Synastry: What Astrology Actually Compares in Relationship Charts

Western astrology and Vedic astrology check relationship compatibility with two different instruments entirely. Synastry overlays two full natal charts and reads the angles one person's planets make to the other's. Vedic matching, ashtakoota, skips the full chart and scores the two people's Moon nakshatras against each other on a fixed point scale. Neither produces the single percentage that dating apps like to print, and neither system treats its own number as a verdict.

Synastry: Overlaying Two Charts

A synastry reading takes both people's natal charts and looks at the aspects between them: where one person's Mars falls relative to the other's Venus, where the Suns and Moons stand to each other, where Saturn from one chart lands on a personal planet in the other's. The luminaries and the Venus-Mars pair carry most of the relational weight. A Sun conjunct Moon between two charts describes an easy sense of being seen. Mars conjunct or square Venus describes attraction, and a square generates just as much charge as a conjunction, often more, since a tense aspect is not automatically a bad one in synastry. The chart does not output a score. It outputs a list of specific contact points, some easy, some friction-heavy, and a synastry reading is the work of describing what each one is actually doing.

A related but separate technique, the composite chart, merges the two natal charts into a single midpoint chart representing the relationship as its own entity rather than comparing two people. Synastry compares; composite charts merge. Most relationship readings lean on synastry and treat the composite as a secondary check.

Ashtakoota: The Vedic Answer

Vedic astrology approaches the same question through a narrower, more mechanical lens. Ashtakoota, literally the eight-fold match, compares the Moon nakshatra of each person across eight categories, covering temperament group, dominant instinct, and psychological compatibility among others, and awards points toward a total of 36 gunas. Traditional kundli matching for marriage looks for a minimum threshold, commonly cited around 18 of 36, before a match is considered workable. It is a faster, more codified system than synastry: two Moon positions and a lookup table, rather than a full two-chart overlay.

Does a Higher Score Mean a Better Relationship?

No, and this is where both systems get flattened by the apps that reduce them to a single percentage. A high ashtakoota score screens out certain traditional incompatibilities; it does not measure chemistry, communication, or how two people actually get along day to day. A synastry chart full of easy trines can describe a comfortable, low-friction pairing that never quite generates heat, while a chart with hard Mars-Venus squares can describe exactly the opposite. Both traditions were built as diagnostic tools for astrologers to interpret, not as pass-fail scores, and neither Western nor Vedic astrology has ever claimed compatibility reduces to one number.

Anyone curious where their own Moon sits, in either system, can look it up for free with a natal chart. For a read on how a specific relationship is trending over the coming month rather than a static compatibility check, a 30-day plan for your relationship works from real transits instead of a fixed score.