Via combusta and the void Moon: the Moon of medieval astrologers

In medieval astrology, both Arabic and Latin, the Moon was regarded as the decisive planet in practical matters. Being the swiftest, she touched every sign and every other planet within a few days, and for this reason she was seen as the «carrier» of their influences towards the Earth. The manuals devoted to choosing favourable moments and to interrogations, that is, answers drawn from the sky at the instant a question was asked, reserved for her numerous and detailed rules.

From Baghdad to Forlì

The rules about the Moon reached Europe through twelfth-century translations of authors such as Abu Ma'shar, known in Latin as Albumasar, and Sahl ibn Bishr, a Jewish astrologer active in the ninth century, whose treatises on elections and interrogations long circulated in Latin libraries. In them the Moon appears as co-significator of almost every question: her condition and movements showed how the matter would unfold.

The most famous Italian heir of this tradition was Guido Bonatti of Forlì, active in the thirteenth century and adviser to Ghibelline lords such as Guido da Montefeltro. His Liber astronomiae, a vast astrological encyclopaedia, gathers and orders the Arabic doctrines in an often combative tone. Dante placed him among the soothsayers in the twentieth canto of the Inferno, with the line «See Guido Bonatti; see Asdente», a sign both of his fame and of the distrust he aroused.

The via combusta

Among the most characteristic notions was the via combusta, the «burnt path», a stretch of the zodiac that medieval texts usually place between the middle of Libra and the middle of Scorpio. When the Moon passed through it she was judged weakened, almost scorched, and undertakings begun on those days were considered ill-favoured. The origin of the doctrine is unclear: various explanations have been proposed, from the fall of the Sun in Libra to the presence of stars thought to be baleful.

Equally feared was combustion proper, that is, the Moon's closeness to the Sun around the new moon, when she disappears in his rays. During that time the Moon was considered powerless, except at the instant of exact conjunction, called cazimi, «in the heart of the Sun», when on the contrary she was thought to be strengthened. These fine distinctions show how astrologers turned into rules a phenomenon visible to everyone: the disappearance and rebirth of the crescent.

The Moon void of course

Another rule concerned the Moon «void of course», in Latin vacua cursus. This was said when the Moon, before leaving the sign she was in, no longer formed any aspect with the other planets. For medieval authors it signalled that the matter would come to nothing, that nothing would happen or that things would stay as they were. Bonatti often lists it among the conditions to be avoided when starting an undertaking.

Besides the void Moon, the treatises list other unfavourable conditions, such as the Moon «besieged» between two malefic planets, slow in motion, or placed in the last degrees of a sign. The list of these impediments, going back in part to Dorotheus of Sidon and Sahl ibn Bishr, became a kind of checklist repeated throughout astrological literature until the seventeenth century, including the Christian Astrology published by the Englishman William Lilly in 1647.

These doctrines do not describe a measurable influence, but they reveal the way of thinking of an age. The Moon, changeable and swift, was perceived as the link between the heavens and human affairs, and each of her conditions became a sign to be interpreted. Reading Bonatti or his Arabic models means watching the birth of a technical language that left lasting traces in European culture.