What Sun-Sign Compatibility Can and Cannot Tell You
Two people trade birth months and ask whether a Taurus and a Scorpio get along. That is a real, computable question, and it has a real, computable answer — just a narrower one than the question sounds like it is asking. This page states plainly what a sun-sign comparison actually computes, what it can honestly tell two people, and what it structurally cannot see once exact birth times and places are available for both.
Nothing below argues that comparing sun signs is wrong. It is one specific, well-defined computation, and it is the coarsest one this system is able to run on a birth chart. The rest of this page is about what a full two-chart comparison adds on top of it, described in the same units — bodies, degrees, orbs, houses — so the difference is something to check, not something to take on faith.
What a sun sign is
A birth chart places ten bodies — Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto — at exact ecliptic degrees, somewhere around a 360° circle. A sun sign takes exactly one of those ten, the Sun, and reports which of twelve equal 30° arcs its exact degree falls into: 0°–30° is Aries, 30°–60° is Taurus, and so on around the circle. This system computes that bucket with the same arithmetic it uses for every other body's sign label — the degree divided by 30, rounded down to the nearest whole number. There is no separate calculation reserved for the Sun.
That needs only a birth date on all but about twelve days a year — the days the Sun is actually crossing one of the twelve 30° boundaries. Away from a crossing, the Sun's roughly 1°-a-day motion means the hour or the city of birth cannot move it across a boundary on its own, so the date alone fixes which arc it falls into. On a crossing day, that same slow motion cuts the other way: the crossing itself can land in the middle of the day, and the hour of birth — together with the birth place, which shifts what that hour means in UTC — decides which side of the boundary the Sun is actually on, the cusp-day question familiar to anyone who has had to ask whether they were born a Libra or a Scorpio. This is where an exact birth time and place earn their keep even for a single body: this system computes the Sun's degree from the full birth instant, the same computation it runs for the other nine bodies, rather than from the date alone, so a cusp-day chart is decided by the actual birth instant rather than defaulting to a guess — with one caveat: the timezone this system resolves for that instant is the standard, non-daylight-saving offset for the birth zone (see below), so a cusp birth during a past daylight-saving period can still come out an hour off.
What a sun-sign comparison can tell you
Because the twelve signs are twelve equal 30° arcs spaced around the same 360° circle, any two of them sit some whole number of arcs apart — 0 through 6, folding the count so that 7 arcs one way around is the same distance as 5 the other way. That reduces to seven possible angular relationships between any two Suns: 0°, 30°, 60°, 90°, 120°, 150°, or 180°. That is a real, if very coarse, geometric fact, and a sun-sign comparison can state it correctly from two birth dates and nothing else.
This system's own synastry engine computes five of those seven angles as scored aspects between exact-degree positions: conjunction (0°), opposition (180°), square (90°), trine (120°), and sextile (60°) — the same five, in the same order, as the table further down this page. A sixth, the 150° relationship some astrologers call a quincunx, is named elsewhere in this same codebase, but only inside a classification set attached to aspects the engine already found some other way — not as an angle this engine's synastry comparison scores on its own. The seventh, 30° apart, does not appear anywhere in this engine's synastry vocabulary at all. So a sun-sign comparison and this system's two-chart comparison are, in that narrow sense, asking a version of the same question about the same body — which of a small set of fixed angles two points sit at — just at very different resolutions.
That makes a sun-sign comparison genuinely well suited to exactly one situation: when a birth date is all either person has. It needs nothing else to run, and what it reports — a fixed angular relationship between two signs — is a real computed fact about the Suns involved, not a guess standing in for one.
What a sun-sign comparison cannot see
Everything past a birth date is invisible to a sun-sign comparison, not because it was left out by mistake but because the computation never takes it as an input. Four kinds of information a full two-chart comparison uses do not exist for it at all:
Nine other bodies, and a much looser match
Moon, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto never enter a sun-sign comparison — only the Sun does. A full comparison checks all ten bodies in one chart against all ten in the other, a hundred cross-chart pairings, against five possible angles each, at orbs of 3° to 6°. A 30°-wide sign bucket, by contrast, tolerates being off by up to about 15° from its centre and still counting as a match — five to fifteen times wider than the orbs this system allows for an actual computed aspect.
Direction — whose Venus, whose Mars
Every aspect this system computes between two charts keeps a direction: your Venus to your partner's Mars is stored and reported separately from your Mars to your partner's Venus, because the two use different pairs of exact degrees, and one can register as an aspect while the other does not. A sun-sign comparison has one Sun on each side and nothing left to point in a direction — "your Sun square their Sun" and "their Sun square your Sun" are the same fact stated twice, because a sign has no owner-specific degree left once it has been rounded away.
