Synastry: What Two Birth Charts Say About Each Other

Synastry is the astrological practice of comparing two birth charts. Instead of reading one chart on its own, an astrologer lays the second chart beside it and looks at how the two meet: which planets line up, at which angles, and where each person’s planets fall in the other person’s chart. This guide explains how that comparison is done, the five aspects astrologers read between charts, what house overlays are, why the birth time matters, and, just as important, what synastry cannot tell you. It is all tradition, offered for reflection and entertainment, and it comes with a free calculator so you can see it on your own details.

What Synastry Is

The word comes from the Greek for “stars together”. Astrologers who write about synastry describe it the same way across very different schools: a side-by-side comparison of two birth charts, looking at how one person’s planetary placements relate to the other’s. It is used for couples, but also for friends, parents and children, business partners and siblings, because the method itself does not care what kind of relationship it is.

A synastry chart has two layers, and most astrologers read both.

Inter-aspects: one chart’s planets against the other’s

The first layer is the inter-aspects, sometimes called synastry aspects. They are the angles between a planet in the first chart and a planet in the second. If one person’s Venus sits at 10 degrees of Leo and the other’s Mars at 10 degrees of Sagittarius, the two are 120 degrees apart, a trine. Every planet in one chart is checked against every planet in the other, so a full comparison looks at more than a hundred pairs.

House overlays: where one person’s planets land

The second layer is the house overlays. Here one chart’s planets are placed into the other chart’s houses, and then the other way round. Astrologers read each house as an area of life, so an overlay is read as the area of one person’s life that the other person’s planet touches. Overlays depend on the houses, which depend on the exact birth time, so this layer needs the time of the person whose houses are used.

Synastry and the composite chart

Synastry is often confused with the composite chart, and astrologers keep the two apart. Synastry leaves both charts intact and compares them point by point. A composite chart blends the two into one new chart, usually by taking the midpoint between each pair of matching planets, and it is read as a chart of the relationship itself. Both start from the same two birth charts; this guide and our calculator do synastry.

What You Need Before You Start

A synastry chart is only as good as the two birth charts underneath it, so the data matters more than the method.

Birth dates, places and times

For each person you need the date of birth, the place of birth and, ideally, the time. The place sets the time zone and the coordinates; the time zone has to be the one in force on that date, including daylight saving time, which is why our calculator uses the historical time zone rules of each birthplace. If you do not know a time, a birth certificate or hospital record often has it.

What changes without a birth time

Most planets move slowly enough that the date alone pins them down. The Moon does not: we computed its daily motion from 1950 to 2009 and it moved between 11.77 and 15.35 degrees a day. Without a time, the Moon’s position is uncertain by that much, and the Ascendant, the Midheaven and the houses cannot be calculated at all. Astrologers who write about synastry make the same point: the rising sign and the houses need the time, and so does the Moon’s exact degree.

Our calculator handles an unknown time by working out every position at the start and at the end of the birth day. A contact that holds all day is listed normally. One that holds for only part of the day is marked as depending on the birth time, with its range of orbs, and the Ascendant, Midheaven and house overlays of that person are left out, with a note saying so.

The Five Aspects Astrologers Read Between Two Charts

Synastry uses the same aspects as a single birth chart. The angles are old ones: Sepharial’s 1910 handbook lists the “sextile of 60 degrees, square of 90 degrees, trine of 120 degrees”, alongside the conjunction at 0 and the opposition at 180. What changes in synastry is that the two ends of each angle belong to different people.

Sepharial also shows how older astrology ranked them: in his handbook the trine and sextile are the good aspects and the square and opposition the difficult ones. Modern writers have softened that language, and the usual reading today is gentler, as the five descriptions below show.

Conjunction (0 degrees)

Two planets at or near the same degree. In synastry, astrologers commonly describe a conjunction as a merging or blending: the two themes act on each other directly. Most add that its tone depends on which planets are involved rather than on the aspect itself, an idea that goes back at least to Sepharial, who judged a conjunction by the planets that formed it.

