The two-person reading
Your month and day place you in one of the twelve tropical zodiac Sun signs, and in one of that sign’s three faces — the ten-degree thirds older astrologers called decans. The year is collected so the form can reject impossible dates, and is then ignored: a Sun sign depends on the day of the year, not on which year it was.
We then read the angle standing between the two signs: 0°, 30°, 60°, 90°, 120°, 150° or 180°. Those seven angles are the classical aspects, each carries its own long-standing reading, and each is set out in full here. The score weights that angle and then adjusts it by the angle between your two faces — which is why two people born under the same sign five weeks apart are not read identically here.
Why it is built on the angle alone
The first version of this engine scored a pairing on three terms — the shared element, the shared quality, and the distance between the signs — and presented them as three independent judgements.
They are not independent. On a twelve-sign wheel the distance between two signs determines the other two: signs four apart always share an element, signs three apart always share a quality. So the three terms were one term counted three times. The consequence was not subtle. The engine could produce only seven distinct scores, all of them between 77 and 90; one of its four labels was unreachable by any real pair of birthdays; and a visitor who tried two different friends was frequently shown the same number.
Everything a compatibility score can honestly say is already contained in the angle, so the rebuilt engine says it once. The variation that used to be faked by triple-counting now comes from the faces, which is the one genuine source of within-sign difference the birth date makes available.
What the number means, and what it does not
The displayed score is a friendly summary of that mix. It is not a probability, a measurement or a forecast. Scores run from the mid-thirties to the low nineties, and the distribution is deliberately uneven, because the classical aspects genuinely fall into a harmonious family and a hard one with a gap between them. Smoothing that gap would mean inventing noise.
A low score is not a worse pairing. It means less of the relationship runs on shared instinct, so more of it has to be said out loud. Six band labels exist and every one of them is reachable — that is checked by a test that evaluates all 133,225 combinations of calendar days, because the previous engine shipped a label no birthday could ever produce and every unit test passed throughout.
The result is also stable and order-independent: the same two dates always return the same reading, and swapping which birthday went in the first box changes nothing except the two names. A fortune that changes on refresh is not a fortune.
The second reading
Below the score and its five cards the same two dates are asked nine more questions. They are there because the angle is one quantity and a thin one: two people born under the same two signs a decade apart share an aspect and share almost nothing below it. Every block says on its own face whether the number came out of the sky or out of a tradition, and the ones that came out of a tradition carry the year that tradition was invented.
What is computed. The midpoint is the halfway point of the shorter arc between the two Sun longitudes, given to the arcminute — a third point with a position of its own, not an average of two opinions. Both birth moon phases come from the real Sun–Moon elongation. The Black Moon is the mean lunar apogee, the far end of the Moon's elliptical orbit; the nodes are where that orbit crosses the Sun's path, which is why eclipses happen there. The Chinese year element comes from the computed lunar new year. The sky today is the Moon's position at the moment the page is open, and it is the only part of the reading that changes.
What is assigned, and when. Reading a midpoint as the relationship itself is Alfred Witte's Hamburg School, 1920s. The temperament reading of a birth phase is Dane Rudhyar, 1967. The apogee's meaning is French and American astrology from the 1970s onward — Sepharial's 1918 "Black Moon" was something else. The nodes are ancient as Rahu and Ketu; their past-life reading in the West arrives through Alan Leo and Dane Rudhyar and is popularised by Martin Schulman in 1975. The life path is not astronomy at all: it adds up the digits of a date written in a calendar Europe adopted in 1582, by a method L. Dow Balliett began around 1900 and Juno Jordan codified in the 1960s. The birth card enters wide use with Angeles Arrien's The Tarot Handbook, 1987.
Two things it does not do. It does not blend the Greek four elements with the Chinese five — they are unrelated systems, so they are shown side by side and never added into one score. And the three questions at the end are not generated by a model. Nothing on this site calls out to anything. They are written, and which three you are shown is decided by the angle, the tempo gap and your two dates, so the same pair always gets the same three.
Putting the same date in both boxes is a supported reading rather than a mistake. The score is withdrawn when you do — there is no angle between a point and itself, and printing a flattering number for one would be measuring zero — and everything below it, which is about one date at a time, still has something to say.
The other six tools
The moon phase pairing computes one quantity — the angle between the Sun and the Moon — and presents it three ways: the name of the phase, the shape of the disc, and what two of them make side by side. The Moon comes from the complete sixty-term lunar longitude series, accurate to about ten arcseconds. No birth time is asked for, so a date near a boundary is flagged rather than guessed at.
The Chinese zodiac computes Chinese New Year rather than looking it up: month eleven is the lunar month containing the December solstice, the new year is month one two months later, and a leap month is found the way the calendar defines it — the month with no major solar term in it. Everything is evaluated at UTC+8. That is the difference between being right and being wrong for everybody born in January and most of February.
The sidereal converter subtracts the Lahiri ayanamsa from the tropical positions to give the Vedic sign, the Moon's rashi and its nakshatra with its pada. Which ayanamsa is named on the result, because that is the only real choice the tool makes.
The group reading applies the same seven angles to every pair among up to eight people, then reports what only a group has: an average, an absence and a distribution.
The rune draw and the I Ching cast are the two that compute nothing astronomical, and they are the two that say the most about themselves. Both derive the draw from the date rather than shuffling or throwing, because a fortune that changes on refresh cannot be checked; both name what that departs from; and the rune pages are explicit that the blank rune appears on no historical object and that the divinatory meanings in circulation are Ralph Blum's, 1982, not the second century.
None of the six invents a new engine. The four astronomical ones are the same two published series — Meeus's solar and lunar theory — asked different questions, which is why they can be checked: the tests compare them to real new moons, real solstices and twenty-nine published Chinese New Year dates rather than to themselves.
What we deliberately leave out
The two-person reading is not synastry. Real synastry compares two complete charts — every body in one against every body in the other, plus the angles and the houses. This tool compares two Sun signs and their faces, from two dates, with no birth time and no birthplace, and it never presents itself as more than that.
Nothing here is a birth chart either, and deliberately so. A birth chart needs an exact birth time and a birthplace and answers a different question; Cosmic Meridian calculates one, in the browser, on the same privacy terms — a different tool with different inputs, not an extension of this score. Building a second chart calculator here would have been two sites under one publisher competing on one query.
No tool on this site predicts an outcome. None assesses a person. None offers medical, mental-health, legal, financial or relationship-safety advice. Nothing here is a substitute for a qualified professional.
Where the calculation happens
In your browser, on this page, and nowhere else. There is no server to send anything to: the site is static files and the astronomy ships with the page.
Your birth dates, birth times and birthplaces are not transmitted, stored, written to a cookie, placed in the URL or kept between visits. The only events recorded are that a calculation started and that it finished — not the inputs, not the result. The privacy notice sets this out in full.
How to use the result
Notice what feels recognisable. Question what does not. The useful part is the conversation you have after reading — not agreement with every sentence. A reading of this kind works the way a good question works: it is valuable because of what you say back to it.