How the sky is cast
The Swiss Ephemeris, and what Sidereal asks of it.
Every position in Sidereal — your natal chart, today’s sky, the wheel that turns between them — is computed by the Swiss Ephemeris, the planetary library that professional astrologers and their software rely on, built on the same planetary theories NASA’s Jet Propulsion Laboratory publishes. It is arithmetic, and it would say the same thing with the power off. No part of it is AI.
What we ask it
Your birth moment is turned into Universal Time and then into a Julian day — the astronomer’s continuous count of days — and for that instant we ask the ephemeris for the longitude and speed of twelve bodies: the Sun, the Moon, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto, Chiron, and the true North Node. With your birthplace, looked up on OpenStreetMap for its latitude and longitude, it also casts the houses in the Placidus system, and with them the angles: the Ascendant rising in the east, the Midheaven overhead. Positions come back to within arcseconds — far finer than the degrees and minutes a chart prints.
The zodiac it measures in
Positions are given in the tropical zodiac, the one Western astrology reads: 0° Aries is the March equinox, and the twelve signs follow from there. Yes — we are called Sidereal and we read the tropical zodiac. The name is about the stars, not the frame of measurement; Why Sidereal tells that story.
Retrogrades, transits, aspects
Speed arrives with every position, so a planet running backward is known from the sign of its own motion, never looked up in a table. Today’s sky is cast at noon each day; a transit is the angle between that running sky and a placement of yours, an aspect the angle between two placements — each found by subtracting one longitude from another and asking whether the difference falls within the orb astrologers allow for a conjunction, sextile, square, trine, or opposition.
When the hour is unknown
If you don’t know the hour you were born, the moment stands at local noon — the conventional compromise. The planets are placed to within a day’s motion, which for most of them is a fraction of a degree; the Moon, moving some thirteen degrees a day, is the one that can change sign on you. The houses and angles, which turn through the whole wheel in a day, are left out rather than invented.
What is not computed
The reading. Everything above is the ephemeris’s; what the Seer makes of it is AI, a reflective lens rather than advice — where we stand on that.