Synastry

Meaning

Synastry is the comparison of two birth charts. Instead of reading one person's chart on its own, the astrologer lays the two side by side, or one around the other on a double wheel, and asks how they meet. The word comes through Late Latin synastria from Greek syn-, "with, together", and astron, "star"; the Oxford English Dictionary records it in English from 1657 [2].

There are three main ways of comparing:

• Inter-aspects. Aspects between a planet in one chart and a planet in the other: one person's Sun square the other's Mercury, and so on.
• House overlays. One person's planets placed in the other person's houses, to show which part of the other's life each planet touches.
• Charts for the pair. A single chart standing for the relationship: the composite chart, made of the midpoints of the two people's planets, and the Davison chart, cast for the moment and place halfway between the two births.

Synastry is not only for couples. Ptolemy applied it to marriage and to friendship and enmity [1], and astrologers use it for parents and children, colleagues and business partners.

Origin and history

Ptolemy. The Tetrabiblos (2nd century) already compares two nativities. In the chapter on marriage Ptolemy says that marriages mostly last when the Sun and Moon of the two charts are in harmonious aspect to each other, by trine or sextile, particularly "by exchange", and even more when the husband's Moon is so placed to the wife's Sun. When the luminaries of the two charts are in disjunct signs, or in opposition or square, he expects quarrels and separation. Benefic planets aspecting the luminaries keep the marriage pleasant; malefics make it quarrelsome [1]. Robbins's note explains "exchange", following the ancient commentator, as the luminaries of one chart in aspect with those of the other: Sun with Sun, Moon with Moon, or each Sun with the other's Moon [1].

The chapter on friends and enemies widens the method. Ptolemy compares the four most important places in both charts, the Sun, the Moon, the Ascendant ("horoscope") and the Lot of Fortune. When they fall in the same signs, or exchange places, they make a firm sympathy; in disjunct or opposite signs, lasting enmity; trine or sextile makes the sympathy less, and square makes the enmity less. He links the kind of friendship to the place that agrees: the luminaries give friendship by choice, which he calls the best and surest kind; the Lots of Fortune, friendship through need; the Ascendants, friendship through pleasure or pain. For passing acquaintances and quarrels he looks at the times when the prorogations (directions) of the planets of one chart reach the places of the other [1]. The core of modern synastry, reading one chart's planets against the other's, is already here.

The twentieth century. Chart comparison became a large part of modern practice, with its own books. Reinhold Ebertin applied the methods of cosmobiology to partners in Applied Cosmobiology (1972). He notes that the two charts used to be traced on transparent paper and laid one over the other, sign on sign; instead he sets both on the 90° dial, keeps only contacts within about 1°, and reads one person's planets against the other's planets and midpoints. He then developed the contact cosmogram, several concentric 90° circles for several people's charts; he refers readers to his book The Contact Cosmogram (1974) for many more examples [5] (see contact cosmogram). Robert Hand's Planets in Composite (1975), with a foreword by John Townley, is described by its publisher as the first major textbook to use the composite technique [4]. Ronald Davison, president of the Astrological Lodge of London, published Synastry: Understanding Human Relations Through Astrology in 1977; according to its publisher it covers methods of comparison including planetary cross-aspects and house interchanges, uses twenty horoscopes as examples, and introduces his "relationship horoscope", now called the Davison chart [3]. Midpoints formed across the two charts, one end in each, are a related technique (see combined midpoint).

How it is used

What is compared. The planets, the Moon's Node and, when both birth times are known, the Ascendant and Midheaven. The Sun and Moon keep the weight Ptolemy gave them. Ebertin's list of factors to watch between partners starts with the Sun, Moon, Venus and Mars and their midpoints, then goes on to Jupiter, Saturn, the outer planets and the Moon's Node [5].

Birth times. Inter-aspects between the slower planets need only the date. The Moon moves about 12° to 15° a day, so without a birth time its position is known only roughly, and the angles and houses not at all. House overlays into a person's chart need that person's birth time.

Chart comparison and relationship charts. Synastry proper keeps the two charts separate and compares them; the composite and Davison charts merge them into one chart that is read for the relationship itself. The two approaches are usually used together.

On this site. The Synastry tool takes two sets of birth details and one house system for both charts. The result opens on a double wheel, with the first person's chart and houses on the outside and the second person's planets in a ring inside it, and tabs for Contacts (the inter-aspects), House overlays (each person's planets in the other's houses), Contact cosmogram (Ebertin's 90° dial for the pair), Mirrors and parallels (one person's planets on the other's antiscia and declination parallels) and Calculation. A switch above the wheel changes the view to the composite or Davison chart, shown either as a triwheel, with the relationship chart in the middle ring between the two birth charts, or as a single wheel. When a birth time is unknown, noon local time is used for that person's planets and their Moon is marked approximate; their houses and angles are left out everywhere, and so are the houses and angles of the composite and Davison charts, which need both times.

Examples

Albert Einstein (14 March 1879, 11:30 local mean time, Ulm; 10:50:02 UT) and his first wife Mileva Marić (19 December 1875, Titel, Vojvodina; 7 December Old Style, the Julian date Astro-Databank also gives [6]). They married in 1903 and divorced in 1919. Mileva's birth time is not recorded; Astro-Databank gives 10 a.m. (rated B, from a biography), estimated from a narrative in a 2013 biography that places the birth on a sunny morning [6]. Here her chart is cast for noon local mean time (10:38:48 UT), so her Moon is only approximate: during that day it moved from 18°37′ Virgo to 1°26′ Libra.

• The luminaries, Ptolemy's test. Einstein's Sun is at 23°30′ Pisces. Mileva's Sun at 27°10′ Sagittarius squares it, 3°40′ from exact. Her noon Moon at 25°04′ Virgo opposes it, 1°34′ from exact, and the opposition holds within 8° for any time that day. Ptolemy would count both as signs of strife rather than of a lasting marriage [1].
• Tight inter-aspects (orbs 8° for conjunction and opposition, 7° for square and trine, 5° for sextile): Einstein's Sun square Mileva's Mercury (23°16′ Sagittarius), 0°14′; Einstein's Jupiter (27°29′ Aquarius) sextile Mileva's Sun, 0°19′; Einstein's Saturn (4°11′ Aries) conjunct Mileva's Node (4°31′ Aries), 0°20′. See inter-aspect.
• House overlays. In Einstein's Placidus houses Mileva's Venus (18°47′ Capricorn) falls in his 7th house and her Sun and Mercury in his 6th. The reverse, Einstein's planets in Mileva's houses, cannot be known without her birth time. See house overlay.

See also

Orb for how far from exact a contact may be; midpoint for the arithmetic behind the composite chart and the contact cosmogram.