Passover & the Paschal Moon

What this is. Passover falls on 14 Nisan, the spring full moon. In antiquity it was fixed by observation — sighting the new moon and the ripening barley — not by calculation, and the fixed Hebrew calendar is later (c. 4th c. AD). So this is an astronomical reconstruction of the first full moon after the spring equinox, not a record of what was observed. The lunar positions hold to minutes even in antiquity; the main uncertainty deep in BC is ΔT (the Earth's slowing rotation), which can shift the time of day and occasionally the date. Dates are given on both the modern Gregorian (season-anchored) and the historical Julian calendars — note how the Julian date drifts later across the millennia as that calendar slips from the seasons. The date of the Exodus is itself debated (early c. 1446 BC vs late c. 1260 BC); the calculator simply runs back to 1500 BC.