Porphyry

Meaning

Porphyry houses take the four angles as they are and share out the zodiac between them. The arc from the Midheaven to the Ascendant is cut into three equal parts, which give the cusps of the 11th and 12th houses. The arc from the Ascendant down to the IC gives the 2nd and 3rd. The other cusps lie opposite.

Precisely: if the Midheaven is at 0° Capricorn and the Ascendant at 15° Aries, the quadrant between them spans 105° of the zodiac. Each house in it is 35° wide, so the 11th begins at 5° Aquarius and the 12th at 10° Pisces. The next quadrant, from the Ascendant to the IC at 0° Cancer, spans 75°, so the 1st, 2nd and 3rd houses are 25° each. Houses in the same quadrant are equal; houses in neighbouring quadrants are not.

This is the simplest way to respect the angles. Every quadrant system agrees on the four angles and differs only on the eight cusps between them. Porphyry divides that space evenly, with no reference to time or to circles of the sky. Its houses grow unequal with latitude, as the quadrants do, but more gently than in Placidus or Koch. At 60° of latitude they range from about 13° to 47° of the zodiac, against 11° to 85° for Placidus.

It also works everywhere. As long as the angles exist, the quadrants can be divided, so Porphyry houses can be drawn inside the polar circles, where Placidus and Koch fail. Its limit is the reverse of its strength: dividing degrees of the zodiac evenly ignores how unevenly they rise, which is what the time-based systems try to capture.

In practice

• A plain quadrant system. Astrologers who want the angles as cusps without choosing a theory of time or space often settle on Porphyry.
• Ancient technique. Some readers of Hellenistic texts use it to judge how close a planet is to an angle, the idea behind angular, succedent and cadent houses.
• High latitudes. For births far north or south, it gives readable houses where Placidus and Koch become extreme.
• A neighbour of Alcabitius. Alcabitius also gives moderate houses at high latitudes, but divides time rather than the zodiac.

Across traditions

• Jyotish has a method, named after the 11th-century astronomer Śrīpati, that divides the quadrants in the same way. It takes the points it finds as the middles of the houses rather than their beginnings.

Origin and history

This is the oldest quadrant method on record. Vettius Valens describes dividing the quadrants between the angles in his Anthology, in the 2nd century. The method takes its name from Porphyry of Tyre, the 3rd-century Neoplatonist philosopher, to whom an Introduction to Ptolemy’s Tetrabiblos is attributed, where the division is set out. It was little used through the Middle Ages, when Alcabitius’ system led. Today it is valued as the plainest quadrant option, and it has gained users with the revival of Hellenistic methods.

How Ulune calculates it

Choose Porphyry in the birth form’s options. The Swiss Ephemeris computes the cusps with its code O. The Ascendant, the IC, the Descendant and the Midheaven are the cusps of the 1st, 4th, 7th and 10th houses.

Porphyry is also Ulune’s fallback. Placidus and Koch have no cusps inside the polar circles, beyond about 66°34′ north or south. When one of them is chosen for a birth there, Ulune builds the houses with Porphyry instead. A note under the chart then says so, for example: “Houses in Porphyry: Placidus has no cusps inside the polar circles.” Progressed angles and the Vertex follow the same fallback. If Porphyry itself cannot be computed, the chart is not cast. In a composite chart in Porphyry, the angles are the midpoint angles and the other cusps keep, inside each quadrant, the average of their places in the two charts.

Correspondences