Synastry is the art of comparing the birth charts of two people to see how they click. It’s useful for all sorts of relationships in our life - love partnerships, friendships, workplace relationships, family relationships, and more. Basically any time two people interact, synastry can shed some light on that interaction.
From what we can tell, this art has been practised pretty much right from the beginning of the tradition of astrology. For example, there are clear and unmistakable references to synastry techniques early in the Hellenistic period, in texts such as Dorotheus of Sidon’s Pentateuch (1st century CE) and Ptolemy’s Tetrabiblos (2nd century CE).
Over the years these techniques became more and more complex and codified, as subsequent authors picked up the ball and ran with it (a pretty common phenomenon in the history of astrology). But the heart and soul of synastry is really simple and doesn’t need to be overwhelming. We just need to remember a few simple concepts.
The Big Three
In the earliest sources on synastry like Dorotheus and Ptolemy, the focus is mainly on what we call "the Big Three” - the rising sign, the sun sign and the moon sign. The authors don’t explain why, but the reasoning seems to be that these placements are the ones that speak most to our personality, our character, and our behaviour, so they are the most important when it comes to considering how two people will get along. And that makes ancient synastry very accessible, because practically everyone with more than a passing interest in astrology knows their sun, moon and rising signs.
For a relationship to be smooth and functional, what you really want to find is some harmony between these placements in each person’s birth chart. If these placements link up in a harmonious way, the people will tend to get along easier, with less friction and less cause for argument and butting of heads. If these placements are discordant, there will be more differences and disagreements between the parties.
What do we mean by harmony? In astrology, we’re really talking about aspect configurations, but aspect doctrine can be complex, especially for beginners.
The purpose of this post is to keep things simple, so instead of worrying about aspects right now, I want to focus your attention on the elements of the signs that your sun, moon and rising fall in.
Elemental pairings
Putting it simply:
fire signs and air signs are harmonious
earth signs and water signs are harmonious
For this reason, a person whose Big Three fall mostly in fire or air signs should tend to get along better with people who also have mostly fire or air signs in their Big Three.
And similarly, a person whose Big Three falls mainly into earth or water signs should tend to get along better with people who also have mostly earth or water signs in their Big Three.
Think of it this way - fire needs air to keep it burning, and earth needs water to produce plant life. Meanwhile, dumping earth or water onto a fire puts it out, and you can’t breathe air if you’re under water or buried in the soil. There is a fundamental linkage between these elements (air and fire, earth and water) that binds them together and makes them more functional.
Working out which elemental pairing predominates in your chart is easy to figure out.
If your Big Three are all in signs of the same element, it’s a no-brainer that this element predominates. For example, a person with Aries rising, Leo Moon, Sagittarius Sun is clearly a fiery individual and should tend to get along better with fiery or airy individuals.
If your Big Three land in signs of two different elements, the element with two placements predominates. For example, a person with Aries rising, Scorpio Moon and Pisces Sun has one fire placement and two water placements in their Big Three, so they are predominantly watery and should tend to get along better with watery or earthy individuals.
If your Big Three land in signs of three different elements, you’ll need to see which elemental pairing predominates (fire-air or earth-water). For example, a person with Aries rising, Virgo Moon and Cancer Sun has one fire placement, one earth placement and one water placement. While no one element predominates, this person tends towards the pairing of earth and water, so they will tend to get along better with watery or earthy individuals.
We can also seem some slightly more complex scenarios emerging here, which can describe better the full range of relationships:
A person who has two placements from their Big Three in air or fire, but has one placement in water or earth, will tend to get along better with airy or fiery people, but will still be able to relate to watery or earthy people too, and on some level they will need those kinds of people around them in order to feel completely supported and understood.
Similarly a person who has two placements from their Big Three in earth or water signs, but one placement in air or fire will tend to get along better with earthy or watery people, but will still be able to relate to airy or fiery people too, and will need some of these types around them to feel supported and understood.