House overlays
A house overlay — one person's point falling inside a specific one of the other's twelve houses — needs an exact birth time and place for whichever person's houses are being checked against, converted into twelve house cusps; each direction is computed independently, so a reading built from one exact birth time and one approximate one can still carry real overlay placements in one direction and none in the other. A sun-sign comparison uses neither a birth time nor a birth place as an input, so there is no version of a house overlay it could produce in either direction, even in principle. It is not a gap in an otherwise similar computation; it is a question the computation was never given the material to ask.
Two people it cannot tell apart
Two people born in early November five years apart share a Sun sign and, by the geometry above, would be reported identically against a third person's Sun. A full comparison for those same two birth dates would very likely find Jupiter — which moves through a sign roughly once a year — in a different sign for the two years, and plausibly Saturn (about once every two and a half years) as well; over a wider gap, Uranus, Neptune, or Pluto, which each take anywhere from about seven to several decades to cross one sign, could differ too. Two people born on the same day, hours apart, keep the same Sun sign but not the same houses: the twelve houses complete a full rotation roughly once a day, so an Ascendant computed a few hours apart can land in an entirely different sign, carrying every house overlay with it. Two people born on the same day and hour in different cities differ again, for a third reason — a different birth place moves the house cusps on its own, which a sun-sign comparison has no way to register, because it never takes a birth place as an input either.
What a two-chart comparison computes instead
A full comparison starts from two independent birth charts, each carrying the same ten bodies, angles, and houses. Nothing in it recomputes a position — it only compares exact degrees that already exist, point by point, in both directions.
The five aspects this system scores, and their orbs
An aspect is counted when the angular distance between a point in one chart and a point in the other falls within a set margin — an orb — of one of five fixed angles:
| Aspect | Angle | Orb |
|---|---|---|
| Conjunction | 0° | 6° |
| Opposition | 180° | 6° |
| Square | 90° | 5° |
| Trine | 120° | 5° |
| Sextile | 60° | 3° |
When a pair of points is close enough to satisfy more than one of those five angles at once, only the tightest match — the one with the smallest orb — is kept, and the others are discarded rather than reported alongside it.
Synastry compares ten bodies this way, in both directions, between the two charts: Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.
Houses, when a birth time allows them
House cusps come from the Placidus method, computed from an exact birth time and place — the timezone behind that birth time is itself resolved from the birth coordinates, not typed in as a guess, though the offset it resolves to is the standard, non-daylight-saving offset for that zone, not a historically accurate one, so a birth during a past daylight-saving period can still be off by an hour. When both people's birth times are exact, the reading places each person's ten points into the other's twelve houses, in both directions. Without an exact birth time for someone, house overlays for that person's chart are left out rather than estimated from a guess — the same rule this system applies everywhere birth-time precision matters.
When a sun-sign comparison is enough, and when it is not
Enough, when a birth date is genuinely all there is
If neither person's birth time or place is available — meeting someone new, a dating profile, a passing conversation — a sun-sign comparison is the one comparison that still runs on what is actually known, and what it reports is a real, if coarse, computed fact rather than a guess dressed up as one.
Not enough, once exact birth times and places exist
Once both people's exact birth times and places are known, a sun-sign comparison stops being a shortcut and starts being a comparison that leaves out information already sitting there: nine bodies, exact degrees and narrow orbs, direction, and house overlays, all computed from data that has already been collected. At that point the question is not whether the fuller comparison is right and the coarser one wrong — it is that one of them is now answerable with everything available, and the other still is not.
What comparing charts does not decide
Nothing here ranks two people
Neither comparison on this page produces a score, a percentage, or a verdict, and neither is meant to. The narrow one sorts two birth dates into buckets; the wide one measures distances between twenty computed positions. Both stop at description. What either result is said to mean for two particular people is interpretive convention rather than measurement, and it is not predictive — no comparison here forecasts how a relationship unfolds or what its future holds.
What this page leaves to a reading
Method is the subject here: what each comparison can reach, and what it cannot. The seven sign-distance relationships above are named, not interpreted, and no individual two-chart aspect is written out. That authorship belongs to an astrologer.
See the fuller comparison applied to two real charts
The mechanics above — ten bodies, exact orbs, house overlays in both directions — are what runs behind every Love Astrology reading.