Acronychal Setting

Definition

Acronychal setting denotes a star's apparent setting on the western horizon at sunset — that is, at the moment the Sun is setting in the west, the star is also setting opposite or near it, having been visible above the horizon during the preceding daylight hours. In the Babylonian observational tradition the acronychal setting is the directly observable partner of a different star's heliacal rising at the opposite horizon, the two phenomena occurring simultaneously to provide a double anchor on the star calendar.

In Tradition

Across Mesopotamian astronomy and the Hellenistic-and-later traditions that inherited its observational vocabulary, acronychal setting is treated as one of the canonical synodic phases of a fixed star or planet, paired with its heliacal rising at the diametrically opposite horizon. The pairing furnishes the cross-check from which Babylonian astronomers calibrated the schematic year.

In Practice

Astronomers and astrologers identify acronychal setting by observing whether a star is on the western horizon at the moment the Sun reaches the western horizon — the star setting with the Sun rather than rising opposite it. Practical use in the Babylonian tradition pairs each heliacally rising constellation with the constellation acronychally setting diametrically opposite, yielding a double-anchor observation that confirms the star calendar. The MUL.APIN compendium tabulates twenty-one such rising-setting pairs across the ecliptic belt, opening with the Pleiades rising paired with Scorpio setting. In the modern fixed-star tradition the same phenomenon is used to mark when a natal star, having been visible during the day, falls out of sight as the Sun also sets.

Historical Origin

Acronychal setting is documented in MUL.APIN Tablet I iii 13-33 (1st-millennium BCE canonical compendium drawing on 2nd-millennium material), where the formula 'The X rises and the Y sets' enumerates twenty-one diametric rising-setting pairs. The Pleiades-Scorpio pair opens the sequence; the Fish-and-Old-Man with Furrow-and-Wild-Dog pair closes it. Hunger and Steele (2019) treat the procedure as a characteristic Mesopotamian contribution to observational astronomy preparing the later Goal-Year methodology.

Etymology

Origin: Greek. Meaning: From Greek ἀκρόνυχος (akronychos, 'at nightfall' — literally 'tip of night'), denoting the moment of sunset that begins the night..

Further Reading

• Hermann Hunger & John Steele, The Babylonian Astronomical Compendium MUL.APIN
• Bernadette Brady, Brady's Book of Fixed Stars