Porphyri was not the inventor of the house system that bears his name. But he did propose a system that has a strong resemblance with it. This system has no name, I will simply call it Porphyri original.
Porphyri wrote a work, the Isagoge, which contains an introduction to the work of Claudius Ptolemy, and is published in the CCAG V. 4 [Cumont, 1904]
In this work, he describes that he wants to divide the quadrants in three equally sized parts but that he wants to use a prespan of 5 degrees.
According to Wilhelm Knappich, Porphyri did not agree with a variable prespan as used by Pancharios and developed the system we describe here. [Knappich, 1978, p. 15].
Walter Koch en Wilhelm Knappich mention that Porpyri based his system on the prespan that was already proposed by Ptolemy [Koch/Knappich, 1959, p.75].
The system is (almost) never used.
The calculation is almost the same as for the commonly used system of Porphyri, you just need to advance MC and ascendant by 5 degrees before you start calculating. You can also calculate the cusps with the commonly used Porphyri system and afterwards advance all results by 5 degrees. The result will be the same.
For the calculation I refer to the example on the Porphyri system.
The location is again Enschede (52º 13′ N and 6º 54′ E). Date and time November 2, 2016 (Gregorian calendar), 21:17:30 UT. The sidereal time is 0:35:23,6 and the obliquity E is 23° 26′ 13,56586091”.
All cusps are advanced by 5 degrees:
MC: 9°37′48” Aries + 5° = 14°37′48” Aries Asc: 3°30'28" Leo + 5° = 8°30'28" Leo cusp 11 : 17°35'21" Taurus + 5° = 22°35'21" Taurus cusp 12 : 25°32'55" Gemini + 5° = 0°32'55" Cancer cups 2 : 25°32'55" Leo + 5° = 0°32'55" Virgo cups 3 : 17°35'21" Virgo + 5° = 22°35'21" Virgo
The remaining cusps will be opposiite the calculated cusps.
- Cumont, Franz and Franz Boll: Catalogus Codicum Astrologorum Graecorum V. Partem Priorem. Codicum Romanorum. Brussel, 1904.
- Knappich, Wilhelm – Tradition und Fortschritt der klassischen Astrologie. In Qualität der Zeit nr. 28/39. Vienna, september 1978.
- Koch, Walter and Wilhelm Knappich – Horoskop und Himmelshäuser. 1. Teil. Grundlagen und Altertum. Göppingen, 1959.