Sun-Moon Compatibility Calculator
Discover astrological compatibility between two people based on their Sun sign (birth date) and Moon sign (approximate lunar position). Enter both birth dates to calculate elemental harmony, aspect angles, and an overall compatibility score.
Person 1
Person 2
Formula
Sun Sign: Determined by tropical zodiac date ranges (e.g., Aries: Mar 21 – Apr 19).
Moon Sign: Approximated using the Julian Day Number (JDN) and mean sidereal lunar period:
JDN = day + ⌊(153m+2)/5⌋ + 365y + ⌊y/4⌋ − ⌊y/100⌋ + ⌊y/400⌋ − 32045
Moon Longitude = ((JDN − 2451545.0) × (360 / 27.32166) + 218.3165°) mod 360°
Moon Sign Index = ⌊Longitude / 30⌋
Aspect Angle: Shortest arc between two sign midpoints (sign index × 30° + 15°). Orb tolerance ±10°.
Overall Score:
Base = (P1Sun→P2Moon × 0.30) + (P1Moon→P2Sun × 0.30) + (Moon-Moon × 0.20) + (Sun-Sun × 0.20)
Element Bonus = ((SunElem + MoonElem) / 2 − 50) × 0.10
Quality Bonus = (SunQuality − 50) × 0.05
Final Score = clamp(Base + ElementBonus + QualityBonus, 0, 100)
Assumptions & References
- Sun signs use the standard tropical zodiac (Western astrology) date boundaries.
- Moon sign calculation uses the mean lunar sidereal period of 27.32166 days and the J2000.0 epoch reference longitude of 218.3165° (Meeus, Astronomical Algorithms, 2nd ed.).
- Moon sign accuracy is ±1 sign near sign boundaries due to the simplified mean-motion model (no perturbation terms). For precise Moon signs, a full ephemeris is required.
- Aspect scores are based on classical Western astrological tradition: Trine (120°) and Conjunction (0°) are most harmonious; Square (90°) and Quincunx (150°) are challenging.
- Elemental compatibility follows the classical four-element system: Fire/Air and Earth/Water are naturally compatible pairs.
- This calculator is for entertainment purposes only and does not constitute professional astrological advice.