Birth Chart Calculator

A birth chart, or natal chart, is a map of where the sky was from your birthplace at the moment you were born. It records the zodiac position of the Sun, the Moon and the eight planets, the two points where the horizon and the meridian cut the ecliptic, and the angular relationships between all of them. Everything a reading draws on comes from that one snapshot, which is why the birth time and place matter: they fix the horizon, and the horizon is what turns a list of planetary positions into a chart with a top, a bottom and an east.

Three layers of information sit in it. The signs say what kind of expression each body takes - Mars in Libra pursues differently from Mars in Aries. The houses say which area of life it plays out in, and they depend entirely on the birth time, since the whole zodiac passes over the horizon every twenty-four hours. The aspects say how the bodies relate to each other: a square between two planets is read as friction between what they represent, a trine as ease. This calculator gives you all three, plus the exact degree of every position and whether Mercury through Neptune were retrograde.

The astronomy is computed rather than looked up, and the test suite checks it against JPL Horizons, NASA's own ephemeris service, at eight dates from 1905 to 2060: the Sun to within 20 arcseconds, the Moon and Chiron to within 10 and Venus to within 6, with Pluto held to 20 at a pinned date. Twenty arcseconds is less than a five-thousandth of the width of a zodiac sign. Where a convention has to be chosen it is stated on the page rather than buried: the zodiac is tropical, houses default to Placidus with whole-sign and equal available, aspect orbs are printed alongside each aspect, and the lunar node is the mean node. Everything runs in your browser and nothing you enter is uploaded.

A worked example makes the output concrete. As a Lilith birth chart calculator, it lists Black Moon Lilith in the Positions table with its degree, sign and house: cast London, 15 June 1990 at 14:30 and Lilith sits at 24°54' Scorpio in the 2nd house, sextile Saturn with an orb of 0.9°. As a tropical birth chart calculator, it measures every sign from the March equinox, which is the Western convention. That chart also shows why the house system matters: the Moon at 16°12' Pisces is in the 5th house under Placidus and the 6th under Whole Sign.