Placidus House System

Placidus is a time-based house system developed in the 17th century and named after the Italian monk and astronomer Placidus de Tito (although it predates him in some forms). It is the default house system in the vast majority of modern Western astrology software, including ZODIA.

The Placidus method works by taking the diurnal arc of a degree of the ecliptic — the time it takes that degree to move from rising on the eastern horizon to crossing the upper meridian — and trisecting it. The intermediate cusps (the second, third, eleventh, and twelfth houses) are computed from these trisections, with the angles (1st, 4th, 7th, 10th) fixed by the ascendant and midheaven.

A practical consequence of the Placidus method is that houses are unequal in size and the inequality grows with latitude. Near the equator, Placidus houses are reasonably even. At high latitudes (above about 66 degrees), the system breaks down entirely because some degrees of the ecliptic never rise above the horizon. For most birth locations between roughly 60°N and 60°S, Placidus produces stable, well-defined houses.

Placidus is not the only valid system, and some modern astrologers prefer Whole Sign houses (where each sign is a full house, no fractional cusps) for their clarity and historical pedigree. The choice between systems is one of the more debated topics in technical astrology. ZODIA itself uses Whole Sign houses — the oldest house system in Western astrology and the one used by most serious practitioners today — but the underlying ephemeris supports Placidus and any other system.