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Hebrew calendar

• The beginning of the Hebrew calendar
• predates Hillel the Nasi. In the sixth century BCE,
• during their captivity in Babylonia,
• the Jews adopted the Babylonian names of months.
• Before this, their months were numbered, not named.
• The Babylonian calendar, like the Hebrew calendar,
• was lunisolar.
• Gradually, over hundreds of years,
• teachers switched from direct observation
• of agricultural events
• to determine when to add an extra month to the year,
• and, to determine the start of each month,
• from direct observation of the first crescent moon
• to adding synodic months
• from the first year of the world,
• Anno Mundi 1, thought to have occurred
• on the sixth of September 3761 BCE
• just before midnight, Jerusalem time.
• In 1178, Maimonides codified rules
• for calculating the points and periods.

Interesting aspects

• It’s good that holidays and rituals
• are kept in synch with the revolutions
• of the earth, moon, and sun,
• but there are many interesting
• aspects to this calendar
• such as counting years from Anno Mundi 1,
• that is, from 3761 BCE in the Proleptic Gregorian calendar,
• having each year start on Rosh Hashanah,
• near the solstice, the time of year
• when God created Adam and Eve,
• counting a year by months and not by days,
• having each day start at sunset
• and so having no fixed length in minutes
• having each month start with a new moon,
• having a new moon start when its crescent is first visible,
• calculating a synodic month to be 29 days,
• 12 hours, and 793 eighteenths of a minute,
• having any extra month start before the last month,
• so that the first month would be in the spring
• having sunset occur when three medium-size stars are visible,
• having different opinions as to when the date changes,
• starting the day either at dawn or at sunrise,
• having an hour be one-twelfth of the time
• from sunrise to sunset, not a fixed number of minutes,
• this being shorter in winter and longer in summer,
• dividing the hour into 1080 parts
• having a seven-day week,
• not coincidentally the number of days
• that God took to create the world,
• and having Sunday be the first day of the week.
• Having a calendar
• is having good reasons
• for all these decisions,
• and then sticking to it.

Inaccuracies

• To correct for the fact that twelve lunar months
• fall short of the days needed for the earth to orbit the sun,
• every two or three years the year gets an extra lunar month
• (that is, for seven of every nineteen years comprising the Metonic cycle);
• however, this year still falls short one day
• (compared to the Gregorian calendar)
• every 231 years, so what the heck?
• One can get really serious about the synodic month
• being about six-tenths of a second too long.
• The actual length of the synodic month
• varies by about thirteen and a half hours,
• but it turns out that the tides
• are making the month longer
• while also slowing earth’s rotation,
• so that no calendar is staying in synch.