The comparison of two birth charts: how one person's planets touch the other's.
You look at which aspects form between the two charts and which of each person's houses the other's planets fall into. Order matters: «her planets in his houses» and the reverse are two different readings, and SideraMea keeps them separate rather than blending them.
How to read a synastry
Synastry is read on two levels. The first is the aspects between one person's planets and the other's: a trine between one partner's Moon and the other's Venus is traditionally read as emotional ease, a square between the two Mars as friction in how each of you acts. The second level is the houses: where the other person's planets fall in your chart shows which part of your life they tend to bring to life. Their planets in your seventh house, for example, touch the way you experience partnership.
The contacts that weigh most involve the luminaries, Sun and Moon, and the angles of the chart, the Ascendant and Descendant above all. A single tense aspect does not define a relationship: you read the whole picture.
Why synastry uses natal orbs
The site calculates synastry aspects with natal orbs, 8° for the conjunction and opposition, 6° for the square and trine, 4° for the sextile, rather than the tighter transit orbs. The reason is that two fixed structures are being compared, not a passing influence: both birth charts stay the same for life, so a contact between them is as permanent as the aspects within each chart.
For the same reason a synastry has no dates. It describes how two people meet, not when something happens between them. For the timing of a relationship astrologers turn to other techniques, such as transits to both charts.
Frequently asked questions
Can synastry tell you if a couple is compatible?
Not in the sense of a score. It shows where two people find each other easy and where they challenge each other, but every relationship has both harmonious and tense aspects, and the tense ones are often the ones that keep the attraction alive. Traditionally synastry is a tool for understanding each other better, not for deciding whether a relationship will work.
What is the difference between synastry and a Davison chart?
Synastry keeps the two charts separate and looks at how they touch. A Davison chart instead creates a third chart, calculated for the midpoint between the two birth dates and the two birthplaces, and reads it as a portrait of the relationship itself. They answer different questions: how the two of you interact, and what character the bond itself has.
Can you do synastry without a birth time?
Yes, within limits. The aspects between planets remain reliable, with some leeway for the Moon, which covers about thirteen degrees in a day. Houses and angles, on the other hand, depend on the time: if one person's is missing, you cannot say which of their houses the other's planets fall into, or use their Ascendant.
See it in your own chart
Calculating your birth chart is free: wheel, planets, houses, aspects and a first reading. All you need is your date, time and place of birth.