Methodology

Each person's chart is cast by this site's own engine, built on the open-source Sakai ephemeris library (MIT licence). It gives the Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto, the true (north) node, the Ascendant and the Midheaven, in the tropical zodiac: signs start at the March equinox.

The birth date and time are turned into a moment in universal time with the time zone rules in force at the birthplace on that day, including daylight saving and wartime clocks. The engine then measures each point as a longitude along the ecliptic.

On four test charts the engine's planet positions agree with NASA JPL Horizons to within about 0.03 degrees, and the Ascendant and Midheaven to within 0.01 degrees. It is valid from 1900 to 2100; a birth date outside those years is refused, not guessed.

The calculation is rarely the weak point. A birth time that is off by four minutes moves the Ascendant by about one degree, and a wrong birthplace or time zone shifts everything. The result is only as good as the birth data entered.

Houses are Placidus. Near the poles Placidus cannot be worked out, so the engine uses Porphyry houses there and the result says so. The composite chart uses Equal houses, counted from its own Ascendant, and labels them as such.

An aspect is counted when the angle between two points is within its orb of 0, 60, 90, 120 or 180 degrees: conjunction, sextile, square, trine, opposition. Each body has its own orb: 8 degrees for the Sun and Moon, 6 for Mercury, Venus and Mars, 5 for Jupiter and Saturn, 3 for Uranus, Neptune and Pluto. The north node, the Ascendant and the Midheaven take 5.

A pair of points allows the average of its two orbs, and a sextile three quarters of that. Two points form one aspect at most, because these orbs never overlap. Trines and sextiles are called harmonious, squares and oppositions challenging; a conjunction is harmonious unless Saturn, Uranus, Neptune or Pluto takes part, or both points are Mars.

A contact counts when at least one of its two points is personal: the Sun, Moon, Mercury, Venus, Mars, the Ascendant or the Midheaven. Contacts between slow planets are shared by everyone born in the same few years, so they say nothing about this couple.

The strongest links are ranked by how much the two points weigh and how close the aspect is to exact. The Sun, Moon, Venus and Mars weigh most; the outer planets least.

Tick that you do not know the time and that person's chart is cast for noon, with no Ascendant, no Midheaven and no houses. The Moon moves about 13 degrees in a day, so an aspect that involves a fast point is marked as depending on the hour when it does not hold at 00:00, at noon and at 23:59 of that day.

The composite chart is the midpoint method: each point is halfway between the two people's matching points, taken along the shorter arc of the circle. The Ascendant and Midheaven midpoints are corrected when they fall on the far side. Houses are Equal, counted from the composite Ascendant. With an unknown birth time the page shows the signs a midpoint can fall in instead of a single degree.

A synastry chart has no total. Any percentage would be a weighting invented for the display, and it would suggest a verdict the chart cannot give. The site lists the contacts, names each as harmonious or challenging, and says in plain words what the tradition says it means.

Each pair page says how often two random birthdays make each of its aspects. The figure was counted once with the engine: 600 random birthdays from 1950 to 2010, every ordered pair of them (359,400 pairs), each aspect tested with the orbs above. It says how common a contact is, not how common a relationship is.

Astrology is not scientifically validated. This site calculates where the planets stood and states what the astrological tradition says a contact means. Read it as a symbolic language for talking about a relationship, not as a prediction and not as advice.

Birthplaces are searched in a list of populated places of 5,000 people or more, from GeoNames (CC BY 4.0), which carries each place's coordinates and time zone. A smaller place is not in the list: pick the nearest larger one. Historical time zone rules come from the IANA time zone database; before 1970 a few merged zones can be off by their old local rule.

If a position, an orb or a sentence is wrong, write to support@pyxl.pro with the page and the birth data. A confirmed correction changes the data, the page and its date together.