Synastry: how to read a relationship compatibility chart

What is synastry?

Synastry (from the Greek syn, “together,” and astron, “star”) is the branch of astrology about how two natal charts interact. An astrologer lays one person’s planets over the other’s chart and looks at two things: the aspects between planets (the angles between them) and the houses the partner’s planets fall in. The first describes how you “sound” to each other; the second, which areas of life your partner enters in your chart.

Synastry does not deliver a verdict of “you’re a match / you’re not.” It answers questions like: where do you feel drawn to each other, where do you understand each other easily, where will there be friction, and what will you need to negotiate? All of this is a language for conversation, not a sentence. The basics are covered in “Natal chart explained: how to read your birth chart”.

Why the Sun sign isn’t enough

Popular zodiac compatibility charts compare only the two Suns, one point out of ten. But every chart has ten planets (Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto), so two charts make a hundred planet–planet pairs, and more with the Ascendant and MC. The Sun sign covers basic self-image and style, but not how a person loves, argues and calms down, which is the job of Venus, Mars and the Moon. The Moon covers about 13° in a day and changes sign every two-plus days, so matching Suns says nothing about it.

That is why a Leo and a Capricorn, whom the charts call a “difficult pair,” can get along wonderfully if their Moons and Venuses match. And two Leos, perfect “on paper,” sometimes cannot find a common language because of their Mars placements. We wrote separately about Sun sign pairings in zodiac compatibility, but that is only the first layer. The real picture comes from comparing charts, which is what follows.

Key contacts between charts

A contact is an aspect between one person’s planet and the other’s. You can read them in both directions: “her Mars to his Sun” and “his Mars to her Sun” sound different. Orbs (the allowed deviation from the exact angle) depend on the planets: the more “personal” the point, the wider the orb. In Oracul’s synastry: Sun and Moon up to 8°, Mercury, Venus, Mars and the angles of the chart up to 6°, Jupiter and Saturn up to 4°, the lunar node 3°, and Uranus, Neptune and Pluto 2°; for a pair the smaller of the two tolerances is used. Here is what astrologers look at first.

Sun–Moon

The classic “I’m accepted” contact. A conjunction, trine or sextile (one person’s Sun and the other’s Moon) gives a sense that your partner understands without words: the Sun feels supported, and the Moon feels that its need for care has been noticed. A square or opposition means different rhythms: one shines and acts, while the other needs things quieter and closer to home, and without a conversation about it both start to feel out of place. This is one of the most heavily weighted pairs in Oracul’s calculation.

Venus–Mars

The classic of attraction: one person’s taste and tenderness meet the other’s desire and drive. A conjunction and opposition are “chemistry” and sparks; a trine and sextile are easy flirtation where it is pleasant to reach toward each other; a square is attraction with friction, where quarrels and making up go together. In Oracul, a Venus–Mars conjunction and opposition are tagged as attraction, a trine and sextile as harmony, and a square as tension.

Moon–Moon

It shows whether your ways of calming down match and what each of you counts as “home.” With a trine or sextile, daily life falls into place without long explanations: a similar rhythm of sleep and meals, the same need for quiet or for company. With a square or opposition, one person under stress wants to be alone while the other wants a hug, and each reads the other’s choice as coldness. Simple agreements about who helps with what on a hard day work well here.

Ascendant and Descendant

If your partner’s Sun, Moon or Venus lies on your Ascendant, you notice each other right away; on the Descendant (the opposite point) your partner “steps into the role” of companion. This contact depends on the exact time: the Ascendant moves about one degree every four minutes, so an error of 20 minutes shifts it by 5°. Oracul calculates these contacts only when the birth time is known.

Contacts with Saturn

A partner’s Saturn touching your Sun, Moon or Venus is described as an “axis of commitment”: support, seriousness, a sense of responsibility, and at the same time criticism, a feeling of being controlled or held back. Oracul’s calculation singles these contacts out. It is not a “bad” aspect: it means your partner strongly affects your stability, and it is up to the two of you whether that becomes reliability or pressure. The orb here is narrow, up to 4°.

Mercury–Mercury

A shared language: do you get each other’s jokes, do you finish each other’s sentences? Supportive aspects give ease of conversation, while a square gives a situation where the words seem the same but the meaning differs. These pairs do well with agreements put in writing.

Contacts with the lunar node

Tradition reads a partner’s planets near your North Node as a meeting that “pushes you to grow” (see how that works in the article on the lunar nodes). It often comes with a feeling that the meeting was “no accident.” We consider that judgment subjective: in Oracul’s calculation the node carries a small weight, counts only with personal planets and within a 3° orb.

Your partner’s planets in your houses

Overlays show where in your life your partner’s planet “unfolds.” The house is the area where you feel it. Read them in both directions: the second person’s planets in the first person’s houses and the first’s in the second’s, since each of you gets a different picture. This requires the exact birth time of both: without it Oracul does not show the Houses section. For the houses themselves, see “Houses in astrology”. Here are six houses that usually matter more than the others in synastry:

For the calculation, Oracul uses these overlays: a partner’s Sun, Moon and Venus in your 7th house add 3 points each to harmony, in the 5th house 2 points each, and Saturn in the 7th is flagged as an axis of commitment. On the Houses screen the app shows each person’s Sun, Moon, Venus and Mars in the other’s houses.

