What synastry is
Synastry is the modern astrological method of laying one person’s birth chart over another’s and looking at the angles, called aspects, between the planets of the two charts. Where a Sun sign comparison asks only whether two signs get on, synastry asks how every planet in one chart relates to every planet in the other. It is used for partners, friends, family and colleagues, and most people use it to start a conversation rather than to reach a verdict.
What the tool does
Enter the date, time and place of birth for two people. The tool finds the positions of the Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto and the mean North Node in the tropical zodiac for each. It then compares every planet in the first chart with every planet in the second and lists each aspect within orb, tightest first, with a short plain reading for the closest twelve.
The five aspects
An aspect is a particular angle between two points on the zodiac circle.
- Conjunction (0 degrees): two planets in the same place. Their themes are felt together and strongly.
- Sextile (60 degrees): a supportive link that works best when both people make a small effort.
- Square (90 degrees): a link of friction. It can bring tension and also the energy to grow.
- Trine (120 degrees): an easy, flowing link that asks little of either person.
- Opposition (180 degrees): a push and pull across the two charts, often magnetic and in need of balance.
The orb is how far from the exact angle the planets can be and still count. We use the same orbs as our birth chart tool: 8 degrees for the conjunction, opposition and trine, 7 for the square and 5 for the sextile when the Sun or Moon is involved, and 6, 6, 6, 5 and 4 otherwise, with 5 at most for the node.
How to read it
The tool counts the flowing links (trines and sextiles), the tense links (squares and oppositions) and the conjunctions, but gives no score. A couple with many flowing links is not bound to be happy, and one with many tense links is not doomed. The links involving the Sun and Moon tend to be the ones people recognise first, and Venus and Mars links are traditionally watched in romance. A tight conjunction or opposition to Saturn, Pluto or the node is often described as a link that feels weighty or fated, which many people find meaningful and others find simply intense.
If you do not know a birth time
Planets other than the Moon barely move in a day, so the tool still works without a time. The Moon moves about thirteen degrees a day, so without a time it is placed at noon local time, and any link involving it is marked as approximate.
What synastry cannot do
It cannot say whether a relationship will last, succeed or end, and no aspect is a warning. Real relationships depend on how two people treat each other. Astrologers also look at composite charts, house overlays and the rising signs, which this tool does not cover. Astrology is a tradition of reflection, not a proven method of prediction.
Frequently asked questions
↳ Do we both need an exact birth time?
No. The planets other than the Moon barely move in a day, so the result is reliable without a time. Any link involving the Moon is marked as approximate when a time is missing.
↳ Is a chart with more tense aspects a worse match?
Not in this tradition. Squares and oppositions are read as friction and spark, and many long partnerships have plenty of them. The tool counts them but gives no score.
↳ What is the difference between synastry and Sun sign compatibility?
Sun sign compatibility compares one placement in each person. Synastry compares every planet in one chart with every planet in the other, so it describes a pair far more specifically.
↳ Which zodiac and node does the tool use?
The tropical zodiac of Western astrology and the mean North Node. Aspects are the same in the sidereal zodiac, because every planet moves by the same amount.
↳ Can synastry tell me if we should stay together?
No. It can give two people something to talk about, but a decision about a relationship belongs to the people in it. If you are worried about your well-being in a relationship, please speak to someone you trust.