Synastry
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What the tradition reads
Synastry is the comparison of two people's birth charts directly against one another, reading the aspects each person's planets make to the other's, which houses of one chart the other person's planets fall into, and how the two charts' angles and rulers relate, all in order to describe the shape of the relationship between them rather than either person alone. Western astrology reads synastry chiefly through aspect: a warm trine between one partner's Venus and the other's Mars is read very differently from a tense square in the same pair, and a cluster of close, easy aspects between the two charts is read as an unusually comfortable fit. Vedic astrology instead formalises much of its synastry into the Ashtakoota system, an eight part points based comparison of the two partners' Moon nakshatras and rashis rather than a free reading of aspects, giving a numerical score out of thirty six that supplements, though does not replace, a fuller look at both charts together. Wikipedia's own account of astrological compatibility groups Western, Vedic and Chinese astrology together as three separate traditions under which this kind of comparison is approached, though the specific method Chinese astrology uses to compare two people's charts is not detailed by the sources consulted for this entry.
The story
Synastry comes from the Ancient Greek sunastria, literally a conjunction or a coming together of the stars, built from the prefix syn, together or with, and astron, a star, so the word names the technique's basic operation as plainly as any term in the astrological vocabulary does: two separate skies brought together and read as one. The technique's most widely cited modern touchstone is the psychologist Carl Jung's study, described in his book Synchronicity, in which he compared the horoscopes of married couples against randomised pairings and found a correlation between the married couples and what astrological prediction would expect. Jung could not find a causal relationship to explain the correlation, even after changing his method and repeating the experiment with the same result, and it was this inability to explain a finding he kept reproducing that led him to propose synchronicity, an acausal connecting principle, rather than any ordinary cause; the study remains the reference point most often cited whenever synastry's modern reputation, rather than its ancient technique, is discussed.
Written from
Corpus key glossary:synastry.