The Alcabitius houses

Where the sources disagree

The system carries his name; it does not owe him its invention. Al-Qabisi uses it and describes it precisely enough for the late Middle Ages to credit him with it, but the oldest clear calculation is read in a chart attributed to Rhetorius, which sources date to the fifth, the sixth or the seventh century depending which one you open. We do not settle it: we write fifth to seventh century, and we say why.

You will often meet a description that runs the house circles through the north and south points of the horizon. It describes Regiomontanus, and here it is geometrically impossible: the Alcabitius circles pass through the celestial poles. The difference is not a matter of words, it changes the cusps, since it decides whether you look for the point whose right ascension equals the division or the one whose oblique ascension does.

The Latin name drifts: Alcabitius, Alchabitius, sometimes Alkabitius, and Italian makes it Alcabizio. We write Alcabitius, the form kept by the Swiss Ephemeris, by the tables of houses and by the encyclopedias; Italian keeps its own, right down to the address of this page.

Beyond roughly 66.57° the Midheaven passes below the horizon and the quadrant to be divided would run past a half turn. A convention is needed, and the sources do not agree. We take the Ascendant at its opposite point, as the Swiss Ephemeris does and as we already do for Porphyry, and the page tells you when the case arises rather than handling it silently.

What we do with it

Nothing, and that is deliberate. Our charts are calculated in Placidus, with Porphyry as the fallback beyond the polar circle, and this page changes nothing in what is delivered to you. Switching schools would move the cusps of readings already written, and would leave our whole body of interpretation talking about quadrants that were no longer its own. This table is here to be consulted, not to be applied behind your back.

Why it earns its place anyway: it is a division of time, read on the celestial equator, and that is the unit in which primary directions are counted, the predictive engine of medieval astrology. With cusps and directions both measured in right ascension, directing a planet to a cusp keeps an arithmetic meaning. That connection is our reading and not a quotation, and we flag it as such. J. D. North, for his part, thought it so universal between 500 and 1500 that he simply called it the standard method (Horoscopes and History, Warburg Institute, 1986).

The cusps you read here do not come from a copied table: they come out of the code that calculates the site's charts, asked four times over. On a calculation that starts from a given ARMC, our values match the published hand calculation to three millionths of an arcsecond. On a calculation that starts from a date, the time chain costs a few hundredths of an arcsecond, shared by Placidus and Alcabitius alike: that is why the page shows only four decimals there.