From your birth time to Universal Time
Everything starts with one instant. Your local birth date and time are converted to Universal Time (UT) using the time zone of your birthplace from the IANA time zone database, the same database built into your browser and operating system. It includes each place's historical rules: daylight saving in the year you were born, wartime clock changes, and local mean time before standard time zones were adopted. The offset used is always shown next to your birth place.
Two edge cases are flagged. When clocks go forward, one hour never happens; a time inside it is marked and the offset before the change is used. When clocks go back, one hour happens twice; the earlier, daylight instant is used by default, and you can switch to the later one. For births before 1970, when records in some places are less certain, the form notes it, and you can enter a fixed UTC offset or another time zone instead.
Planet positions
Positions of the Sun, Moon and planets come from Astronomy Engine 2.1.19, an open-source (MIT license) library built for arcminute accuracy, which the tests below check against NASA JPL. For every instant the site computes the apparent geocentric right ascension and declination (equator and equinox of date), which astrocartography uses, and the apparent ecliptic longitude and latitude, which the zodiac uses. No parallax or refraction is applied, following the standard astrocartography convention.
Chiron comes from a table of apparent positions every five days from 1900 to 2100 published by NASA JPL's Horizons system, interpolated with a four-point cubic; outside those years Chiron is left out. The lunar nodes use the osculating (true) node from the Moon's orbit, with the mean node as an option. Black Moon Lilith is the mean lunar apogee, projected onto the ecliptic.
Tests
The site's automated tests compare every planet, the Sun and the Moon against reference positions from JPL Horizons at six dates between 1879 and 2026, and Chiron at the dates from 1900 on. All agree to within 0.5 arcminutes; the largest difference measured is 0.24 arcminutes. House cusps, the Ascendant, Midheaven and Vertex are checked against the Swiss Ephemeris, an independent astrology library, for eight charts on five continents, to within 0.01°. The mean node and Black Moon Lilith agree with it to within 0.01°, and the true node to within 0.05°. Time conversion is tested on historical cases, including 1969 British Standard Time, India's half-hour offset, Nepal's 45-minute offset and New York's local mean time before 1883.
How lines and parans are drawn
For each body, the MC line is the meridian where the body's right ascension equals the local sidereal time; the IC line is the meridian opposite. The ASC and DSC lines are the places where the body is on the geometric horizon: at each latitude, the hour angle where the cosine equals minus the tangent of the latitude times the tangent of the declination. The tests confirm, through an independent calculation of altitude and azimuth, that every sampled point of every line puts its body exactly on the horizon or meridian.
Parans are the latitudes where one body's line crosses another's. The tests confirm that at every paran both bodies are angular at the same moment.
Distances and strength
Distance from a place to a line is the shortest great-circle distance on a sphere of mean Earth radius (6,371 km). Strength bands follow common astrocartography practice and are this site's stated convention: strong within 250 km, moderate to 600 km, mild to 1,000 km. Rankings weight each line by a smooth curve that falls to about half at 450 km. Paran latitudes count within about 160 km, at a lower weight than lines.
Houses and charts
Houses use Placidus by default, calculated by iterating each cusp's semi-arc, with Whole Sign, Equal and Porphyry available. Placidus cannot be calculated inside the polar circles, so Porphyry is used there and the chart says so. Charts use the tropical zodiac. Aspects use common orbs: 8° for conjunctions and oppositions, 7° for squares and trines, 5° for sextiles, 2° wider when the Sun or Moon is involved and narrower for points.
Timing: sign dates, returns, transits and solar returns
Several tools ask when something happened rather than where a planet was. They all work the same way: step through time at a pace no body can outrun (a day for Venus and Mars, two hours for the Moon, longer for the slow planets), then narrow each crossing by halving the interval until it is known to the minute. The dates a planet entered and left its sign, Venus, Mars, Chiron, node and Lilith returns, and the moment of each solar return are all found this way.
The results are checked against the Swiss Ephemeris. Venus and Mars sign changes agree to within about two minutes, solar returns to within two minutes, the nodes' and Lilith's sign changes to within three hours (their formulas differ slightly between programs, and the slow true node wobbles), and mean node returns to within three hours. The transit search is checked against the separate Saturn return calculation, and every exact transit is tested to be within 0.01° of exact.
Transits use tight orbs so the list stays meaningful: 1° for the Moon and the node, 1.5° for the Sun, Mercury, Venus and Chiron, 2° for Mars, Uranus, Neptune and Pluto, and 2.5° for Jupiter and Saturn. The year ahead lists exact aspects from Jupiter, Saturn, Uranus, Neptune, Pluto and Chiron to your natal planets, points and (with a birth time) angles.
Composite and solar return charts
The composite chart places each planet and point at the midpoint of the two people's positions, on the shorter arc. The composite Midheaven is the midpoint of the two Midheavens, and the Ascendant and houses are derived from it at a latitude: where the couple lives, if entered, or halfway between the two birthplaces' latitudes. When either birth time is missing, the composite angles and houses are left out.
A solar return is cast for the moment the Sun's apparent longitude equals its value at birth, found in a window of three days either side of the birthday, for the place you choose. Its houses follow the same house system settings as the birth chart. The "where to spend your birthday" ranking scores cities with the solar return's lines and parans exactly as Best places scores your birth chart.
Places and maps
Birth places and city searches use GeoNames (CC BY 4.0): more than 160,000 populated places, each with its IANA time zone. Rankings use the cities of 100,000 people or more plus national capitals, outside the polar circles. The world map is drawn from Natural Earth (public domain) in a Miller cylindrical projection, which keeps MC and IC lines straight.
What the interpretations are
The written meanings of lines and signs describe astrological tradition: how astrologers commonly read each combination. They are offered for reflection and curiosity. There is no scientific evidence that planetary positions at birth affect how life goes in a particular place, and nothing here is a prediction or professional advice. If you find an error in a calculation, please let us know.