← Encyclopedia of Astrology · lesson 85/235
Porphyry houses
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What it is
Porphyry is the oldest and simplest of the quadrant house systems. It takes the four angles as given — the Ascendant, Imum Coeli, Descendant and Midheaven — and divides each of the four quarters between them into three equal parts of zodiacal longitude.
So if the Ascendant is 10° Virgo and the Imum Coeli is 4° Sagittarius, the arc between them is 84 degrees; each of the 1st, 2nd and 3rd houses is 28 degrees long, and the 2nd and 3rd cusps fall at 8° Libra and 6° Scorpio.
How it differs
- Both angles are exact cusps. The Ascendant opens the 1st house and the Midheaven the 10th, as in every quadrant system — unlike equal houses or whole sign houses, where the Midheaven floats.
- Houses are equal within a quadrant, unequal between them. The three houses of one quarter are always the same size, but a long quadrant has three long houses and a short one three short houses, so house sizes change abruptly at each angle. Signs can be intercepted.
- It needs nothing but the angles. No spherical trigonometry is involved beyond finding the Ascendant and Midheaven, so it can be worked out by hand from a chart already drawn.
- It never fails. Because it only divides the zodiac between points that always exist, it keeps working at high latitudes, where Placidus and Koch break down — though there, the angles themselves can fall very unevenly, and the houses with them. Much astrological software uses Porphyry as its fallback when Placidus cannot be calculated.
Where it comes from
It is named after Porphyry (c. 234–305), the Neoplatonist philosopher who wrote an introduction to Ptolemy's Tetrabiblos and described the method there. He did not invent it. The astrologer Vettius Valens, writing a century earlier, already describes dividing the quadrants in this way, attributing the idea to an otherwise unknown authority called Orion. In Hellenistic practice it seems to have been used less for the topics of life — which were usually counted by whole signs — than for judging how strongly placed a planet was, by its distance from the angles.
The method passed into Indian astrology too, where the Sripati system starts from Porphyry-style cusps and shifts them so that each angle falls in the middle of its house.
When astrologers reach for it
- As a middle way, for those who want the angles as cusps without the distortion of the time-based systems.
- At high latitudes, as a quadrant system that still works.
- With Hellenistic technique, alongside whole sign houses, for assessing whether a planet is close to an angle and so strongly placed.
- When only a printed chart is available, and intermediate cusps are needed quickly.
In a reading here
Porphyry is one of the seven house systems Viastaro offers. Charts use Placidus unless another system is chosen, as a default for the account and per person in the reading settings.