Synastry calculator: what happens when two charts meet

A synastry calculation overlays two birth charts in one wheel and extracts the aspects each person's planets make to the other's. How far your Venus sits from their Mars, whether their Saturn touches your Moon, whose planet falls into whose house — the physics of the relationship lives exactly here.

For accurate results you need both birth times. Without a time the planetary aspects still calculate, but house overlays and the Moon's degree can drift, so read house interpretations cautiously in a timeless synastry.

What to look at first

• Moon contacts: emotional safety. Moon-Moon harmony feels like speaking the same language; Moon-Saturn feels like holding yourself back.
• Venus-Mars: the attraction axis. When it is mutual, the chemistry rarely burns out.
• Sun-Moon: the classic longevity signature. One person naturally feeds the other's emotional need.
• Saturn contacts: weight and durability. Saturn binds, but it also brings duty, testing and timing.
• House overlays: their Sun in your 7th puts you in partnership mode; in your 12th the relationship turns private, blurry and rescuing.

How to read the aspect types

Conjunction merges, opposition mirrors, square triggers and amplifies, trine flows, sextile offers an opening. A square or opposition is not 'bad' — those aspects are usually what gives a relationship movement. A synastry built only on trines is comfortable, but often short on the tension that creates desire.

Orb matters. Keep 8 degrees for Sun and Moon, 5-6 for personal planets and around 3 for outer planets, and the reading sharpens. A contact under 1 degree becomes the central theme of the relationship — both people feel it the moment they meet.

The common mistake: reading one aspect alone

Plenty of people write off a relationship because of a single Venus-Saturn square. Synastry is a sum: with three nourishing Moon contacts in the same chart, one Saturn square does not end anything — it only tells you where you have to grow up. Map the overall texture first, then descend into individual aspects.

Synastry vs composite vs Davison

• Synastry: two separate charts compared. It answers 'what do you do to me?'
• Composite: a third chart built from the midpoints of both. It answers 'who are we?'
• Davison: a real moment in time between the two birth dates and places. It behaves like the relationship's own birth chart and can receive transits.

Reading all three together is the healthiest approach: synastry shows chemistry, composite shows the relationship's character, Davison shows timing.