PLACIDIANHOUSESYSTEM
It is easy for a person looking at an astrological chart for the first time to make the incor-
rect assumption that the 12 “pie pieces” are the 12 signs of the zodiac. These lines indi-
cate the house divisions (sign divisions are usually not represented), which can begin or
end at different places in different signs. Astrologers disagree about how to draw the
houses, although most agree that the first house should begin on the eastern horizon and
the seventh house (180° away) should begin on the western horizon. Also, the great
majority of systems begin the tenth house at the degree of the zodiac that is highest in
the heavens and the fourth house at exactly 180° from the cusp of the tenth house.
The Placidian house system was developed by a 17th-century Italian monk
and professor of mathematics named Placidus de Tito. In this system, the house cusps
between the ascendant and the midheaven are obtained by trisecting the time it takes
a degree of the zodiac to rise from the eastern horizon to the midheaven. Owing to the
widespread availability of Placidian tables of houses, this was the most popular house
system in the early 20th century, and it still enjoys widespread use.PLACIDUS
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.
Sources:
Holden, James H., and Robert A. Hughes. Astrological Pioneers of America.Tempe, AZ: Ameri-
can Federation of Astrologers, 1988.
Placidus. Physiomathematica sive Coelestis Philosophia Naturalibus hucusque desideratis ostensa prin-
cipiis....[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.Placidean House System
[532] THEASTROLOGYBOOK