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.

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.

In 1657 he was appointed professor of mathematics at the University of Pavia, a position he held for the rest of his life.

English translations of Placidus' Primum Mobile were published by Manoah Sibly (1789) and John Cooper (1814).