Our Methodology
We use the VSOP87 planetary theory — the analytical model behind professional astronomical almanacs.
Sun-sign index
This index uses only Sun signs, elements, modalities and whole-sign spacing. It is an interpretive scale, not a measured probability of attraction, marriage or relationship success.
Start with the element value below. Add 5 for cardinal–mutable pairs; subtract 10 for different signs of the same modality. Add 10 for the same sign. Add 8 for signs six places apart, 5 for two places apart, and subtract 5 for three places apart (using the shorter distance around the zodiac). Limit the total to 40–100. These weights are editorial choices, not fitted to relationship outcomes.
- Fire + Fire
- 85
- Fire + Earth
- 50
- Fire + Air
- 90
- Fire + Water
- 45
- Earth + Earth
- 80
- Earth + Air
- 55
- Earth + Water
- 85
- Air + Air
- 75
- Air + Water
- 60
- Water + Water
- 90
78 unique pairs cover all 144 ordered sign selections. Reversing two signs opens the same guide.
VSOP87 Ephemeris
The Gold Standard in Planetary Position Calculations
VSOP87 (Variations Séculaires des Orbites Planétaires) is a mathematical model developed by the Bureau des Longitudes in Paris. It calculates planetary positions with exceptional accuracy by modeling the gravitational interactions between all planets in our solar system.
VSOP87 is an analytical planetary theory developed at the Bureau des Longitudes in Paris and used in professional astronomical almanacs and software. Its theoretical accuracy is around one arc-second for the inner planets and about ten arc-seconds for the outer planets.
How We Calculate Your Chart
Time Conversion
We convert your birth date, time, and timezone to Julian Date—the standard astronomical time format that accounts for leap years, timezone changes, and historical calendar adjustments.
Planetary Positions
Using VSOP87 algorithms, we calculate the precise ecliptic longitude of each planet at your birth moment. This includes the Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.
House Cusps & Angles
Based on your birth location's latitude and local sidereal time, we calculate house cusps, Ascendant, Midheaven, and Vertex using the Placidus house system (the most widely used in Western astrology). We support 10 house systems total.
Aspect Calculation
We identify geometric relationships between planets (conjunctions, oppositions, trines, squares, sextiles) and calculate their orbs for precise aspect interpretation.
Verification Process
We don't just trust our calculations—we verify them.
Supported planetary outputs are compared with astro.com reference values in automated tests using a 0.5° tolerance; this is a test threshold, not a universal accuracy guarantee.
The reference suite includes charts with documented birth times, including historical examples such as Einstein.
Our calculations are updated regularly to account for the latest astronomical constants and corrections.
Accuracy testing
The app uses deterministic VSOP87 calculations and IANA timezone data. Supported outputs are compared with astro.com reference values in automated tests using a 0.5° tolerance.
Related Resources
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