Prosthetikos and Aphairetikos

Definition

These are the two poles of planetary speed measured against a planet's own average motion. A planet running faster than its mean daily motion is prosthetikos (Greek προσθετικός, "additive"); one running slower is aphairetikos (ἀφαιρετικός, "subtractive"). The pair reads speed as a tilt in timing — quick to deliver, or slow and delayed.

In Tradition

Brennan documents the pair as a speed condition: the additive planet was taken to bring its significations sooner or earlier in life, the subtractive one to deliver them later, with delay or sluggishness. He notes that the words were sometimes applied instead to direct versus retrograde motion, the author's intent not always clear in the surviving texts. This is about velocity, distinct from the morning-or-evening-star phase that tracks a body's position relative to the Sun.

In Practice

Compare a planet's current daily motion against its average: faster is additive, slower is subtractive. Read the additive one as inclined to produce its themes earlier or more readily, the subtractive as inclined to delay or drag them out. Keep this separate from solar phase — the pair is about how fast a body travels, not whether it rises before or after the Sun. And bear Brennan's caution in mind: some sources stretch the terms to cover direct versus retrograde instead.

Historical Origin

The pair is described in Chris Brennan, Hellenistic Astrology (chapter 7, "Planets," in the planetary-speed section, p. 234), which sets out the additive/subtractive distinction and flags the occasional alternate use for direct versus retrograde motion.

Etymology

Origin: Greek. Meaning: additive (prosthetikos) and subtractive (aphairetikos).

Further Reading

• Chris Brennan, Hellenistic Astrology
• Vettius Valens, Anthology