If you just remember these simple rules about the harmony between earth and water signs, and the harmony between air and fire signs, you will be able to start thinking about your relationships through this synastry lens and start comparing the charts of other people too.
Layers of personality
Each placement (Sun, Moon and rising) describes a different layer of the psychology. This can be helpful if you want to really dig into synastry and understand which parts of one chart are harmonising with which parts of the other chart. For example, what does it mean if one person’s Sun and another person’s Moon sign are in harmony, but their rising signs are not?
I want to keep things as simple as possible here, so for the purpose of this article here’s some very basic delineations for the rising sign, the Sun sign and the Moon sign based on traditional astrology:
As the centre of the heliocentric solar system, and the orbital “tether” of Mercury and Venus in the geocentric cosmos, the Sun represents a point of focus or orientation for the person born at that time. The Sun is also associated with ideas of authority, governing and mastery. And so in the astrology of personality the Sun sign signifies things our goals and values, our focus, our orientation in life, an area of personal mastery or authority, and the governing principles in our life.
The Moon is traditionally connected with the instinctive and animal nature within our souls, the most basic needs and drives that inform our impulses and desires. Hence the Moon sign is associated with our needs and desires. It shows what we need in order to feel happy, secure, and fulfilled, and in the context of relationships it can show what we need from other people. For example, a fixed Moon sign needs stability, a cardinal Moon sign needs action, and a mutable Moon sign needs variety.
While everyone born on the same day will share the same Sun sign, and usually the same Moon sign too, the rising sign changes every two hours. This makes the rising sign the part of the birth chart most intimately associated with the person born at that time. The rising sign is what sets you apart from other people born in the same season as you, what makes you unique. In Hellenistic astrology the rising sign was called “the helm” in the sense of the helm of a ship. Today a better analogy might be the steering wheel or the driver’s seat in a car. And so the rising sign indicates how we move through life. It shows how we approach life, how we tackle life’s problems, how we show up in the lives of other people, the way we interact with the world and with other people, our thinking style, our problem-solving style, and it can even indicate how we move about physically.
Taking this all into consideration, we can think more deeply about the synastry connections that can be formed between these three key points in the astrology of personality:
Harmony between Sun signs can show an alignment of values, shared goals, and a shared focus or direction in life
Harmony between Moon signs can show an alignment of needs and desires, and a deep understanding of the emotional space of another human being, their impulses and drives
Harmony between rising signs can show an alignment of action in the world, a shared method of interacting and problem solving, working well as a team or partnership, living together in a way that flows easily and is not jarring or confusing
Harmony between Sun and Moon signs shows that the goals and values of one person are in alignment with the needs and desires of the other person
Harmony between Sun and rising signs shows that the goals and values of one person are in alignment with the approach to life of the other person
Harmony between Moon and rising signs shows that the needs and desires of one person are in alignment with the approach to life of the other person.
The reason why
The reason these signs are harmonious is because the aspect configurations between these signs are productive for relationships.
In traditional astrology there are five aspects: the conjunction (copresence), the sextile, the square, the trine, and the opposition.
The conjunction (or copresence) is when two placements fall in the same sign. For example, two placements in the sign Aries are copresent, and if they’re close by degree they’re conjunct.
The sextile is when two placements are in signs with one sign in between them. For example if one placement is in Aries and the other placement is in Gemini, there is one sign (Taurus) between them and they are in a sextile aspect.
The square is when two placements are separated by two signs between them. For example if one placement is in Aries and the other is in Cancer, there are two signs (Taurus and Gemini) between them and they are in a square aspect..
The trine is when the two placements are separated by three signs between them. For example if one placement is in Aries and the other is in Leo, there are three signs (Taurus, Gemini and Cancer) between them and they are in a trine aspect..
The opposition is when the two placements are separated by 5 signs between them, so that they sit opposite each other in the zodiac (hence the name). For example if one placement is in Aries and the other is in Libra, there are five signs (Taurus, Gemini, Cancer, Leo and Virgo) between them and they are in an opposition aspect..