The composite chart, and why Oracul doesn’t build one

The composite is another method. For each pair of planets it takes the midpoint between the two people’s positions: the composite Sun is the midpoint between the Suns, the composite Moon is the midpoint between the Moons, and the result is one symbolic “relationship chart.” The composite describes the couple as a separate entity, and many astrologers read it alongside synastry. Oracul reads only synastry: contacts between two real charts, house overlays and three scales. The composite stays out of the calculation: it is not built anywhere in the app’s code, and we will not describe what does not exist.

How Oracul calculates compatibility

The calculation is designed so you can check it by hand. Each contact has a weight: pair weight × aspect sign × orb falloff. Pairs have different weights: 5 for Sun–Moon, Moon–Moon, Venus–Mars and contacts of the Sun, Moon and Venus with the Ascendant and Descendant; 4 for Sun–Sun, Venus–Venus, Moon–Venus, Mars–Mars and Saturn with the Sun, Moon or Venus; 3 for Mercury–Mercury, Mercury–Sun, Mercury–Moon, Sun–Venus, Sun–Mars and Moon–Mars; Jupiter gets 2, and the lunar node and outer planets get 1.5. The aspect sign: trine +1, sextile +0.8, square −1, opposition −0.6; a conjunction is usually +1, but for Saturn, Mars and the outer planets it is negative. The falloff is cos² of the ratio of the orb to the tolerance, so an exact contact weighs a lot and one at the edge of the orb almost nothing.

These add up to three scales out of 100: harmony is the sum of supportive links, tension is the sum of links that need working through, and attraction is the total strength of all contacts, that is, how strongly the charts touch each other. The overall score is the balance of harmony and tension, calibrated on a sample of random pairs: about 10% of pairs score below 35, 10% above 75, and a typical pair gets around 60. So a score of 60 is an ordinary good relationship, not “average.” The score is not a probability of happiness: it is a balance of links. If the birth time is not given, the Moon is counted with reduced weight.

How to work out a contact by hand: an example

Take two hypothetical people. The first was born on August 14, 1996 and the second on March 3, 1994; for the example we use planet positions on those dates from ephemeris data (for the second person, at 09:00 UTC). Let’s check one contact: the first person’s Mars and the second person’s Sun.

1. 1
   Convert positions to longitude

   Longitude = sign number × 30° + degrees within the sign. The first person’s Mars is at 12°53′ Cancer: Cancer is the fourth sign (index 3), so 3 × 30 + 12.88 = 102.88°. The second person’s Sun is at 12°33′ Pisces: Pisces is index 11, so 11 × 30 + 12.55 = 342.55°.

2. 2
   Find the difference and fold it to 180°

   342.55 − 102.88 = 239.67°. The angle is over 180°, so we take the “short way” around the circle: 360 − 239.67 = 120.33°.

3. 3
   Compare with the aspect angles

   The nearest angle is 120° (a trine). The deviation from the exact angle is the orb: 120.33 − 120 = 0.33°.

4. 4
   Check the tolerance

   In Oracul the orb for a Mars–Sun pair is the smaller of the two tolerances: 6° for Mars and 8° for the Sun, so 6°. 0.33° fits with room to spare, so the contact is very exact.

5. 5
   Work out the weight

   Weight = pair weight × aspect sign × orb falloff. For Sun–Mars the pair weight is 3, a trine gives +1, and the falloff is cos²(π/2 · 0.33/6) ≈ 0.99. Total 3 × 1 × 0.99 ≈ +2.98: a strong supportive contact.

Doing the same for the other pairs gives this couple’s strongest contacts: the first person’s Mars to the second’s Sun (trine, orb 0.33°, weight +2.98); the first person’s Venus to the second’s Saturn (trine, orb 2.03°: a strong Saturn contact in a supportive aspect); the first person’s Sun to the second’s Moon (square, orb 3.74°): an example of “different rhythms,” whose weight is muted because no birth time is set and the Moon counts half as much. And the first person’s Sun to the second’s Mercury (opposition, orb 1.19°) points to different ways of putting thoughts into words. Notice that one couple has an exact trine, a square and an opposition all at once, and that is normal.

Common mistakes when reading synastry

• One aspect as a verdict. A Moon–Moon square is not a “cross” on the couple but a specific area: you need to talk through how each of you experiences stress. A chart has a hundred contacts, and none decides everything.
• Fear of Saturn. A Saturn contact means “serious,” not “bad.” It gives stability and makes you treat the relationship responsibly; problems arise when Saturnian criticism goes unspoken.
• Searching for “perfect compatibility.” A couple without a single challenging contact among a hundred planet pairs is rare, and its absence is not a mark of quality. Smooth relationships with no challenge can be boring, too.
• Ignoring an imprecise birth time. With a half-hour error the Ascendant and houses shift by about 7° and into neighboring areas; the Moon without a time can be relied on only with caveats.
• Looking from one side only. A contact works for both: “your Mars to his Sun” is felt by him, too.
• Comparing scores between couples. A score describes the balance of contacts within one couple, not a ranking of couples against each other.

Your pair’s signs and elements in a second

Want to try the simplest version? Enter two dates and the site will show the signs and the combination of elements. That is the first layer; real synastry needs the birth time and city of both people, which we will ask for in the app. To see what happens next, look at the sample “Compatibility report by chart”; we also make a couple forecast showing how transits affect the two of you. More on this on the compatibility page, and on aspects in “Aspects in the natal chart”.

Frequently asked questions