(redirected from Placidus, Saint)


(religion, spiritualism, and occult)

Placidus de Titis (1603–1668) was an Italian mathematician and astrologer best known for the house system that bears his name. He joined the Olivetan Order when he was 21. He was a reader of mathematics and physics at the University of Padua for some years, and he was appointed professor of mathematics at the Milanese University in Pavia in 1657, a position he held for the rest of his life. He was also an astrologer to some prominent religious and political figures of the time.

Placidus attributed the initial inspiration for his system of division to a remark made by Ptolemy in Tetrabiblos. Ptolemy equated different semidiurnal arcs because they are equivalent to the same number of temporary hours. Analogically, Placidus reasoned, the twelfth-house cusp should begin at one-third of the semidiurnal arc above the horizon, the eleventh-house cusp at two-thirds of the semidiurnal arc above the horizon, and so forth.

Although mistaken, Placidus was convinced he had discovered Ptolemy’s lost method of determining houses, and he began to write books in which he described the new system. This system was adopted by John Partridge but rejected by most other English astrologers. At the beginning of the revival of astrology in England in the late 18th century, Manoah Sibly published English translations of Placidus’s Primum Mobile. The system of Placidus became the dominant system in England, and later the Placidian system was passed to France and Germany. Beyond his house system, Placidus was the inventor of secondary and tertiary directions. He also promoted the use of transits to both the natal and the progressed positions of the planets.


Holden, James H., and Robert A. Hughes. Astrological Pioneers of America. Tempe, AZ: American Federation of Astrologers, 1988.
Placidus. Physiomathematica sive Coelestis Philosophia Naturalibus hucusque desideratis ostensa principiis [Physico-mathematical (questions) or Celestial Philosophy set forth by means of natural principles hitherto lacking …. ] 2d ed. [Revised by his pupils Brunaccio and Onorati from the 1650 edition.] Milan: Fran. Vigoni, 1675.
Placidus. Tabulae Primi Mobilis cum … Triginta clariss. natalium Thematibus. [Tables of the Primum Mobile with thirty horoscopes of famous births.] 2d ed. Milano: Fran. Vigoni, 1675. (Originally published 1657.)