Synodic Month

greek: συνοδικὸς μήν (synodikos mēn) · latin: mensis synodicus · babylonian: (System A/B mean-synodic-month value 29;31,50,8,20 days)

Definition

The synodic month is the time-interval between two successive lunar conjunctions with the Sun (equivalently, two successive new moons or two successive full moons) — the full cycle of lunar phases as seen from Earth. In Babylonian mathematical astronomy the mean synodic month is given as 29;31,50,8,20 sexagesimal days (≈ 29.530594 days), the same value Hipparchus later used; modern astronomy gives the mean synodic month as 29.530589 days, agreeing with the Babylonian figure to roughly 0.4 seconds.

In Tradition

Across Babylonian, Hellenistic, and modern astronomical traditions, the synodic month is read as the basic time-unit of the Moon's relationship to the Sun and the foundation of the lunar calendar. Hunger and Pingree note the synodic-month length 29;31,50,8,20 days is the value column G of Babylonian System B yields, and that this length was passed forward into Greek mathematical astronomy through Hipparchus.

In Practice

Practitioners use the synodic month in several registers. As a calendar unit it underlies the Babylonian, Hebrew, and Islamic lunisolar calendars; the Metonic 19-year cycle (235 synodic months ≈ 19 tropical years) is the basis of intercalation. As a timing unit in horoscopic astrology it grounds the lunation cycle (new moon, first quarter, full moon, last quarter), eclipse cycles (the Saros = 223 synodic months), and lunar-phase mundane and natal interpretation. Babylonian mathematical astronomy organized planetary and lunar prediction around integer multiples of synodic months: 1 month, 6 months, 12 months, and 223 months — the latter being the Saros eclipse-recurrence interval that ties anomalistic and synodic cycles together.

Historical Origin

The 29;31,50,8,20 day value for the mean synodic month is attested in Late Babylonian System A and System B mathematical-astronomical tablets (5th c. BCE through Seleucid period). Hipparchus knew the value (2nd c. BCE) and transmitted it into Greek astronomy. Ptolemy's *Almagest* (c. 150 CE) preserves the figure within its lunar theory.

Etymology

Origin: Greek. Meaning: Meeting / conjunction month.

Further Reading

  • Hermann Hunger & David Pingree, Astral Sciences in Mesopotamia
  • Francesca Rochberg, The Heavenly Writing