Sextile (60 degrees)

Two planets about a sixth of the circle apart. The sextile is commonly described as a supportive, friendly contact, an opportunity that tends to show up when both people use it.

Square (90 degrees)

Two planets a quarter of the circle apart. Astrologers commonly describe the square as friction: two themes that want different things and press against each other. Many also describe that friction as a source of drive and growth, which is why a square between two charts is not read as a sign that something is wrong.

Trine (120 degrees)

Two planets a third of the circle apart, usually in signs of the same element. The trine is commonly described as the easiest of the five, a contact where the two themes flow and support each other with little effort.

Opposition (180 degrees)

Two planets on opposite sides of the zodiac. The opposition is commonly described as a polarity, attraction and tension at once, with each side highlighting what the other lacks.

How close is close enough: orbs

An aspect rarely lands on the exact degree, so astrologers allow a margin called an orb. Orbs vary between astrologers and programs, and no single table is standard. Our calculator uses the table Astrotheme publishes for its compatibility charts:

The same page gives the reason synastry orbs are usually kept tighter than birth chart orbs: “Much smaller orbs are used in forecasting and synastry work because these techniques require aspects to be more exact if they are to be effective.” We add one rule of our own: contacts with the North Node, the Ascendant or the Midheaven are held to 3 degrees, because those points shift with small errors in the birth data. Whatever the table, astrologers agree on the principle that the tighter the orb, the stronger the contact is considered.

The Contacts Astrologers Usually Read First

Every planet is compared, but astrologers who write about synastry keep naming the same few pairs as the place to start. These are traditional emphases, not rules.

Sun and Moon

Contacts between one person’s Sun and the other’s Moon are among the first that astrologers look at, often described in terms of emotional understanding. Comparing the two Sun signs, and the two Moon signs, is the simplest version of the same idea.

Venus and Mars

Venus and Mars are the classic pairing in relationship astrology. Some writers say that all the planets are analysed in a synastry reading but that Venus and Mars carry a little more weight, read as affection meeting desire, or chemistry.

Moon and Moon

Moon to Moon contacts are one of the pairs astrologers most often name, read in terms of emotional rhythm, comfort and the sense of feeling at home with someone.

The Ascendant, Saturn and the nodes

Some astrologers give special attention to contacts with the Ascendant, read in terms of first impressions and how two people come across to each other. Saturn contacts are often read in terms of commitment and structure. Contacts with the North Node draw attention from some modern astrologers too, read as a sense of shared direction.

Each of those node contacts has a page of its own, with what guides say about it and how often it turns up between random pairs of charts in our own count: the Sun, the Moon, Venus, Mars, Saturn, Pluto, the Ascendant and the Descendant on the other person’s North Node. Taking those eight points together, 23.3% of 400,000 random pairs had at least one of them within 3 degrees of the other person’s node, in one direction or the other.

Why outer planet contacts count for less

Uranus, Neptune and Pluto move so slowly that people born a few years apart often have them in the same signs, so contacts between them are shared by a whole generation. Astrologers usually give those contacts less weight than contacts involving personal planets, and our calculator lists them separately as generational contacts.

House Overlays in Plain Words

A house overlay asks a simple question: if you placed one person’s planets into the other person’s chart, which houses would they land in? Because each of the twelve houses is traditionally tied to an area of life, from self and resources in the first two to community and solitude in the last two, the answer is read as the area of one person’s life that the other person’s planet touches. For what each house stands for, see our guide to the twelve houses in astrology.

Reading an overlay both ways

Overlays run in two directions, and astrologers read both: first person A’s planets in person B’s houses, then B’s planets in A’s houses. The two directions rarely match. One person’s Sun can fall in the other’s 7th house, the house traditionally tied to partnership, while the second person’s Sun lands somewhere quite different in the first chart. Overlays need a birth time only on the side whose houses are used, so with one time known you still get one direction.

