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ADR 0007: Athenian-calendar conversion fidelity

Status: Accepted (2026-04-29)

Context

The Antikythera mechanism's back dial uses the Olympiad cycle, but ancient Greek users would have spoken about dates using the Athenian (Attic) calendar — twelve lunar months, year starting at the first new moon after the summer solstice, archonship year (e.g. "the year of Eponymos Apollodorus").

Three components matter:

  1. Lunar month name + day — derivable from a uniform synodic-month model with ±1-day error vs. observed Athenian dates.
  2. Intercalary "Poseideon II" — the 13th lunar month inserted in some years to keep the lunar year aligned with the solar year. Athenian intercalation was non-systematic.
  3. Archonship year — the most useful date marker for historical research, but requires the canonical archon list (Develin 1989 covers -510 to -301 BCE; gaps and disputed reigns outside that period).

Decision

v0.1.0 returns:

  • ✅ Attic month name + day (uniform synodic-month model; ±1-day caveat documented).
  • ❌ Intercalary "Poseideon II" — not modeled; lunar drift folded into year-counter.
  • ❌ Archon name — archon: None; the Develin 1989 table is not bundled.

v0.2.0 will add the archon table for -510 to -301 BCE; outside that range we will raise UnknownArchonError rather than guess.

Consequences

  • Historians using jd_to_athenian for v0.1.0 get the lunar-month structure but must look up archonship from external sources.
  • The approximation_note field in the response makes the limit explicit.
  • v0.2.0 archon-table addition is a non-breaking minor-version bump (adds a field; doesn't remove or rename).

Alternatives considered

  • Bundle Develin 1989 in v0.1.0. Rejected on scope grounds — the archon table is a multi-month digitisation effort with disputed-reign judgement calls; defer to v0.2.
  • Use a less authoritative archon list. Rejected — we'd ship wrong dates rather than honest "we don't know."
  • Refuse to answer Athenian queries entirely. Rejected — the Attic-month structure is genuinely useful even without the archon name.

References

  • Pritchett, W. K. & Neugebauer, O. (1947). The Calendars of Athens.
  • Develin, R. (1989). Athenian Officials 684–321 B.C.