Methodology

The calculator places the Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto, the true lunar node, the Ascendant and the Midheaven. The south node is the point opposite the true node. The zodiac is tropical: Aries begins at the March equinox.

Planet positions come from the ephemeris series bundled with the calculation engine. They are worked out on this site's own server from the date, the time, the coordinates and the time zone you send, and the result is not stored.

The engine covers the years 1900 to 2100. A date outside that range gets a message instead of a chart.

On the test charts, planet positions agree with NASA JPL Horizons to within about 0.03 degrees, and the Ascendant and the Midheaven to within 0.01 degrees. The true node differs by about 0.04 degrees. The test charts are four births checked against Horizons, and two published charts of well-known people checked against their recorded data.

That is the accuracy of the figures, not a claim about every chart: a birth time known to the minute fixes the Ascendant to a fraction of a degree, a birth time known to the hour does not. The page on birth time accuracy shows how fast the angles move.

Placidus is the default. Whole Sign, Equal and Porphyry are offered too, and the house systems page compares them. Whole Sign and Equal houses are worked out from the Ascendant, Porphyry from the Ascendant and the Midheaven, and each is checked by hand-worked cases.

Placidus has no cusps inside the polar circles. When you ask for Placidus at such a latitude, the calculator falls back to Porphyry and says so on the result.

The calculator finds the conjunction, opposition, trine, square, sextile and quincunx between the planets, the true node, the Ascendant and the Midheaven. Each type has one orb, the same on every chart, listed on the aspects page and asserted equal to the engine's by a test.

The clock time you enter is read in the time zone of the birth place, using the IANA time zone database with its history, so a birth in 1943 uses the rules of 1943. When a clock change skipped an hour or repeated one, the calculator flags the time and tells you which reading it used.

Births before 1970 in places whose zone the database has merged with another can be off, because the merged record keeps only the rules that agree since 1970. If the birth certificate states a local time, check the chart against it.

If you do not know the birth time, the calculator casts the chart for noon, hides the Ascendant and the houses, and marks a planet that can change sign during the day.

Place names and coordinates come from GeoNames, licensed under Creative Commons Attribution 4.0 (CC BY 4.0). The list holds places of 5,000 people and more, with Latin-script names, and coordinates rounded to two decimals, about one kilometre.

The list is downloaded to your browser the first time you click the birth-place field, and the search runs there: what you type is not sent to the site. If your town is not listed, choose the nearest larger town, or enter coordinates and confirm the time zone the calculator suggests.

There is no Chiron and no asteroids. There is no sidereal zodiac yet: every position is tropical. The calculator makes no predictions and gives no dates of future events.

Koch, Regiomontanus and Campanus houses are not offered until they have been checked against published charts.

Astrology is not scientifically validated. The site calculates positions, which is astronomy, and states what astrological tradition says about them. Its readings are short, plain-language descriptions meant for reflection, and none of them is advice or a forecast.

The date on a page is the day its facts or its words last changed, the same date the sitemap reports. Rebuilding the site without changing a page leaves its date alone.

Reports of errors go through the contact page. A confirmed correction changes the data, the page and its date together.