The sun and moon keep different time, and reconciling them is the oldest problem in the sacred calendar. The answer, found by Meton of Athens: 19 solar years almost exactly equal 235 lunar months — they disagree by only about two hours per cycle. After 19 years the full moon returns to the same date. It sets the Hebrew calendar and the date of Easter. Slide the years and watch them re-align.
The Metonic cycle: 19 tropical years = 6939.60 days, and 235 synodic months = 6939.69 days — a mismatch of only ~0.09 day (~2 hours) per 19-year cycle, so the lunar phases recur on the same solar dates. It underlies the Hebrew calendar’s 7-leap-months-in-19 rule and the computus for Easter (the ‘golden number’). A fail-loud self-check throws unless 19 solar years and 235 synodic months agree to within a few days and round to 235. ◆ real positional astronomy, node-verified.
LIT: the 19:235 near-coincidence is exact astronomy. AMBER: which calendar rules a culture built ON it (leap-month placement, Easter) are conventions; the residual ~0.09 day/cycle slowly accumulates — the cycle is very good, not perfect.