What an overlay adds to the aspects

Astrologers often describe the division of labour like this: the aspects describe how two people interact, and the overlays describe where in life that interaction is felt. That is why most synastry readings use both layers rather than choosing one.

A Worked Example With Two Fictional Charts

To show what the method produces, we ran two invented people through our calculator: Alex, born on March 14, 1992 at 8:30 AM in Chicago, and Sam, born on August 2, 1990 at 9:15 PM in Lisbon. Neither is a real person.

Alex has the Sun at 24° Pisces, the Moon at 27° Cancer and 21° Taurus rising. Sam has the Sun at 10° Leo, the Moon at 27° Sagittarius and 21° Aquarius rising. With the orbs above, the two charts make 40 contacts, 6 of them between the slow outer planets and nodes.

The tightest personal contacts

The two Ascendants form a square with an orb of 0.3°, the kind of close contact astrologers would read in terms of first impressions and two different ways of meeting the world. Alex’s Moon sits within half a degree of Sam’s Jupiter. Alex’s Mercury makes a trine to Sam’s Sun (orb 0.6°), and Alex’s Mars is conjunct Sam’s Ascendant (orb 2.0°). Alex’s Sun is square Sam’s Moon (orb 2.8°), one of the Sun and Moon contacts astrologers look at first, and Alex’s Venus makes a sextile to Sam’s Moon (orb 3.7°).

What is not there

There is no contact at all between Venus and Mars in either direction. We computed 2,000 random pairs of birth dates from 1950 to 2009 with the same orbs, and a Venus and Mars contact turned up in 50.1% of them, so Alex and Sam are simply in the other half. Astrologers would not read the absence as a problem; it is one pairing among dozens.

The overlays

Alex’s Sun and Venus both fall in Sam’s 1st house, and Alex’s Jupiter in Sam’s 7th. Going the other way, Sam’s Sun falls in Alex’s 4th house, traditionally the house of home and roots, and Sam’s Mercury in Alex’s 5th, the house of creativity and play.

What a Synastry Chart Cannot Tell You

This is the part most synastry pages skip, and it matters more than any single aspect.

No score and no verdict

Any two birth charts meet many times. In our 2,000 random pairs, the average was 31 contacts between the planets and nodes of the two charts, from 13 at the fewest to 50 at the most, and every single pair had at least one contact between personal planets. A list of contacts, then, says little on its own about whether two people suit each other. Astrologers who write about synastry tend to stress the same thing in their own way: no single aspect tells the whole story, and people with challenging contacts between their charts can still have rich relationships. That is why our calculator shows no compatibility score or percentage.

What the research says

Synastry is part of astrology, and astrology has not held up when tested scientifically. Wikipedia’s summary of the research says that scientific tests “have shown it to have no scientific validity or explanatory power”. Synastry is best treated as a symbolic language for talking about two people, which is how astrologers who use it responsibly present it, and how we present it here.

Where synastry is useful

Read that way, a synastry chart is a prompt for conversation. It can put words to a difference two people already notice, or suggest a question to talk through together. It should never be the reason to start, stay in or leave any relationship.

Numerology’s Take on Two People

Astrology is not the only tradition that compares two people. Numerology does it with birth dates alone, by comparing two life path numbers and how the tradition reads the pairing. If you want that angle as well, the numerology compatibility calculator shows both numbers and all 81 pairings in one grid.

Try It on Two Real Charts

Start with each person’s own chart, since synastry only adds to what each chart says alone: the birth chart calculator gives you the planets, houses and aspects for one person. Then put the two together in the synastry chart calculator, which draws the bi-wheel, the aspect grid and the house overlays, and marks anything that depends on a missing birth time.

If you would like the two charts written up for you, the compatibility reading puts both birth charts and both sets of numbers side by side: your life paths together, the Sun, Moon and rising signs compared, Venus and Mars, and the aspects where your charts meet, in an illustrated PDF. Like everything on Starvela, it is offered for reflection and entertainment, never as a prediction.