How we calculate

Every calculator here has a pass mark: a check against a source that does not share its code. A page only goes into search engines when its tool meets its mark. The results below come from the last test run, 2026-11-03.

Sky positions

The Sun, Moon and planets come from astronomy-engine (MIT licence), as apparent geocentric longitudes on the tropical zodiac, measured on the true ecliptic and equinox of the date, with light-time and aberration applied. We use the JPL Horizons model of Delta T (the gap between Earth rotation time and atomic time) so far-future dates match Horizons. Pass mark: Sun to Pluto and the Moon within 1 arc-minute of NASA JPL Horizons at 30 dates from 1900 to 2100. Measured: 300 of 300, worst 16.9 arc-seconds (neptune 1968-10-27 08:40).

Lunar nodes and Black Moon Lilith are our own code from Meeus, Astronomical Algorithms, chapters 47 and 50. The true node adds five periodic terms to the mean node; Lilith is the mean lunar apogee only. Meeus worked examples: 15 of 15. Pass mark for the true node: within 0.25 degree of the JPL Horizons lunar node, and within 1.9 degrees of the mean node over 1900-2100 (Horizons' own node reaches 1.89 degrees from the mean). Measured: true node within 0.208 degrees of JPL Horizons on 80 dates (pass mark 0.25); largest true - mean node 1.878 degrees over 1900-2100 (pass mark 1.9).

The Ascendant and Midheaven use Greenwich apparent sidereal time (IAU 2006 with the equation of the equinoxes) plus your east longitude, and the true obliquity. Pass mark: a second formula path, finding the ecliptic point on the horizon and on the meridian through astronomy-engine's own coordinate rotations, agrees to 0.01 degree for 50 random times and places from 60 S to 60 N. Measured: worst 0.00 arc-seconds over 50 random times and places, 60 S to 60 N.

Houses: Placidus by default, Whole Sign and Equal on request. Placidus cannot be drawn above about 66 degrees latitude; there the chart uses Whole Sign and says so. Checks: 40 Placidus charts: cusps 1 and 10 equal Asc and MC, opposite cusps 180 degrees apart, cusps 11, 12, 2, 3 meet the semi-arc definition to 0.01 degree; 70 N switches to Whole Sign.

Aspect orbs

Orbs are 2 degrees wider when the Sun or Moon is involved. The same table is used for birth charts, synastry, composite charts and transits. Composite positions are midpoints along the shorter arc.

Time zones and places

Your local birth time is turned into UTC with your own browser's time-zone data (Intl.DateTimeFormat), for the IANA zone of the place you pick. A time that did not exist because clocks jumped forward, or that happened twice because they went back, makes the calculator ask which one you mean. Every sky result shows the zone, offset and UTC used, with an offset override, and births before 1970 get a reminder that older records can be wrong.

Pass mark: 40 local times (New York 1965, London 1971 British Standard Time, Chicago 1974 winter daylight time, India 1945, Sydney, Phoenix, Sao Paulo, every kind of changeover day and more) give the same offset as Python zoneinfo built from the IANA source, in Chromium, Firefox and WebKit (our stand-in for Safari).

Amsterdam 1935 summer, +1:19:32. Before 1937 the Netherlands kept Amsterdam Mean Time, 19 minutes 32 seconds ahead of GMT (+1:19:32 in summer). The full IANA database (with its historical backzone file) has this, but the trimmed copy built into browsers treats Amsterdam like Brussels and gives +1:00. For Dutch births before 1937, type the offset +01:19:32 in summer or +00:19:32 in winter.

Places come from the GeoNames cities15000 list (CC BY 4.0): towns of 15,000 people or more, with coordinates and the IANA zone. For a smaller place, pick the nearest town or type coordinates. Spot checks: 20 of 20 spot checks: country, coordinates within 0.01 degree of the dump, zone.

Chinese zodiac

The animal year starts at Lunar New Year. We compute the dates ourselves from new moons and solar terms in China time (Beijing local mean time before 1929), with the leap-month rule. Against the Hong Kong Observatory tables: 200 of 200 Lunar New Year dates match the Hong Kong Observatory tables.

Numerology

Pythagorean letter values: A, J and S are 1, B, K and T are 2, through I and R at 9. Each word of a name is reduced to one digit first; the words are added and reduced again, and the master numbers 11, 22 and 33 are kept at that last step. For the life path, the year of a birth date is reduced to one digit, while the month and day are reduced but keep 11 and 22 when they land on one (day 11, 22 or 29, month 11), so a total of 33 can occur. When adding every digit at once gives another answer, the result shows both. Y counts as a vowel when no other vowel sits next to it, unless you choose always or never. Karmic debt is flagged when 13, 14, 16 or 19 appears before the last reduction. Accents fold to base letters; other scripts are not supported yet. The Chaldean page uses the Chaldean table (1 to 8). Each tool's own cases are listed below.

Every calculator