Synastry and Compatibility: What a Sun-Sign Match Can and Cannot Tell You

Synastry is the comparison of two whole birth charts: you take every position in one person's chart, every position in the other's, and measure the angle between each pair. Those angles are called cross-aspects, and they are the substance of the technique. A sun-sign compatibility score is a different and much smaller thing — it takes one placement from each side, out of the ten bodies a chart computes, and returns a number identical for every pair of people on Earth who share those two signs. Both are worth understanding. Only one of them is a description of two specific charts, and even that one is a vocabulary rather than a verdict.

Synastry is arithmetic before it is interpretation

A birth chart is, underneath the artwork, a list of numbers: an ecliptic longitude from 0° to 360° for the Sun, the Moon and each planet, plus the degree rising over the eastern horizon if the birth time is known. Synastry lays two of those lists side by side and subtracts. If the difference lands close to 0°, 60°, 90°, 120° or 180°, the pair is in aspect, and how far it sits from exact is the orb. The subtraction is identical to the one used inside a single chart — see aspects explained — and the only thing that changes is that the two numbers came from two different people.

Worked example. One person's Venus at 12° Taurus is at 42° of longitude, because Taurus begins at 30°. The other person's Mars at 9° Capricorn is at 279°, because Capricorn begins at 270°. The difference is 237°, which is more than half a circle, so subtract it from 360° and you get 123°. The nearest classical angle is 120°, the trine, so this is a Venus–Mars trine with a 3° orb. No meaning has been assigned yet. Anyone with both charts can check that number and get the same answer.

Do that for every combination and you get a grid. The Zodiux two-chart calculator compares seven bodies per person — Sun, Moon, Mercury, Venus, Mars, Jupiter and Saturn — which is 49 ordered cells, and it counts a contact when the separation is within 7° of a conjunction, trine or opposition, 6° of a square, or 5° of a sextile.

Before a long list of contacts starts to feel like evidence, price it. Folded into the 0°–180° range a separation can occupy, those five orb windows claim about 50° of it: 7° at the conjunction end, 10° for the sextile, 12° for the square, 14° for the trine, and 7° at the opposition end, the two ends being narrow because a separation cannot run past 0° or 180°. That is 28% of the range, so on a uniform assumption chance alone puts roughly fourteen contacts on the 49-cell board for any two charts at all. The assumption is imperfect — Mercury and Venus are tethered to their own Suns, so some cells are not free to take any value — but the direction holds, and it is the same conclusion the single-chart version of this calculation reaches. A long list is a wide orb, not a strong connection.

The grid is ordered, and that matters more than it sounds. Person A's Venus to person B's Mars is a different cell from person A's Mars to person B's Venus, and Western practice reads the two differently: the planet doing the contacting and the planet being contacted are not interchangeable roles. Zodiux computes both cells and labels each by name, so the direction is visible in the output. It does not yet interpret them differently — the interpretation text is looked up under a sorted pair key, so both directions print the same sentence. A sun-sign score cannot express the distinction at all, having only one placement per side.

Which contacts carry weight, and what limits each

Not every cross-aspect is read equally, and on this site the weighting is not a matter of taste you have to take on trust: the calculator carries a published weight per planet pair, and the table below prints them. A higher weight means that contact moves its dimension score further. The reasons behind the ordering are mostly about how fast each body moves, which is a physical fact rather than a preference.

That last row deserves the numbers. Uranus circles the zodiac in about 84 years, Neptune in about 165 and Pluto in about 248, which is roughly 4.3°, 2.2° and 1.5° of movement per year. Any two people born within three years of each other therefore have Neptunes about 6.5° apart or closer — inside the 7° conjunction orb this site uses — purely because they were born in the same stretch of a decade. So does a whole classroom, and a whole commuter train. A Neptune conjunction between two thirty-year-olds is a fact about the mid-1990s, not about them.

That is why the calculator stops at Saturn. Including the outer planets would add contacts to almost every pair, making every reading look richer while distinguishing nobody. Leaving them out is a smaller, more honest instrument.

House overlays, and the choice they depend on

The second layer of synastry is the overlay: instead of measuring angles, you drop one person's planets into the other person's house wheel and ask which area of life each one lands in. Their Mars in your fourth house reads differently from their Mars in your tenth. It is a genuinely different question from the aspect grid, and most full synastry readings do both.

It also rests on a choice many sites never mention. Placidus, Koch, Equal and Whole Sign produce different cusps from the same birth data, so the same planet can land in the eleventh house under one system and the twelfth under another — which means an overlay reading is partly a settings choice. There is no measurement that settles it. What houses are covers the systems and house systems compared works the disagreement through in degrees.

Zodiux computes no house cusps under any system, so it produces no overlays. Publishing one would mean silently picking a system on your behalf. Until the site takes a public position it reports what it can measure — sun, moon, rising and every planet by sign — and leaves the overlay to you. Birth time is the practical constraint either way: the Ascendant advances about 1° every four minutes on average, so a time rounded to the nearest hour is already uncertain by about 7.5° in each direction, a 15° window, half a sign. That average also hides a lot, because the rate varies with latitude and with which sign is rising. Birth time and time zones sets out where the error actually concentrates, which is historic daylight-saving rules rather than the astronomy.

