How the charts are calculated

Everything on this site starts from one calculation: where the Sun, Moon and planets stood at a given moment, seen from a given place. This page sets out how that is done, how accurate it is, where the techniques built on it come from, and where calculation ends and interpretation begins.

The astronomy

Positions come from the Swiss Ephemeris, the planetary engine published by Astrodienst and used by most professional astrology software. It compresses the ephemeris of NASA's Jet Propulsion Laboratory — the same numerical model of the solar system that spacecraft navigation relies on — into files small enough to ship with the site. The files bundled here cover the planets, the Moon, Chiron, the four main asteroids and nine further minor planets, among them Eris, Sedna and Pholus.

Charts use the tropical zodiac, measured from the spring equinox. Houses are Placidus unless you choose another system; the lunar node is the true node and Black Moon Lilith the mean apogee, both switchable where a tool offers it. A birth time is converted to Universal Time with an offline historical time-zone atlas, including daylight saving and the local mean time used before standard time zones existed.

How accurate it is

On 13 September 2026 the site's positions were compared with NASA JPL Horizons — the reference NASA itself publishes — for fifteen bodies on four dates between 1750 and 2026: 54 comparisons in all. The median difference was 0.05 arcseconds. An arcsecond is 1/3600 of a degree; the Moon's disc is about 1,800 of them across.

For modern dates every body agreed to within 0.6 arcseconds. In 1750 the planets differed by up to 1.4 arcseconds and Chiron by 4.7, because Chiron's orbit is chaotic that far back. Two limits are real rather than small: Chiron is only available from the year 675, and Ceres, Pallas, Juno and Vesta only between 1800 and 2399, because no longer-range files for them exist.

What depends on you

The largest source of error in any chart is the birth time. The Ascendant moves about one degree every four minutes, and the houses move with it. When the time is not known, the chart is cast for noon and the reports say so: they leave out the Ascendant, the houses and everything measured from them rather than presenting a guess as a fact.

Where the techniques come from

Beyond the chart itself the site computes about a hundred standard techniques of Western astrology, each taken from the tradition that defined it rather than invented here:

• Ptolemy, Tetrabiblos (2nd century): the aspects, the essential dignities, the Egyptian and Ptolemaic bounds, and the animodar.
• Vettius Valens, Anthology (2nd century): zodiacal releasing, annual profections, the decennials and the Hermetic lots.
• Persian and medieval astrology: the firdaria and the almuten of the chart.
• William Lilly, Christian Astrology (1647): the considerations of horary astrology and the table behind the trutine of Hermes.
• Bernadette Brady, Brady's Book of Fixed Stars (1998): parans, measured with her orb of half a degree of arc, and the character of the stars.
• The Hamburg School (Alfred Witte, early 20th century): the eight Uranian points.
• Modern astronomy for the rest: lunations and eclipses, the saros series (checked against NASA's eclipse records), ingresses, stations and planetary hours.

The calculations are covered by more than five thousand automated tests, many of which check a result against something outside the site's own code — a published eclipse date, a NASA position, a rule stated in the source — so that a later change cannot quietly alter it.

Calculation and interpretation

Everything above is calculation, and it can be checked: a position, an aspect, a date. What a position means is interpretation. The readings are written for this site from the astrological tradition and assembled by the engine according to what your chart actually contains — which planets, in which signs and houses, in which aspects. No astrologer reviews an individual reading before you see it.

Astrology is an interpretive tradition, not a science, and nothing here predicts events or guarantees outcomes. It is offered for reflection and entertainment. It is not medical, legal or financial advice, and the site never describes a chart as showing a health condition.

If something looks wrong

If a position, a date or a statement on the site looks wrong to you, please write through the contact page with the details. Calculation errors are treated as bugs and fixed, and the fix is guarded by a test.

Figures measured on 13 September 2026.