Synastry compares two natal charts and looks at the angles between one person's planets and the other's. This is not a sun-sign compatibility quiz: the calculation works with the actual planetary longitudes at each partner's moment of birth, so the result is unique to that specific pair. Below is what such a calculation actually gives you, which contacts carry more weight than others, and what synastry should not be expected to do.
What a cross-chart aspect is
An aspect is the angle between two points of the zodiac. In a natal chart aspects are found between one person's planets; in synastry, between the planets of two people. If the first partner's Venus sits at 12° Taurus and the second partner's Mars at 14° Leo, the angle between them is 92°, which is a square with a 2° orb. Every such pair is a separate channel of interaction: Moon to Moon is about emotional rhythm, Mercury to Mercury about how easily you negotiate in words, Venus to Mars about attraction. Direction matters: the first partner's Venus to the second partner's Saturn and the first partner's Saturn to the second partner's Venus are two different contacts with different meanings.
Which contacts weigh more
The weight of a contact comes from three things. First, the bodies themselves: Sun and Moon weigh most, then Venus and Mars, then Mercury, Saturn and Jupiter, and least of all the outer planets and fictitious points. Second, the aspect type: conjunction and opposition are stronger than trine and square, and those are stronger than sextile and the minor aspects. Third, the orb, meaning the deviation from the exact angle: an aspect with a 0.5° orb works noticeably harder than the same aspect with a 6° orb. That is why one exact contact between the luminaries often means more than five wide aspects between Uranus and Neptune.
Harmony and tension: why a square is not a verdict
Trines and sextiles give ease: in those areas partners understand each other without explaining. Squares and oppositions give friction, and friction is usually what movement grows out of: a Moon opposite Saturn forces you to learn to talk about needs, something a couple with nothing but trines may never learn. The conjunction is a separate case: it amplifies both planets, and its tone depends on which planets they are. Venus conjunct Jupiter reads as generosity, Sun conjunct Saturn as the pressure of duty, anything conjunct Pluto as excessive intensity. A pair with no tense aspect at all is rare, and relationships without friction are more often dull than happy.
The compatibility index and what it actually measures
The calculator shows a number from 0 to 100. It is a weighted balance: the summed weight of harmonious contacts against the summed weight of tense ones, mapped to a scale centred on 50. Fifty means a typical pair, not a neutral one: across a sample of four hundred random pairs the median sits exactly there, and eighty percent of pairs fall between 36 and 64. So 40 is not a disaster and 70 is a genuinely rare predominance of harmonious contacts. What matters more is this: the number measures the composition of aspects, not the quality of a relationship. It knows nothing about how two people handle friction, and that is what decides whether a couple lasts.
House overlay and why birth time is needed
Besides aspects, synastry looks at which houses of one chart the other person's planets fall into. A partner's Sun in the fifth house is one story, in the twelfth another, even when the aspects are identical in both cases. But houses depend on the ascendant, and the ascendant on the exact time of birth: an error of a quarter of an hour shifts the cusps by a few degrees, an error of two hours moves a planet into the neighbouring house. If the time is unknown, it is correct to read only the aspects between planets and to draw no conclusions about houses, the ascendant or the MC at all.
Composite and Davison charts
Synastry looks at two people separately and at the links between them. A composite does something else: it builds a third chart from the midpoints of both partners' planets, that is, a chart of the relationship as a phenomenon in its own right. A Davison chart is built for the average time and average place of birth of the two people, so it has a real moment and real houses. These methods do not replace synastry; they answer a different question: not how we interact but what this relationship is in itself. In practice they are taken as a second step, once the aspects have been worked through.
Limits of the method
Astrology is not a science, and synastry is an interpretive model, not a forecast. The planetary weights, the sign rules for conjunctions and the normalisation coefficient are our own convention assembled from several traditions: consensus exists only on the general hierarchy, while the specific numbers differ from school to school. The compatibility index should not be read as a verdict, and certainly not as grounds for a decision about a relationship. The sensible use is different: see where a pair has it easy and where it is predictably hard, and put that into words.
The calculation is free and runs in your browser: birth data does not go to the server. Planetary positions are computed by Swiss Ephemeris to arcsecond accuracy.
