Zodiac Compatibility Calculator

Sun sign compatibility scores the angular separation between two signs on the 360 degree zodiac. Because each sign occupies exactly 30 degrees, the count of signs between two placements fixes the aspect: 0 signs apart is a conjunction, 2 a sextile at 60 degrees, 3 a square at 90, 4 a trine at 120, and 6 an opposition at 180. Those are the classical aspects, chosen because they are simple divisions of the circle by one, six, four, three, and two. Signs 1 or 5 apart, the semi-sextile at 30 degrees and the quincunx at 150, were not aspects in traditional practice at all; such signs share neither element nor modality and were said to be in aversion, unable to see one another. That is why this calculator scores them lowest, in the 35 to 55 band, while trines land near 95. Element and modality form the second layer. Trine signs necessarily share an element, and square and opposition pairs necessarily share a modality, so the bonuses applied here for fire with air or earth with water partly restate the aspect rather than adding independent information. The larger limitation is the input: a Sun sign compresses an entire chart into one of twelve boxes, ignoring the Moon, Venus, Mars, and the chart angles that traditional synastry compares aspect by aspect. The percentages themselves are a modern convention with no basis in the astrological tradition, which described aspects qualitatively rather than scoring them. Controlled research, including analyses of large sets of marriage and divorce records, has found no association between sun sign pairings and relationship outcomes.

The aspect doctrine is Hellenistic rather than Babylonian. Babylonian astrology worked with omens and used the zodiac as a coordinate system, but the geometry of sympathetic and antipathetic angles appears with the Greek astrology assembled in Egypt in the last two centuries BCE. Ptolemy set out the reasoning in the Tetrabiblos in the second century CE, deriving the aspects from ratios of the circle in a manner that borrows openly from Pythagorean harmonic theory, and treating trine and sextile as harmonious because they link signs of the same gender while square and opposition generate tension. Later Greek writers including Porphyry formalized the doctrine of aversion for the 30 and 150 degree separations. Comparing two charts, the practice called synastry, is ancient in principle, but the point-by-point compatibility reading grew with the twentieth-century consumer astrology market. Sun sign columns, which began as newspaper features in the 1930s, reduced the whole system to birth month for a mass audience, and compatibility charts pairing the twelve signs followed as magazine and paperback fare. Percentage scores of the kind produced here are newer still, an artifact of web and app astrology from the 1990s onward, where a single number is easier to display than a paragraph of nuance. Working astrologers generally treat sun sign matching as a starting shorthand and rely on full chart comparison, while the empirical literature since the 1970s has supported neither approach.