Photismos

Definition

Photismos (Greek φωτισμός, "illumination") is Valens's word for the Moon's lit fraction — and, in his table-construction, the number tied to it. The two senses fold together. The value 14 he adds at each break in a calculating sequence stands for the Moon's illumination, since twice 14 is 28, close to the synodic month. The same word names the actual light the Moon shows at a birth, measured as its distance from new or full.

In Tradition

Valens uses photismos in two linked ways. Numerically, he states that the added factor of 14 in his sequence "indicates the illumination of the moon," pairing it with a separate increase he calls the "fingers" of the Sun. In the phase sense, the lit portion is set by how many degrees the Moon stands from new or full. That is its photismos, which Riley marks for English readers with the editorial bracket "<=phase>." The construction-number is grounded in the Moon's real light at the nativity.

In Practice

This is one of Valens's technical building-blocks rather than a standalone chart factor, so you meet it inside his table methods. The useful idea is that the Moon's illumination is being quantified — how far it stands from the new or full moment — and that number feeds his calculations. When you see "illumination" in Valens, read it as this measured lit-fraction, distinct from the broader lunar-phase doctrine of waxing and waning that other authors treat in more general terms.

Historical Origin

The term is attested in Vettius Valens, Anthology Book VIII (pp. 141-142, in Mark Riley's translation), where it carries both the numerical-construction sense (Book VIII.2) and the moon-phase sense (Book VIII.3); Riley supplies the editorial bracket "<=phase>" to mark the second.

Etymology

Origin: Greek. Meaning: illumination; an illumining.

Further Reading

• Vettius Valens, Anthology
• Chris Brennan, Hellenistic Astrology