Synastry Chart

How this synastry chart is calculated

Synastry is the astrological comparison of two birth charts; our guide to synastry explains the aspects, the house overlays and what the comparison can and cannot say. Each chart here is calculated exactly as in our birth chart calculator: planet positions from an astronomical ephemeris tested against the Swiss Ephemeris, the historical time zone of each birthplace, and the tropical zodiac. Then every point in the first chart is compared with every point in the second.

Points compared. The Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto, and the mean North Node, for both people. The Ascendant and Midheaven are added for anyone whose birth time you know.

Aspects. The five major aspects: conjunction, sextile, square, trine and opposition. Their angles are the classical ones; Sepharial's 1910 handbook lists the "sextile of 60 degrees, square of 90 degrees, trine of 120 degrees". Minor aspects such as the quincunx are left out.

Orbs. We use the orb table Astrotheme publishes for its compatibility charts: conjunction 10 degrees, opposition 8, trine 6, square 6 and sextile 4.5, the same for every planet. Synastry orbs are usually kept a little tighter than birth chart orbs, and this table follows that habit for everything except the conjunction. We add one rule of our own: contacts with the North Node, Ascendant or Midheaven are held to 3 degrees, because those points shift with small errors in the birth data.

When a birth time is unknown

Without a time we cannot know where the Moon was to within about 13 degrees, and the Ascendant, Midheaven and houses cannot be calculated at all. So we work out every position at the start and at the end of that birth day. A contact that holds all day long is listed normally. A contact that holds for only part of the day is marked depends on birth time, with its range of orbs. The Ascendant, Midheaven and house overlays of that person are left out, and the result says so.

What astrologers traditionally look at

Astrologers who write about synastry usually describe two layers. The first is the inter-aspects, the angles between one person's planets and the other's. The second is the house overlays, where one person's planets fall among the other person's houses, read in both directions; that layer needs both birth times. Many start with a handful of contacts: Sun and Moon, Venus and Mars, Moon and Moon, and the Ascendant.

The same writers tend to stress that no single aspect tells the whole story, and that people with challenging contacts between their charts can still have rich relationships. This page follows that rule: it shows no score and no compatibility rating.