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Synastry Chart: Compare Two Birth Charts
A synastry chart compares two birth charts and lists the angles, called aspects, between one person's planets and the other person's planets. Enter the birth details of two people. The calculator compares both charts and lists the aspects between them, strongest first.
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What a synastry chart is
Synastry is the astrology of relationships. Instead of looking at one birth chart on its own, it compares two, and looks at the angles between one person's planets and the other person's planets. Those angles are called aspects.
Synastry is more detailed than comparing sun signs. Two people with sun signs that are said to clash can still have many easy connections between their Moons, Venus and Mars, and two people with matching sun signs can have very few. This kind of birth chart compatibility looks at the whole picture.
How the comparison is calculated
First, we calculate both birth charts from the birth date, time and place, using the open-source astronomy-engine library and the tropical zodiac. Then we compare every planet in one chart with every planet in the other, from the Sun to Pluto, plus the ascendant if both birth times are known.
We look for the five major aspects. A conjunction is 0 degrees apart, with an allowed orb of 8 degrees. An opposition is 180 degrees, orb 8. A trine is 120 degrees, orb 7. A square is 90 degrees, orb 6. A sextile is 60 degrees, orb 4. When the Sun or Moon is involved, the orb grows by 1 degree, because astrologers traditionally give the two lights more room.
- Trines and sextiles are counted as flowing.
- Squares and oppositions are counted as tense.
- Conjunctions intensify whatever they touch.
How the synastry chart works: rules and sources
Where astrologers disagree, we pick one rule and say which. These are the rules behind your result:
| Rule | Value we use | Where it comes from |
|---|---|---|
| Aspects | Conjunction, opposition, trine, square, sextile | The five major aspects of classical astrology |
| Orbs | Conjunction and opposition 8, trine 7, square 6, sextile 4 degrees, plus 1 with Sun or Moon | Common orbs in modern practice; our choice |
| Weighting | Personal points (Sun, Moon, Venus, Mars, Ascendant) highest, outer planets lowest, tighter aspects higher | Our own rule, stated so you can check it |
| Scores | No percentage | No traditional method produces one |
| Unknown birth times | Ascendants left out | Our convention |
How the aspects are weighted
Not every aspect counts the same. We give more weight to the Sun, Moon, Venus, Mars and the ascendant, because these are the most personal points in a chart. Mercury counts a little less, Jupiter and Saturn less again, and the slow outer planets only lightly, since they describe a whole generation more than an individual.
An aspect also counts more the tighter it is. A trine that is exact counts fully, and one at the edge of its orb counts very little. From all this, the result shows three simple measures: how much flows easily, how much creates friction, and how strongly the two charts touch overall.
Reading your synastry
Tense aspects are not bad news. In synastry they often go along with attraction and growth, because they are where two people challenge each other. Flowing aspects make things feel easy, but a relationship with only easy contacts can also feel flat.
Your free result lists every aspect between the two charts, with a description of the main connections. The personal reading for $29 is about one chart, yours, in full.
Birth times and limits
If one or both birth times are unknown, we leave out the ascendant and use noon positions, which can move the Moon by several degrees. The aspects between the slower planets stay reliable.
Synastry describes patterns of connection between two charts. It does not decide whether a relationship will work. That depends on the people in it.
Questions
Frequently asked questions
What is a synastry chart?
A synastry chart compares two birth charts and shows the aspects between one person's planets and the other's. It is used in astrology to look at relationships of any kind.
How do I read a synastry chart?
Start with the strongest aspects, which we list first. Pay most attention to aspects involving the Sun, Moon, Venus, Mars and ascendant. Flowing aspects show ease, tense ones show friction and attraction.
Is birth chart compatibility the same as synastry?
They use the same comparison. Synastry lists every aspect between two charts. Our birth chart compatibility calculator turns the same data into a shorter report, placement by placement. Both are more detailed than comparing sun signs.
Do I need both birth times?
Not necessarily. Without birth times we leave out the ascendant and use noon positions. Most aspects between planets are still accurate, though the Moon can move by several degrees during the day.
What orbs do you use?
Conjunction and opposition 8 degrees, trine 7, square 6 and sextile 4. We add 1 degree when the Sun or Moon is involved.
Does a lot of tension mean we are not compatible?
No. Tense aspects often come with strong attraction and are where two people grow. Astrologers look at the balance, not at a single number.
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