What synastry is, and what the score is actually made of

Synastry · every body in chart A against every body in chart B · same-body pairs count · one score, centred on 50, from the contacts underneath it

01Two charts, and every pairing between them

Synastry starts from two ordinary birth charts, computed exactly as any single chart is: date, time and place in, positions and aspects out. What makes it a different calculation is not a different kind of astronomy — it is the same aspect arithmetic, run across two separate sets of longitudes instead of one. Inside a single chart, an aspect is found by comparing every body to every other body once, skipping a body against itself, because a planet cannot be at an angle to its own position. Across two charts that restriction does not apply, and dropping it is not an oversight — it is the entire point. A Sun in one chart sitting close to a Moon in the other is a real, meaningful contact between two people, and so is a Sun in one chart sitting close to the Sun in the other. Synastry runs the full cross product: every body in the first chart checked against every body in the second, same-body pairs included, because those are often the pairings a synastry comparison is asked about first.

For ten bodies in each chart, that full cross product is a hundred pairings before a single aspect has even been checked — considerably more raw comparisons than the roughly forty-five unique pairings a single chart's internal aspect search has to make. That difference in scale is the practical reason a synastry reading tends to surface far more contacts than a natal reading of either person alone: it is not that two people generate more astrology between them by some multiplying effect, it is that the arithmetic is genuinely comparing more pairs.

02The same orb table, applied twice over

Each of those hundred pairings is checked the same way a single chart's aspects are: the angular separation between the two longitudes is measured, compared against each of the nine aspect angles — conjunction at 0°, sextile at 60°, square at 90°, trine at 120°, opposition at 180°, and four minor angles besides — and kept if it falls within that aspect's stated orb. The orb table is the same one a natal chart uses, including the wider tolerance the Sun and Moon receive: any pairing involving either luminary gets its orb multiplied by 1.25 before the comparison is made, so a Sun-involving conjunction is checked against 10° rather than 8°. Only the closest aspect for a given pairing is kept, exactly as within a single chart — a Sun and a Moon that happen to sit near both a sextile and, more loosely, a quincunx are recorded once, under whichever angle they are actually closest to.

What synastry deliberately does not carry is an applying-or-separating direction. That distinction is a fact about motion over time — whether two bodies are still closing toward an exact angle or have already passed it — and it only means something for a single, moving chart measured against a specific date. Two birth charts are two frozen moments each; nothing between them is in motion relative to a shared "now," so there is nothing to apply or separate. Every contact a synastry comparison finds is simply present, at whatever orb it sits at, permanently, in the same way a natal aspect is permanent within one chart.

Each contact that survives the orb check gets the identical strength score a natal aspect gets: tightness — how close the orb sits to an exact hit, scaled linearly from the maximum tolerance down to zero — multiplied by importance, the average of the two bodies' stated weights, where the Sun and Moon sit at 1.0, Mercury, Venus and Mars at 0.8, Jupiter and Saturn at 0.7, the outer three at 0.5, and the smaller points lower still. A Sun square Mars sitting almost exact carries real weight; a Vesta quincunx Pholus sitting near the edge of its narrow orb carries very little. That is the same ranking logic a single chart uses to decide which of its own aspects are worth a sentence, applied here across two people instead of one.

03What the number is actually made of

The contacts a synastry comparison finds get combined into a single score between 0 and 100, and the arithmetic behind that number is worth stating plainly rather than leaving as a black box. Each contact contributes its strength score, multiplied by a weight that depends on its nature: a harmonious contact — a trine or a sextile — contributes its full strength, positively. A challenging contact — a square, an opposition, a quincunx or one of the harder minor angles — contributes at two-thirds the magnitude, negatively, rather than as a full mirror image of a harmonious one. A neutral contact, the conjunction and the two smallest minor angles, contributes nothing to the running total at all, though it is still listed among the contacts found. Treating friction as three-quarters as costly as ease is rewarded is a deliberate choice: a chart full of tension is not the same relationship as two charts with nothing between them at all, and scoring the two identically would erase that difference.