I’m focusing on whole-sign aspects rather than degree-based aspects here because (1) it’s easier and (2) the ancient astrologers like Ptolemy and Dorotheus used whole-sign aspects a lot.
Now, if you look at the examples above, you might notice something interesting:
the trine joins signs of the same element. For example Aries is in a trine with Leo, which are both fire signs.
the sextile and opposition join signs of elements that are harmonious. For example Aries is a fire sign and it’s in a sextile with Gemini and an opposition with Libra, which are both air signs.
the square joins signs of elements that are not harmonious. For example Aries is a fire sign and it is in square with Cancer and Capricorn, which are water and earth signs respectively.
In traditional astrology the sextile and trine were seen as harmonious or “good” aspect configurations. This is because the signs ruled by the benefic or “good” planets (Venus and Jupiter) are configured to the signs ruled by the sun and moon by sextiles and trines. For example Sagittarius is ruled by the benefic Jupiter and it is in a trine to Leo, the sign ruled by the Sun.
Meanwhile the square and opposition were seen as difficult or “bad” aspect configurations, because the signs ruled by the malefic or “bad” planets (Mars and Saturn) are configured to the signs ruled by the sun and moon by squares and oppositions. For example Capricorn is ruled by the malefic Saturn and it is in an opposition with Cancer, the sign ruled by the Moon.
There is one wrinkle in this scheme that I want to iron out. Above I said that fire and air signs get along, and water and earth signs get along, but some of these sign pairings look at each other by opposition, and the opposition aspect is supposed to be a difficult or “bad” aspect. And yes, that’s generally true.
But in synastry, an opposition can be a good thing, and here’s the reason why. In our birth charts, the house associated with marriage and relationships is the 7th house, and that house sits opposite to the rising sign and the 1st house. So when it comes to relationships, there is something fundamentally important about the opposition aspect - it speaks to the experience of relationship as a meeting with The Other, and the profound impact of truly seeking to know and understand another human being. And that can be challenging at times, but it also forms a close and powerful bond between the two people if they are prepared to show up and be honest and real with each other in that space of mutual understanding.
So yes, the opposition joins air signs to fire signs, and it joins earth signs to water signs, and yes the opposition is a difficult aspect - but remember, the core elemental nature of these signs is a harmonious pairing, and the opposition is the aspect that joins the part of the chart most associated with “us” (the 1st house) with the part of the chart most associated with “the other” (the 7th house), so in the context of relationships there is something fundamentally connective about the opposition.
And so amidst the difficulty of that oppositional encounter, a powerful and supportive bond can be formed. The air fans the flames, the water fertilises the earth, and the two become stronger and more productive as a result.
Caveats
My intention with this article is to give you a quick and easy rule of thumb to get you started looking into synastry. It’s not meant to be a comprehensive guide or to cover all possible scenarios.
I focused on the elements because I think that’s something most people can understand quickly, while the full range of aspect configurations between charts can get very complex.
The benefit of simplicity is that it’s easy to use and easy to understand, but the price of simplicity is that it always fails to cover the variety of life.
If you go looking, you will find plenty of people whose Big Three are mainly earthy or watery who will be happily married to someone whose Big Three are mainly fiery or airy. And you’ll also find best friends who have similar clashes in these placements.
There are always exceptions to these rules, because we are focusing here on only three placements instead of bringing in the entire birth chart. The existence of such examples does not mean these rules are invalid - it merely demonstrates that they are simple and cannot cover the full range of potential synastry combinations.
Further reading
If you want to learn more about traditional synastry, you can check out these traditional source texts:
Ptolemy, Tetrabiblos - Book IV, Chapter V and Chapter VII
Dorotheus, Carmen Astrologicum - Book II, Chapter 5, lines 13-16
Sahl ibn Bishr, Book of Nativities, Chapter 7.5