Synastry is the comparison of two birth charts. It sets the positions in one chart against the positions in the other and asks which planets form aspects across the two charts, and which houses of one chart receive the other person’s planets. The charts are calculated separately, and synastry compares the results.
Astrologers use synastry for partners, but also for friends, relatives, and colleagues. Its meanings are interpretive traditions. A strong contact between two charts does not measure compatibility or predict how a relationship will go, and a difficult one does not rule a relationship out. A composite chart, built from the midpoints of two charts, is a different technique.
The synastry calculator needs both birth dates, both local birth times, and both birthplaces. It lists the aspects from the first chart to the second, including contacts between the same planet in each chart, and shows the house overlays in both directions. Enter only birth details that you have permission to use. The comparison is calculated in your browser, and the details are not sent to a server.
Each chart can be opened on its own in the birth chart calculator. The seventh house and the Descendant are the chart areas most often linked to partnership, and the guide to aspects explains how angles and orbs are measured.
In a chart
Suppose one person has Venus at 12° Taurus and the other has the Moon at 14° Taurus. The two positions are 2 degrees apart, which most readers treat as a close conjunction within the orb. If the second person’s house cusps put 12° Taurus inside their seventh house, the overlay reads: the first person’s Venus falls in the second person’s seventh house. Change either birth time by an hour and the aspect may survive while the house overlay moves, because houses depend on the clock time and the angles.
Where you see it on Astrologican
These free tools and pages calculate or list it, so you can see it in a real result.