Your synastry chart, computed.
Enter two birth moments and get every cross-aspect between the charts with its orb and direction, the luminary contacts that headline a reading, and the house overlays. No account.
Angles between two skies.
Both charts are cast the same way as the single-chart calculator: birthplace geocoded, IANA timezone resolved, local time converted to a true UTC instant, geocentric positions from the VSOP87 planetary theory and a Meeus lunar solution. The engine is checked in CI against seven Rodden-rated reference charts — see the accuracy test set.
Then every body in chart A is compared with every body in chart B. Where the angle between them falls within orb of a recognised aspect, the pair is recorded with its exact separation and whether the two bodies are closing that angle or opening it. Sun and Moon get the wider luminary orbs, as in natal work.
| Aspect | Angle | Orb | Luminary orb |
|---|---|---|---|
| ☌ conjunction | 0° | 8° | 10° |
| ⚹ sextile | 60° | 4° | 6° |
| □ square | 90° | 7° | 9° |
| △ trine | 120° | 7° | 9° |
| ☍ opposition | 180° | 8° | 10° |
House overlays use whole-sign houses: each person’s Rising sign is their first house, and the other person’s Sun, Moon, Venus, Mars and Saturn are counted into those twelve signs. The written paid synastry reading works from exactly this table and adds the argument; the sign-pair compatibility pages are the twelve-by-twelve approximation for readers without a birth time.
This overlay compares two natal charts. A daily horoscope is a tropical one-sign column, not a two-chart overlay. Sky now is the live tropical sky for this instant, not either person's birth. Moon today is the current Moon's sign and phase — not how today's Moon sits in either chart.
This overlay compares two natal charts. The 2026 Mercury retrograde calendar is dated station and shadow windows for the sky, not how Mercury sits in either natal. The 2026 eclipse calendar is four dated solar and lunar maxima, not eclipse transits to either chart.
A two-chart overlay is not a natal pipeline. How Charting Stars calculates birth charts is the natal pipeline that casts each birth instant: VSOP87, IANA historical timezones, and the Lahiri/Vimshottari layer — not the angles between two people. Einstein's birth chart in Hellenistic, Vedic, and BaZi is one 1879 natal read three ways — not a two-chart relationship overlay.
Cross-aspects are not a one-sign trait page. The zodiac sign encyclopedia lists one person's standing tropical personality — not the angles between two charts.
Common questions.
What is a synastry chart?
A synastry chart lays one person's natal chart over another's and reads the angles between them: Person A's Moon square Person B's Saturn, A's Venus conjunct B's Sun, and so on. Those cross-aspects, plus where each person's planets fall in the other's houses, are the raw material of relationship astrology.
How is synastry different from sign compatibility?
Sign compatibility compares two Sun signs — twelve by twelve, the same answer for everyone born in those months. Synastry uses the exact birth time and place of both people, so it compares actual planetary positions to the degree and is different for every pair.
Which aspects and orbs does this calculator use?
The five Ptolemaic aspects — conjunction, sextile, square, trine, opposition — with the same orbs the natal engine uses, widened when the Sun or Moon is involved. Each cross-aspect is marked applying (the angle is closing) or separating (it is opening) from the two bodies' rates of motion.
What are house overlays?
Each person's Sun, Moon, Venus, Mars and Saturn is placed into the other person's whole-sign houses, counted from that person's Rising sign. A's Venus in B's seventh house is read very differently from A's Venus in B's twelfth. Both birth times are needed for this.
Do I need an account or an exact birth time?
No account. An exact birth time matters: the Moon moves about half a degree an hour and the Rising sign changes every two hours, so an approximate time shifts both the tight luminary contacts and every house overlay. If someone's time is unknown, tick “I don't know my birth time” for them: their chart is cast for local noon, and their Rising, the house overlays into their chart and every aspect to their Moon are left out rather than guessed.