A composite chart is a different object again

Synastry describes an interaction: your chart, their chart, and the angles between them. A composite chart tries to describe the relationship itself as a single entity with its own chart. The two techniques answer different questions and are not substitutes. Zodiux computes neither; both are named here so you know what you are looking at when another site shows you one.

The midpoint composite

Take each pair of matching points — the two Suns, the two Moons, the two Mercurys — and compute the midpoint of their longitudes. The result is a full set of positions you can draw as a chart. It looks like a birth chart and reads like one.

It is not one. No moment in history had the sky in those positions; the midpoint of two real charts is an average, and averages of angles are not themselves observations. There is also a genuine ambiguity built into the method: any two points on a circle have two midpoints, 180° apart. The usual convention takes the shorter arc, but that is a convention, and taking the other one produces a chart with every position flipped to the opposite sign. A composite is a construction whose rules you have to state.

The Davison relationship chart

The alternative averages the inputs rather than the outputs. Take the midpoint in time between the two birth moments and the midpoint in latitude and longitude between the two birthplaces, then cast an ordinary chart for that time and place. Two people born in 1994 and 1998 get a chart for 1996; two people born in Lisbon and Warsaw get a chart for a point somewhere in central Europe.

The advantage is that a Davison chart is a real moment — the sky genuinely looked like that, and the positions come from an ephemeris rather than from arithmetic on two other charts. The cost is that it is a chart of a place and time neither person has any connection to.

Two compatibility models, on two adjacent URLs

This site runs two different pieces of software under the same word, and telling them apart is most of the point of this page. A pair page such as Aries and Leo is a static file generated at build time from two sign names. The calculator at compatibility is a different model that computes both real charts in your browser. They can and do disagree, and neither is a measurement of a relationship.

The left-hand column is deterministic and history-free. Aries with Leo returned 85 last year and will return 85 for every Aries–Leo pair alive, because two sign names are its only input; there is no first person, no second person and no date beyond two birthdays. The narrowness in the table is the same fact seen from another angle — a model with two inputs has very little it can vary. Every constant behind it, and one pair worked end to end, is published at how the compatibility scores are calculated.

The right-hand column is a better instrument, not a clean one. It uses fourteen real positions instead of two labels and will give two different couples two different answers. But it still folds a sun-sign element term into the score, still prints one interpretation line for both directions of a contact, and still has nothing particular to say about Jupiter. Those seams are stated here rather than buried, because a tool that hides them invites more confidence than it has earned.

What a sun-sign number cannot see

The clearest way to judge a compatibility score is to line it up against the questions people actually bring to it, and mark which ones it has any information about.

Comparing two real charts, in order

If both birth details are available, the sun-sign score stops being the interesting object. Below is the order a synastry reading conventionally works in. Run each chart at the birth chart calculator first; every position it reports is computed in your browser.

A working order for comparing two charts

1. Get both birth times as precisely as you can, and write down how precise that is. A time good to the minute is a different object from a time somebody remembers as the afternoon, and pretending otherwise is where most bad synastry starts.
2. Write both sets of positions in degrees, not signs. Signs are 30° wide and a contact is a matter of a few degrees. Working in signs alone will invent aspects that are not there and hide ones that are.
3. Compare the two Moons, then Sun to Moon in both directions separately. A's Sun to B's Moon is one cell and the reverse is another; do not merge them. These are the three cells this site's own calculator weights most heavily.
4. Then all four Venus and Mars combinations, noting the orb on each. A 1° contact and a 7° contact are not the same finding.
5. Then Mercury to Mercury, remembering it is partly determined by the Sun contact you have already looked at.
6. Then every Saturn contact, and ask before reading it: is this about these two people, or about their being close in age?
7. Sort everything by orb, tightest first. A grid with twenty contacts is not twenty findings. The tight ones carry the reading; the rest is background.
8. Stop at Saturn unless you have a specific reason not to, for the reason set out above.
9. Only then consider house overlays — and name the house system out loud, because Placidus and Koch will move planets between houses on a fifteen-minute correction and Whole Sign will not.
10. Write down what the reading does not cover. Everything in the last row of the table above. A synastry grid describes two charts; it does not describe two lives.

None of this requires software beyond something that reports accurate longitudes. It requires patience and a willingness to write down orbs.

A vocabulary, not a verdict

A sun-sign compatibility score is not useless, and the case for it does not depend on it being secretly accurate. Its use is that it hands two people a shared, neutral, external set of words for a dynamic they may already have noticed. Saying that a pairing scores low on communication and high on chemistry is a prompt to talk about something specific, and it is a prompt nobody has to defend, because it obviously came from a table rather than from either person's opinion of the other. That is the function Zodiux claims for it, and the only one.

The caution scales up rather than disappearing when you move to full synastry. A cross-aspect grid uses far more information and is genuinely a description of two specific charts. It is still not a description of two specific people: consent, effort, timing, circumstance and history do not appear anywhere in an ephemeris, and those are what relationships are made of. How to use astrology tools responsibly covers where that line gets crossed in practice, and what Zodiux does not claim is the complete list, written plainly.