How to read a synastry chart: Jo and Ade, step by step

A synastry chart sets two birth charts side by side and lists every angle between one person’s planets and the other’s. The idea is simple. The list isn’t. Jo and Ade, two people from our example circle, have 30 contacts between their charts, and most of them barely register. This guide reads that list in seven steps: which points count, how wide an orb to allow, and how to tell a tight contact from a loose one. Follow along with your own pair in our birth chart compatibility calculator.

Step 1: Cast both charts

Each chart needs a date, a place and, if you have it, a birth time. Jo was born on 23 May 1991 at 2:20pm local time in Lisbon. Ade was born on 30 September 1989 at 5:05pm local time in Lagos. If you haven’t cast a chart before, our free birth chart calculator does it in a minute.

Use the same house system for both, and keep one rule straight: in every line below, the first name owns the first planet. “Jo’s Venus square Ade’s Moon” is not the same contact as “Ade’s Venus square Jo’s Moon”.

Step 2: Choose the points that count

Charthood compares thirteen points from each chart: the ten planets from Sun to Pluto, the North Node, the Rising sign and the Midheaven. Every Sun-to-Pluto planet of one person is checked against every point of the other, then the five major aspects are measured.

One pair is deliberately dropped. Two outer planets (Uranus, Neptune, Pluto) or the node touching each other don’t count, because everyone born within a few years shares them. Jo’s Neptune is in Capricorn and Ade’s is in Capricorn, yet that pairing is left out of the list. It says something about the years 1989 and 1991, not about Jo and Ade.

Step 3: What orb should I use?

An orb is how far an aspect is from exact. A trine is 120°, so two planets 117° apart make a trine with a 3° orb. Wider orbs find more contacts and make each one weaker. Here is what the engine allows:

• Anything with a Sun or Moon: up to 6°.
• Other pairs: up to 5°.
• Sextiles: up to 3°.
• Uranus, Neptune, Pluto and the node touching a personal point: up to 3°.

Those limits produce 30 contacts for Jo and Ade. Other tools use other numbers, so don’t panic if a friend’s chart shows more. Positions themselves are far finer than any orb. The astronomy library behind our planets says this about its precision:

One arcminute is a sixtieth of a degree, so the real uncertainty in a synastry chart is the birth time, not the planet positions. The Wikipedia entry on astrological aspects covers the tradition behind orbs if you want the background.

Step 4: Sort by tightness. What counts as tight?

Put the contacts in order, tightest first. Here are Jo and Ade’s top eight. The weight column is the engine’s own measure of how much a contact counts: 1.00 for an exact aspect, falling to under 0.15 at the edge of its orb.

The tightest is Jo’s Venus square Ade’s Moon, only 0.14° from exact. A rough rule that works well: inside 1° is tight, 1–3° is clear, and past 3° is background. Jo and Ade have 10 contacts under 1°, 13 between 1° and 3°, and 7 beyond 3°.

Step 5: Which planets matter most?

Not all 30 contacts are equal. Start with the ones where both ends are personal points: Sun, Moon, Mercury, Venus, Mars or Rising. Jo and Ade have 9 of those. They describe how two people actually feel, talk and want things day to day.

Next come personal points touched by Jupiter or Saturn, which add ease or weight. Last come the slow outer planets, which count when they land on a personal point. Only 12 of the 30 contacts feed one of the five scores, because the scoring rules pick particular planet pairs. The rest are real aspects that the tally simply doesn’t use. Our aspects guide covers what each of the five angles feels like.

Step 6: Read each contact as a sentence

Say each contact aloud as planet, aspect, planet, then ask what each end stands for. Take the tightest one. Jo’s Venus is how Jo shows affection and what Jo finds lovely. Ade’s Moon is how Ade feels looked after. A square is a tense angle, so the two don’t fit neatly: Jo’s way of showing care can miss what Ade needs. At 0.14° it is hard to ignore, but it is a prompt for a conversation, not a fault.

Now one that pulls the other way. Jo’s Saturn trine Ade’s Sun (0.66°) is steady and supportive: Jo’s sense of structure sits easily with Ade’s sense of self. And Jo’s Moon is conjunct Ade’s Mars (1.67°), so Jo’s feelings and Ade’s drive land on the same spot of the zodiac. Strong, and it can go either way.

Notice that three contacts, with three different tones, sit inside one relationship. That is normal. A synastry chart is a mixed bag by design.

Step 7: Group the contacts into themes

Finally, sort the contacts that count into the five themes our calculator uses. Each score starts in the middle and moves with each contact, more when it is tight.

Warmth is 41, below the middle, because the tight Venus–Moon square pulls it down harder than the Sun–Moon conjunction lifts it, and a loose Moon–Jupiter square pulls a little more. Staying power is 64 and Talk is 63. Spark sits at 62, and Friction at 34. In plain words: a friendship with steady foundations and easy conversation, where affection needs a little translating. That is a reading to test against what you know of the two people, not a verdict.

Common mistakes

• Counting every contact. Of 30, a fraction really shapes the scores. Start with the 10 under 1°.
• Mixing up whose planet is whose. Always say the first person’s planet first.
• Reading squares as bad and trines as good. A square can be the spark that keeps two people interested.
• Treating shared generational planets as a bond. Everyone born around 1990 has Neptune in Capricorn.
• Using one contact to decide a relationship. It is a conversation starter. See how the same method works on a couple in Robin and Sam’s worked example.

Where to go next

You now have the method: cast both charts, choose the points, set the orbs, sort by tightness, weigh the planets, read each contact as a sentence and group them. Try it on your own pair in the calculator and compare what it lists with the steps above. If you want to see where each person’s planets land in the other’s houses, our house overlays case study picks up from here. And to see how the five scores spread across a whole circle, read the assessment of all fifteen pairs. Planet positions come from astronomy-engine.