Zodiac / Compare
Synastry Chart: Compare Two Birth Charts
Create a two-person synastry chart with a bi-wheel, inter-aspects, traditional themes and transparent contact counts. Birth times and cities are optional.
Synastry Chart: Compare Two Birth Charts: Compares the planets in two real birth charts using five major aspects. See the closest contacts, a bi-wheel and counts of traditionally harmonious and challenging contacts, with conjunctions separate. Invite someone to add their own data without putting birth details in the link. A synastry chart compares two charts; it does not create a composite chart, which uses midpoints. Runs 100% locally in your browser with zero server file uploads.
- Category
- Astrology & fortune
- Runs
- In your browser
- Cost
- Free · no sign-up
- Availability
- Ready to use
Runs entirely in your browser
Compare two birth charts through their closest planetary contacts. Explore the geometry and its traditional themes, without a compatibility percentage.
Birth time and city are optional. Blank times use local noon in the selected time zone, initially your browser’s zone. Choose a city or change the time zone under Advanced. The Moon can change sign within a day.
Compare together
Invite links reveal only computed planet signs, plus a first name if you opt in. They never contain birth dates, times, places or exact degrees. Anyone with the link can read those signs.
Astrology is a cultural practice, not a scientific method for measuring relationships. All calculations run in your browser. City data: GeoNames, CC BY 4.0. GeoNames attribution
How synastry aspects are computed
Tropical, geocentric apparent ecliptic longitudes are calculated for the Sun through Pluto and the mean lunar node. The ASC and MC also depend on place and are included only when both times and places are supplied. Astronomy Engine (https://github.com/cosinekitty/astronomy), an MIT-licensed library, provides the astronomical calculations.
For each A–B pair, reduce the absolute longitude difference to the shorter arc from 0° to 180°. Compare that angle with 0°, 60°, 90°, 120° and 180°. Accept the contact when its orb meets the published 6°/4° rule, with 3° for all sextiles. Each directed A–B pair counts once; A Sun–B Moon and A Moon–B Sun are distinct contacts.
Invite comparisons use whole-sign distance: 0, 2, 3, 4 and 6 signs apart correspond to the five major aspects. They do not supply exact orbs or rank closeness. GeoNames (https://www.geonames.org/) provides city and time-zone data under CC BY 4.0. All calculations use the browser.
Traditional aspect meanings
This table summarises conventional symbolism in original wording. The historical account of aspects in Ptolemy's Tetrabiblos, Book I (https://penelope.uchicago.edu/Thayer/E/Roman/Texts/Ptolemy/Tetrabiblos/1A*.html), records a tradition rather than scientific evidence. Modern planet-pair themes extend that tradition.
| Aspect | Exact angle | Traditional theme | Count group |
|---|---|---|---|
| Conjunction | 0° | Combination and emphasis | Separate |
| Sextile | 60° | Cooperation and exchange | Harmonious |
| Square | 90° | Friction and contrast | Challenging |
| Trine | 120° | Ease and shared expression | Harmonious |
| Opposition | 180° | Polarity and contrasting perspectives | Challenging |
The result groups Sun–Moon, Venus–Mars, Moon–Moon, Sun–Sun, Venus–Venus, Mercury–Mercury and Saturn contacts by aspect type. Repeated contacts in a category produce one theme row per type. These neutral themes are not judgements about either person.
How to use it
- Enter both birth dates, with optional times, cities and first names.
- Compare planetary contacts and traditional themes, with visible orbs and counts.
- Save a card or invite someone using computed planet signs only.
Privacy & limitations
Calculation happens in your browser. Birth dates, times and places are never included in invite links or share cards. Invite links contain planet signs and a first name only if you opt in. Optional names otherwise stay in this browser; card names require a separate opt-in.
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Frequently asked questions
What is synastry?
Synastry compares the positions of planets in two birth charts. An inter-aspect links a planet in person A's chart with one in person B's. Astrological traditions attach symbolic themes to those angles. This tool compares charts; it does not calculate a midpoint composite chart.
Do we both need birth times?
No. A blank birth time uses local noon in the chosen time zone. Without a city, the initial zone is your browser's; change it under Advanced if needed. The Moon can move enough within a day to change contacts. ASC and MC appear only with both birth times and both places. Houses are not compared.
What orbs does this synastry calculator use?
Conjunctions, squares, trines and oppositions allow 6° when either point is the Sun or Moon, and 4° otherwise. Sextiles always allow 3°. The orb is the absolute difference between the separation and the exact aspect angle. Contacts are sorted by orb, with the closest ten marked.
What do harmonious and challenging mean?
In tradition, trines and sextiles suggest ease or cooperation; squares and oppositions suggest friction or polarity. Conjunctions emphasise the themes of both planets and are counted separately. These labels describe cultural symbolism, not relationship quality. Counts are not percentages or predictions.
Why isn't the invite link as exact as entering both birth details?
To protect birth details, a link carries only each planet's sign and an optional first name. Exact degrees, dates, times, places and angles are absent. The recipient sees approximate whole-sign aspects, using the shortest distance between signs. They can share their own link back; for a full comparison, enter both birth details together in one browser.
Is synastry scientific?
No. Planet positions and angular separations can be calculated astronomically, but astrological interpretations and relationship compatibility are not scientifically established. This is a cultural and entertainment tool, with no predictions or relationship advice.
Free tool · runs in your browser · no account required