Synastry Chart: Compare Two Birth Charts

Synastry is the old astrological art of laying one person’s chart over another’s: your Venus against their Mars, your Moon against their Sun, and so on, reading the angles between them. Those angles, the aspects, show where the two of you click, where the attraction comes from and where you keep rubbing each other the wrong way.

Each chart is calculated for that person’s birth date, time and place. In the bi-wheel above, Person A sits on the inner ring and Person B on the outer, and the lines across the middle are the aspects between them.

Start with the personal planets. A Sun–Moon contact shows how one person’s core meets the other’s emotional needs. Venus–Mars is the classic chemistry line. Moon–Moon and Moon–Venus are about comfort and affection, and a planet on the other person’s Ascendant often brings that feeling of instant recognition.

Trines, sextiles and many conjunctions flow easily. Squares and oppositions create friction, and whether that friction helps you grow or just wears you down is up to the two of you. The score gives the Sun, the Moon and the tightest aspects the most weight, so read it as a rough sense of the connection’s overall tone.

Synastry keeps the two charts separate and looks at how they touch. A composite chart merges them into one new chart, built from the midpoints between your planets and theirs, with its own Sun, Moon and Ascendant. It describes the relationship as if it were a third person.

Synastry tells you how you affect each other, and the composite tells you what the relationship is like on its own. Plus gives you both.

The calculator is free with no limit on how many pairs you try. You get the bi-wheel, every aspect between the charts, the balance of easy and hard contacts, and the synastry score.

Plus turns that into a written reading, aspect by aspect, under headings such as emotional bond, communication, attraction, friction and long-term potential. It also adds the composite chart, saving and PDF export.

The Moon changes sign about every two and a half days and the Ascendant about every two hours, and both matter a great deal in a relationship chart. If one of you doesn’t know your birth time, the chart still runs with noon as a stand-in, but treat the Moon and the angles with caution.

In the US the time is usually on the long-form birth certificate; elsewhere, try the hospital record or a baby book.

Contacts that run in both directions weigh more than one-sided ones. If your Venus touches their Mars and their Mars touches your Venus, that is a mutual pattern; if only one side reaches across, often only one side experiences it strongly. That asymmetry explains a large share of the cases where two people describe the same relationship completely differently, and it is directly visible in the bi-wheel because you can see which ring the line starts from.

The classical weighting puts Sun, Moon, Venus, Mars and the Ascendant first, because those describe identity, emotional need, affection, desire and the way you meet each other. Contacts between outer planets are usually generational rather than personal: two people born the same year share them with everyone else born that year, so they say little about this particular pair.

House overlays are the third layer and the one most often skipped. Where their planets fall in your houses says which part of your life they occupy, and it is not symmetric: their Sun may land in your tenth house while your Sun lands in their fourth. Both are true at once, and they describe two quite different experiences of the same relationship.

Synastry cannot tell you whether to stay, and any reading that says so has stopped describing the chart. What it describes is where the ease is and where the friction is, and the friction is not a prediction of failure: plenty of durable relationships are built on squares, and plenty of easy charts describe connections nobody bothered to maintain.

It also cannot compensate for a missing birth time on either side. The Moon, the Ascendant and every house overlay depend on it, and those are the placements a synastry reading leans on hardest. With one time missing you still get the planetary contacts; with both missing, the reading is far less complete than it looks, and it helps to know what is left out.

Finally, a synastry chart describes a pair, not a person. It says nothing about whether either of you is ready for what it describes, which is a question the individual natal charts are closer to answering. Reading the two charts separately first, then the synastry, is the order most working astrologers use for exactly that reason.

A last note on what to do with a difficult synastry chart, because the question comes up more than any other. Hard aspects between two charts describe friction with a specific subject, and knowing the subject is most of the value: a Saturn contact that keeps producing the same argument about commitment is a different problem from a Mars contact that keeps producing the same argument about pace. Neither is a reason to leave and neither resolves itself by being understood. What the chart offers is a name for the recurring conversation, which is generally more useful than being told the relationship is easy.