Sun and moon in the calendar

Israel counted its months by the moon. A month began with the new moon, when the first thin crescent was seen at dusk, and lasted 29 or 30 days; the Hebrew word chodesh means both “new moon” and “month”. The great feasts fell at the full moon in the middle of a month: Passover on the fourteenth day of the first month and Unleavened Bread on the fifteenth (Leviticus 23:5–6). A psalm calls for the ram’s horn “at the New Moon, and at the full moon on the day of our Feast” (Psalm 81:3).

Twelve moons make only about 354 days, some 11 days short of the sun’s year, so an extra month had to be added from time to time — in later Jewish practice a second Adar, seven times in nineteen years. Papyri from Elephantine in Egypt suggest Jews there used such a cycle in the 400s BC. The oldest month names, such as Abib, the month of ripe grain (Exodus 13:4), follow the farm year; after the exile Babylonian names such as Nisan came in. By later tradition in the Mishnah, fire signals from the Mount of Olives announced the new crescent.