Synastry explained: what astrology says about two charts together

Synastry is the technique of putting two natal charts next to each other and reading how the planets in one chart relate to the planets in the other. It is the part of astrology that actually addresses "are we compatible" - in a way that goes beyond Sun-sign matching.

This guide gives you the working model. By the end you can open the Astrolas compatibility checker with two real birth dates and know what to look at first.

Synastry does not score relationships. It maps where two people will surprise each other - and where they will repeat what they already knew.

What synastry is (and is not)

Synastry is not "your Sun sign plus their Sun sign equals compatible / incompatible." That is the trivial version found in magazines. Synastry compares every planet in chart A to every planet in chart B - so you might have 30 to 60 inter-chart aspects between two people, each telling a different story.

Synastry also does not predict whether a relationship will work. It describes:

• What energy gets activated when you are around each other
• What you naturally see and admire in each other
• What you struggle to communicate about
• Which parts of each of you the other amplifies, soothes, or triggers

A relationship can have hard synastry and work through it, or have easy synastry and never get off the ground. Synastry shows the wiring, not the outcome.

The first three things to look at

When you open a synastry chart, look for these three things in order:

1. QSun-WMoon contact. Where does one chart's Sun touch the other chart's Moon? This is the deepest signature of "I see you." Sun-Moon aspects (conjunction, trine, sextile, square, opposition) are the strongest indicator that the two people will recognize something fundamental in each other.

2. Venus-Mars contact. Venus is what you find attractive; Mars is what you pursue. When one person's Venus touches the other's Mars, there is a strong magnetic charge. The orb (how exact the angle is) tells you how loud it is.

3. Aspects to angles (Ascendant, Descendant, MC, IC). If one person's planet falls on the other's angles, that planet's energy will be a recurring part of how the relationship operates. Saturn on the Descendant of a partner produces a different relationship than Venus on the Descendant.

The placements that matter most

Beyond the Big Three above, watch for:

• Saturn aspects - the long-term, the lessons, the structures. Saturn-Venus, Saturn-Sun, Saturn-Moon are often present in relationships that endure (and in relationships that crush each other).
• Pluto aspects - transformation, obsession, power dynamics. Pluto-Venus relationships in particular tend to be intense, formative, and not always easy to leave.
• Jupiter aspects - growth, generosity, "you make me feel hopeful." Jupiter contacts to personal planets often correspond to relationships people describe as expansive.
• Moon-Moon aspects - the emotional resonance. Two compatible Moons run on the same emotional clock.

Houses are half the story

A planet in chart A also falls into a specific house of chart B when overlaid. That house tells you where in life this person activates this part of you.

For example: their Mars in your 7th house tends to be a person who comes in as a defining partner or rival. Their Venus in your 10th house is a person who literally raises your public image. Their Saturn in your 4th house is a person who reshapes your sense of home.

Astrolas's compatibility view shows the house overlay automatically.

What synastry cannot tell you

• It cannot tell you whether to date someone.
• It cannot tell you whose fault a fight is.
• It cannot tell you when to leave.

What it can do is give you a vocabulary for what is going on between the two of you, so you can stop guessing about whether the friction is real or imagined. Friction with clear synastric basis is easier to address than friction you cannot name.

Orbs are tighter between two charts

Inside a single natal chart, many astrologers allow 6 to 8 degrees for major aspects. In synastry most people narrow that to about 3 to 5 degrees for planets, and 1 to 3 degrees for contacts to the Ascendant, Descendant, MC, and IC. The reason is volume: with two full charts, a wide orb produces dozens of aspects and they all start to sound important.

A useful habit is to sort the list by orb and read the top ten only. An exact Moon-Venus trine at 0°40' will show up in the relationship almost daily. A Mars-Neptune square at 4°50' may barely register.

Look for the double whammy

The strongest synastry themes appear in both directions. If her Venus trines his Mars and his Venus trines her Mars, the attraction runs both ways and is hard to ignore. If only one direction exists, one person tends to feel the pull more than the other.

The same applies to harder contacts. One Saturn square between the charts is a lesson. Saturn from each person squaring the other's Sun is a structure the relationship has to be built around.

A quick walkthrough

Say Person A has the Moon at 12° Cancer and Person B has the Sun at 10° Capricorn. That is a Sun-Moon opposition with a 2° orb, so it goes near the top of the list.

What it describes: B's sense of purpose sits directly across from A's emotional needs. B may feel A wants more closeness than B has time for; A may feel B is always on the way somewhere else. There is also real attraction in it, because oppositions describe two halves of one axis. Cancer and Capricorn are both about building a stable life, from home and from the world respectively.

Next, check where B's Sun falls in A's houses. If it lands in A's 7th, B shows up in A's life as a partner figure almost by default. If it lands in A's 10th, B may be tied to A's career. That one placement changes how the opposition plays out day to day.

Synastry vs composite charts

Synastry keeps both charts separate and compares them. A composite chart takes the midpoint of each pair of planets and builds a single third chart for the relationship itself. Synastry answers "what does each person trigger in the other?" The composite answers "what does this relationship become as its own thing?" Most astrologers use synastry first and bring in the composite when a couple has already been together for a while.

What to do next

Open the Astrolas compatibility checker and run two real birth dates. Look for any of the Sun-Moon, Venus-Mars, or angles-aspects described above. You will see them or you will not - and either case is meaningful information.

For a step further, read Sun-Moon compatibility: the cheat sheet.