The Placidus House System is a system of house division that utilizes the movement over time on the ecliptic, or the sun’s apparent path through the sky. Each degree of the zodiac has a diurnal arc, i.e., how far it moves in a day. This, divided in two, becomes the point of the meridian or midheaven. The Placidus system then measures, for a specific time and location, how far a degree moves when it has traveled 1/3th of its diurnal movement from Ascendant to Midheaven, and this becomes the cusp of the 12th house. How far it has moved when it has travelled 2/3rd of its diurnal movement becomes the cusp of the 11th house, and so on.
The system was named after Placidus de Titis, an Italian monk and mathematician, though he did not actually invent the system. It was based upon a system of house division articulated by 12th century Hebrew astrologer Abraham ibn Ezra, who believed that the system was in fact the system originally described by Hellenistic astrologer Claudius Ptolemy in his primary work, the Tetrabiblos.
In Placidus’s most well-known book, Physiomathematica sive Coelestis Philosophia (roughly translated Physico-Mathematics or Celestial Philosophy), published in 1650, Placidus re-articulated and endorsed Abraham ibn Ezra’s theory regarding the use of Ptolemy’s system. Placidus’s book, which was banned by the Roman Catholic Church, made its way to Protestant England where it was embraced by prominent astrologers such as John Partridge and began to supplant the established form of house division, the Regiomontanus system (which was now understood to be based on a misinterpretation of Ptolemy). After this time, English astrologers published tables of Placidus Houses to make them readily available, and the system became widely promoted and utilized.
The Placidus House System is criticized for its failure to operate correctly in extreme latitudes, because certain degrees are “circumpolar” (never touching the horizon), therefore planets falling in those degrees would not be represented on the birth chart. However, it still has become the dominant house system for astrologers in the 20th and 21st Centuries. It remains the default house division system for astro.com.
Further Study:
Anthony Louis, Why are Placidus Houses So Popular? November 29, 2019.
The Astrology Podcast, Episode 244, How Did Placidus Become the Most Popular House System?, Chris Brennan & Anthony Louis, February 24, 2020.
Article Information:
- Author(s): Claire Rootjes
- Editing or additional contributions: Chris Brennan
- Originally published: October 26, 2020
- Last updated: October 26, 2020
- Cite this article: Claire Rootjes, “Placidus House System,” The Astrology Dictionary, October 26, 2020, http://theastrologydictionary.com/p/placidus-house-system/
