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A synastry chart is the comparison of two birth charts placed side by side: where do your Suns land relative to each other, what does your Moon say to theirs, and where do Venus and Mars (the planets that govern attraction and pursuit) line up across your two charts? This calculator takes two birth dates, optional times, and birth cities, computes both charts in your browser using the Astronomy Engine ephemeris, and surfaces the most useful structural overlaps for thinking about how you click.
Everything is computed client-side. Your birth data, and your friend's, partner's, ex's, or first-date's birth data, is never stored on our servers. The result lives only in the URL bar (yours to share or yours to keep private).
What synastry actually compares
- Sun and Sun: how the two of you radiate energy. Same-sign suns feel like meeting yourself; opposite suns are the classic mirror dynamic; same-element suns share an instinctive rhythm without much translation.
- Moon and Moon: how the two of you feel safe. Moons matter for day-to-day comfort and conflict repair, often more than Suns over the long run. A shared element here is one of the quieter green flags.
- Venus and Mars: the romance pair. We look at Person A's Venus against Person B's Mars (and vice versa) and pick the closer of the two cross-pairings as the headline. This is where chemistry, not character, tends to show up.
- Shared elements and modalities: counting how many of your placements share a Fire, Earth, Air, or Water element (and Cardinal, Fixed, or Mutable modality) gives you a structural read on how compatible your underlying tempos are.
What this calculator does not do
Synastry traditionally also includes orb-based aspects between every planet across both charts, twelve houses with cross-overlays, composite midpoints, and Davison relationship charts. We deliberately stick to the sign-grain comparison here so the output is something you can actually read in one sitting; aspect-grain synastry is on the roadmap.
The output is descriptive, never deterministic. "Spicy" and "remarkable" are flavor labels, not verdicts. Real relationships are made of choices, not charts.
Examples
Two summer-born friends, Cleveland 1985 and Tokyo 1992
- Person A birth date 1985-09-05
- Person A birth time (optional) 14:30
- Person A birth location Cleveland, Ohio, United States
- Person B birth date 1992-07-14
- Person B birth time (optional)
- Person B birth location Tokyo, Japan
Person A's late-summer Cleveland birth lands a Virgo Sun (Earth, Mutable, precise and observant). Person B's mid-July Tokyo birth lands a Cancer Sun (Water, Cardinal, tender and protective). Earth and Water are a classically harmonious pairing: Earth gives Water shape and steadiness, Water softens Earth's edges. The Sun-Sun blurb reads as compatible, and the side-by-side placements table will show you where the rest of the picture (Moon, Venus, Mars) lands. Person B's birth time is unknown, so their Rising shows as "needs time" but every other comparison still runs cleanly.
Same-sign Suns: two August Leos
- Person A birth date 1988-08-10
- Person A birth time (optional) 09:15
- Person A birth location London, United Kingdom
- Person B birth date 1991-08-14
- Person B birth time (optional) 21:45
- Person B birth location Sydney, Australia
Both born in mid-August, both Leo Suns (Fire, Fixed, warm and generous). The Sun-Sun blurb reads as same_sign: two Leos in a room mean shared appetites for being seen, similar repair patterns, and the recognizable risk of mirror fatigue when you both want the spotlight on the same evening. Where they actually differ tends to show up in the Moon (which moves about 13 degrees per day, so the two charts will diverge there) and in Venus and Mars. The side-by-side table makes those differences explicit.
Frequently asked questions
What exactly does a synastry chart show?
A synastry chart is two birth charts overlaid on the same wheel so you can see where each person's Sun, Moon, and planets fall relative to the other's. The most useful comparisons for everyday self-reflection are sign-overlaps (do your Suns share an element?), the Venus-Mars cross (where attraction shows up), and shared element and modality counts (whether your fundamental tempos line up).
Do we both need to know our birth times?
No. Birth time only affects two things: the Rising sign and the whole-sign houses. Sun, Moon, Venus, Mars, Mercury, and Jupiter are all computed correctly from the date and the city alone, and these are the placements that drive the sun-sun, moon-moon, and Venus-Mars comparisons this calculator highlights. If either of you knows your time, great; if not, the comparison still runs.
Is synastry the same as a horoscope-compatibility quiz?
No. The "are Aries and Cancer compatible?" articles you find online are sun-sign-only generalizations. Synastry uses both full birth charts, so it can tell you something a Sun-vs-Sun matrix can't: that a friction-y Sun pairing might be balanced out by a Moon pairing that's an easy match, or that two compatible Suns might still struggle because their Venus and Mars are in awkward signs.
What does it mean when our Suns are opposite signs?
Opposite-sign Suns sit on the same axis of the zodiac (Aries-Libra, Taurus-Scorpio, Gemini-Sagittarius, etc.). The trait one of you finds easiest is the trait the other has to work at, and vice versa. That's why oppositions often feel magnetic at first: you're seeing your own opposite, mirrored back. Done well, oppositions are growth pairings. Done poorly, they become tug-of-war. Awareness is most of the work.
We're both heavy-Water charts. What does that mean for us?
Sharing a dominant element means you both speak the same instinctive language: in your case, feeling-first, intuitive, often reading each other without speaking. The upside is a comfort that's hard to fake. The risk is that you can amplify each other's emotional weather and isolate from cooler, more structured input. Balance it by keeping Earth or Air friends close, and doing the occasional reality-check.
How accurate is the chart math?
The planet positions come from the Astronomy Engine library, which is accurate to better than one arc-minute over the next century, far more than the 30 degree-per-sign granularity this calculator displays. Coordinates and historical timezones come from Geoapify, so the UTC offset for any past date (including DST rule changes) is handled correctly. The synastry classifications themselves are sign-grain rather than orb-grain by design, see the intro for the rationale.