Placidus de Titis (also de Titus, Latinization of Placido de Titi, pseudonym Didacus Prittus Pelusiensis;1603–1668) was an Olivetan monk and professor of mathematics, physics and astronomy at the University of Pavia from 1657 until his death. Placidus popularized the system of astrological houses now known as the "Placidian system", current in modern astrology. He did not invent the method; it is acknowledged by the 12th century Hebrew astrologer Abraham Ibn Ezra as the system employed by Ptolemy, an attribution that was accepted by Placidus.
Placidus was born in Perugia, into the Titi noble family. His father died early, and he was looked after by his mother Cecilia. He studied at the University of Padua where his uncle Girolamo de Titi was professor of theology. The Duchy of Milan at the time was owned by Habsburg Spain, administered by Archduke Leopold Wilhelm of Austria. The Archduke showed strong interest in science, especially occult sciences of alchemy and astrology, and Placidus dedicated his astrological house tables to him. He died in Pavia.
Physiomathematica sive coelestis philosophia (1650), Placidus' magnum opus, first published as Quaestionum physiomathematicarum libri tres, under the pseudonym of Didacus Prittus Pelusiensis, second edition by C. Francobacci und A. Scirota (pseudonyms of two of Placidus' students, F. Brunnaccio and F. M. Onorato)
Nuncius astronomicus (1654)
Il corriere astronomico. (1656)
Tabulae primi mobilis cum thesibus et canonibus (1657)
Commentaria in Ptolemaeum de siderum judiciis (1658)
De siderum judiciis, 2 vols. (1660, 1665)
De diebus decretoriis et aegrorum decubitu, 2 vols. (1661, 1665)
Ephemerides coelestium motuum (1661-1665)
Tocco di Paragone, onde evidamente appare che l' astrologia nelle parte concesse da S. Chiese é vera scienza naturale, nobile e utile quanta la filosofia (1666), in defense of astrology as a natural science.