allowed orb of a recognised aspect angle. It is straightforward arithmetic on two sets of ecliptic longitudes, followed by determining where each person's planets fall in the other's houses.

Why do synastry tools give different aspect lists?

Because orbs are not standardised. A practitioner allowing 8 degrees for major aspects finds considerably more inter-chart aspects than one allowing 4, and traditions also vary orbs by body and by aspect type. An aspect grid is only meaningful alongside the orb table used to produce it.

Can synastry be done without birth times?

Only partially. Aspects between slow-moving planets can be approximated from dates alone, but anything involving the Moon cannot, since it moves about 13 degrees per day. House overlays and Ascendant relationships are impossible without accurate times for both people, and those are a substantial part of the technique.