Calendar systems supported by antikythera-spectral¶

The package converts between Julian Day and four calendar systems. Each has caveats; this document records them once so a future user reading the bridge JSON understands what they're looking at.

Gregorian (proleptic)¶

The modern civil calendar, extended backward before its 1582-10-15 introduction (the proleptic Gregorian rule set). For dates before 1582-10-15, the proleptic Gregorian and the actual historical Julian calendar differ; for dates before 1500 CE, the offset is 10 days.

Use jd_to_gregorian(jd) if you want a calendrically clean modern-style date for any JD. Use jd_to_julian_calendar(jd) if you're working with historical sources whose dates are in the Julian calendar.

Round-trip: gregorian_to_jd(year, month, day, …, era='CE'|'BCE') is exact.

Julian (historical)¶

The actual calendar in use 45 BCE through 1582-10-15 CE. Differs from proleptic Gregorian by 10–14 days depending on era.

Use jd_to_julian_calendar(jd) when reading dates as they were originally recorded by historical observers.

Olympiad¶

The four-year cycle Greek historians used to date events: "in the third year of the 169^th Olympiad." Olympiad I.1 is the date of the first Olympic Games, 776-07-01 BCE proleptic Julian = JD 1438178.0.

The Antikythera mechanism's back dial physically displays the Olympiad cycle; Freeth, Jones, Steele & Bitsakis 2008 (Nature 454:614) reconstruct the dial layout. Year-in-Olympiad is 1..4.

Athenian (Attic)¶

Twelve lunar months, year starting at the first new moon after the summer solstice:

Hekatombaion (mid-Jul .. mid-Aug)
Metageitnion
Boedromion
Pyanepsion
Maimakterion
Poseideon  (mid-Dec .. mid-Jan; intercalary slot is "Poseideon II")
Gamelion
Anthesterion
Elaphebolion
Mounichion
Thargelion
Skirophorion (mid-Jun .. mid-Jul)

Caveats (per ADR 0007)¶

The Attic calendar was observational — Athenian magistrates declared the new month after seeing the new moon. Rome's later calendar reforms (45 BCE Julian) and even Athenian intercalation practice introduced systematic deviations from a uniform synodic-month model.

antikythera-spectral v0.1.0 returns the Attic month + day from a uniform 29.530588861-day synodic model. Expected ±1-day uncertainty vs. observed Athenian dates.

The intercalary "Poseideon II" month (the 13^th month inserted in some years to keep the lunar-year aligned with the solar year) is not modelled in v0.1.0; lunar-cycle drift is folded into the year-counter instead of an explicit intercalation rule.

The archonship name (e.g. "the year of Eponymos Apollodorus") is the most useful Athenian date marker for historical research, but it requires the canonical archon table (Develin 1989, Athenian Officials 684-321 B.C.). v0.1.0 does not bundle this table; archon is set to None. v0.2.0 will add it for the period -510 to -301 BCE; outside that range we will raise UnknownArchonError rather than guess.

For research-grade Athenian dates, cross-check against:

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

ΔT and time-of-day precision¶

gregorian_to_jd(year, month, day, hour, minute, second) returns a JD on the assumption the input is Universal Time. For Hellenistic-era queries, the ~3-hour ΔT uncertainty (see DELTA_T_MODEL.md) means asking for "noon at -200-09-22" produces a JD that's ±3 hours uncertain at the level of the actual TDB time the ephemeris would use.

This is fine for day-level queries (the kind the Antikythera mechanism actually predicts) and a cause for caution on hour-level queries.