All of those weighted contributions are summed into a single net figure, and the final score centres that figure on 50 rather than on 0: no contacts found at all scores exactly 50, because that is not a bad outcome or a good one, it is simply nothing known — two charts that happen to share very little geometry. The net total is then scaled against a fixed saturation point, six units of net weighted strength, which is enough on its own to push the score to its ceiling of 100 or its floor of 0. That ceiling is deliberately fixed rather than left to climb indefinitely with the number of contacts found: two charts compared with every extended body included would otherwise simply generate more raw contacts than the same two people compared with only the ten core bodies, and a score that kept climbing with sheer contact count would reward a bigger comparison rather than a better one.

A real pair of charts makes this concrete. Two birth dates — one from mid-1990, one from early 1988, both computed for Istanbul — produce 47 contacts between them once every body in one chart is checked against every body in the other. The five strongest include a Moon semisextile Mercury at an orb of 0.39°, a Sun sextile Venus at 1.23°, a Mercury quincunx Mars at 0.33°, a Moon conjunct Sun at 3.15°, and a Jupiter quincunx Mercury at 0.42° — a mix of tight harmonious contacts, a very tight challenging one, and a wider neutral one, which is exactly the kind of mixed picture forty-seven contacts across two full charts actually produces. Summed and scaled the way the arithmetic above describes, that particular pair scores 48 out of 100 — closer to the neutral middle than to either end, from a pair with a genuinely large number of contacts between them. There was no reason to expect a high number and no attempt made to produce one; the arithmetic ran once and returned what it returned.

04What it is not

A synastry score is not a verdict on a relationship, and treating it as one is the overclaim this particular number attracts more than almost any other in the whole product. It is a single lossy summary built by compressing dozens of individual contacts into one figure, and a caller that shows the number without the contacts underneath it is misusing what the arithmetic actually produced. A 48 is not "below average," because the scale was never built against a population of real couples to be average or below it against — it is centred on 50 by construction, as the score two charts with nothing shared between them would get, which makes 50 a statement about the arithmetic's own baseline rather than a judgment about how relationships typically compare.

Nor does a low score predict difficulty and a high one predict ease. The score is an index into a list of specific, checkable contacts — this Venus near that Mars, at this orb, this Saturn near that Moon, at that one — and the honest use of it is to read the list the number came from, not to stop at the headline figure the way a compatibility percentage on a dating profile invites someone to.

The cruder relative of this whole calculation is worth naming directly: a sign-to-sign compatibility chart, the kind that reduces two entire people to their Sun signs and looks up whether fire trines air or water squares earth. That is a real technique with a real basis — elements that sit a trine or a sextile apart from each other do share an easier geometric relationship than elements a square or an opposition apart — but it uses two data points out of the roughly hundred pairings a full synastry comparison actually checks. Two people's Sun signs being an easy elemental match, or a difficult one, says nothing at all about where their Venuses, Marses, Saturns or Moons actually sit relative to each other, which is where the bulk of the 47 contacts in the example above actually came from.

05Where you see it in your own chart

Every contact a synastry comparison finds carries its own aspect type, its own orb to the degree, and its own strength — the same detail a natal aspect table shows, doubled across two people. Computing each half of the comparison starts the same way a single chart does: date, time and place for each person, run through the same engine that produces every other Lizastro chart, which is the actual precondition for a synastry comparison rather than a separate calculation with its own rules.

The worked pair above — 48 from 47 contacts, deliberately unremarkable rather than flattering — is a real, checkable example rather than an illustration invented for this page, computed by the same engine a reader's own two charts would run through.

→ Compute your own chart first · Read the full methodology

06Related reading

The orb article covers the tolerance and strength arithmetic this piece leans on directly — the same table, the same tightness-times-importance formula, applied here across two charts instead of one. Libra, the sign most associated with comparison and partnership, is worth reading alongside this piece for where synastry sits most naturally among the twelve.

Updated 2026-07-08.