What is synastry, and how is it different from a compatibility score?
Synastry is the astrological practice of comparing two people's birth charts, planet by planet, to describe how they relate. A compatibility score is a summary: a tool picks some chart factors, weighs them and turns them into one number. The score is quicker to read. Synastry shows the detail behind it.
What does synastry actually look at?
Synastry starts with two full birth charts, each calculated from a birth date, time and place. Then it compares them in two main ways.
- Inter-aspects. These are the angles between one person's planets and the other person's. If your Moon sits close to their Venus, astrologers in the Western tradition read that as a link between your emotional needs and how they show affection. Kerykeion's guide says these connections "describe the felt experience of being together." (Kerykeion)
- House overlays. "Each person's planets land in specific houses of the other person's chart." (Kerykeion) Your Sun falling in their 7th house, for example, is read as touching the area of partnership in their chart.
A synastry reading also looks at each chart on its own first. Two people's patterns only make sense once you know each person's.
How is a compatibility score made?
It depends on the tool, and a tool may not publish its formula. One way a score can be built:
- Pick a list of factors, such as aspects between the Suns, Moons, Venus and Mars.
- Give each a positive or negative weight.
- Add them up and scale the result to a percentage or a number out of 10.
The choices in steps 1 and 2 are the tool's choices. Two tools can read the same two charts, pick different factors and weights, and give you very different numbers. Neither number is the "true" one.
Some scores are simpler still and only compare Sun signs. That leaves out the Moon, the rising sign and every other planet, so two Leos born days apart get the same score with anyone.
What does each one miss?
A score hides its reasons. If you see 62%, you don't know which factors pushed it up or down, so you can't tell if it is based on anything you care about.
Synastry gives you the reasons, but it is long and can contradict itself. One pair of planets may read as easy and another as tense, and the reader has to decide what to weigh. That is interpretation, not measurement.
Both share a bigger limit. Neither a score nor a detailed comparison tells you whether a relationship will last, be happy or be safe. In a test using census data on about ten million married couples in England and Wales, researcher David Voas found that the pairing of star signs was "just what we'd predict on the basis of chance." (IFLScience) That study looked at Sun signs only, not full synastry, but no study we know of shows that full synastry predicts relationship outcomes either.
What does this look like for a real couple?
Take two people, Ana and Ben.
- A Sun-sign app sees Ana as a Taurus and Ben as an Aquarius. Those signs are square to each other, so it shows a low score.
- A synastry reading looks further. Ana's Moon is close to Ben's Venus, and Ben's Saturn sits on Ana's Sun. In Western astrology the first is read as warmth and the second as weight or responsibility.
The score said one thing. The synastry gave a mixed picture with specific things to talk about. What it doesn't do is tell Ana and Ben whether to stay together. That is their call, based on how they actually treat each other.
How do I use synastry well?
- Use real charts. Both people need a correct birth date, place and ideally time. Without a time, the rising sign and houses are uncertain, so house overlays are guesses.
- Use the same settings for both charts: the same zodiac, the same house system, and for Vedic charts the same ayanamsa.
- Treat it as a conversation starter. A placement that reads as "tense" is a question to ask each other, not a warning.
- Never make a big relationship decision because of a chart. How someone behaves tells you far more than where their Mars was.
How does Mystogian handle compatibility?
Mystogian's Bonds feature reads compatibility from two people's real charts, each calculated with astronomy code on our server from their birth date, time and place. The AI never calculates planet positions.
FAQ
- Is synastry the same as a composite chart?
- No. Synastry compares two separate charts. A composite chart blends them into one chart for the relationship.
- Do we both need a birth time?
- It helps. Without one, many planet-to-planet aspects are still usable, but aspects to the Moon need checking because the Moon moves fast, and house overlays and the rising sign are unknown.
- Why did two apps give us different scores?
- They likely use different factors and weights. Ask each app what goes into its score.
- Is a high score a good sign for the relationship?
- It tells you what that tool's formula found. It says nothing proven about how the relationship will go.
Sources
- Kerykeion, "Synastry: introduction": https://kerykeion.net/content/learn-astrology/synastry-introduction
- IFLScience on David Voas's study of 20 million birthdays of married couples: https://www.iflscience.com/editors-blog/a-study-of-20-million-people-investigated-the-truth-behind-star-sign-compatibility/