Birth chart compatibility calculator

Comparing two birth charts is called synastry. A synastry chart sets one person's planets against the other's and looks for the angles between them: your Venus on their Sun, your Moon square their Mars. Astrologers read each contact as a way the two of you meet, easily with trines and sextiles, intensely with conjunctions, and with friction you have to work with in squares and oppositions.

Both charts are cast from the real sky with astronomy-engine, in the tropical zodiac. StarMap then checks every planet in one chart against every planet in the other for five aspects, conjunction, opposition, trine, square and sextile, allowing up to eight degrees between the personal planets, the Sun to Mars, and three to five degrees when a slower planet is involved. The contacts are listed tightest first, and a shared Sun, Moon or rising sign is named.

The sky-match score, from 0 to 100, adds those contacts up: conjunctions and trines weigh most, contacts between personal planets count twice, closer aspects count more than wide ones, and a shared Sun, Moon or rising sign adds a little. It measures how much the two charts talk to each other. It is not a verdict on a relationship.

Compare anyway. A chart without a time is cast for noon, so its Moon may be up to about eight degrees out and it has no rising sign; the result says which chart was drawn without one. The Sun and the planets from Mercury to Pluto move too slowly for a missing time to matter much. For the two placements astrologers look at first in a relationship, there are the Venus sign and Mars sign calculators, and for well-known pairs, celebrity couples with their two charts side by side.