You two,
to the degree.
Enter two birth dates to see the exact angles between your Sun, Mercury, Venus and Mars and theirs, and which retrogrades through 2030 land on both charts. Free, computed in your browser from the JPL DE421 ephemeris.
Pick the offset of the place and season of each birth, for example UTC−05:00 for New York in winter. Without a time, noon is used and pairs near the edge of an orb are marked ≈.
What the angles mean
Each person's Sun, Mercury, Venus and Mars sit at a degree of the ecliptic. The calculator measures the shortest arc between each of your four degrees and each of theirs, sixteen pairs, and names the classical aspect when the arc is close to 0°, 60°, 90°, 120° or 180°. The tightest one is the pair most astrologers would read first.
The second part is what no zodiac-sign compatibility chart can tell you: when Mercury, Venus or Mars, during a retrograde, crosses a degree in both charts in the same cycle. Those weeks are the ones tradition treats as shared reviews.
Angles are measured between apparent geocentric ecliptic longitudes from JPL DE421, the same ephemeris as every date in Cazimi. Orbs: 8° for conjunction and opposition, 6° for square and trine, 4° for sextile. No scores and no sign readings: only how far apart the degrees are.
The data
Positions come from a daily DE421 table of the Sun, Mercury, Venus and Mars, 1900–2031, the same file the Cazimi iPhone app uses, so the web and the app show the same degrees; retrograde cycles come from /v1/planets/cycles.json. Everything runs in your browser and nothing you enter is sent anywhere.
Questions
How is chart compatibility calculated here?
From the exact degrees of the Sun, Mercury, Venus and Mars at each birth, sixteen pairs in all, using classical orbs: 8° for conjunction and opposition, 6° for square and trine, 4° for sextile. There is no score; for example, for births on May 14, 1990 at 08:30 UTC and March 1, 1988 at 18:45 UTC, Venus at 11°31′ Aries and Mars at 5°38′ Capricorn are 95°53′ apart, a square 5°53′ from exact.
What if we don't know our birth times?
Leave the time empty and noon is used, so a birth on that day is computed as September 3, 1992 at 12:00 UTC. The Sun, Venus and Mars move about a degree a day or less and Mercury up to two, so only pairs near the edge of an orb can change; those are marked ≈.
Is this zodiac sign compatibility?
No. Two people with the same Sun sign can be 1° or 29° apart, and that changes the aspect: births on April 20, 1990 at 12:00 UTC and May 20, 1990 at 12:00 UTC both have the Sun in Taurus, 29°04′ apart. The calculator works from degrees, not signs, and gives no horoscope or score.