Marking time

Calculating when Jewish holidays occur is now pretty easy, but it didn't used to be that way.  Since the months started on the new moon, and the holidays started at sunset in Jerusalem, messengers were dispatched to let other communities know when the festivals should be celebrated.  When the second temple was destroyed by the Romans, there were no more messengers, so many communities (the diaspora) started celebrating festivals over two days, just to be sure.  (Even today, they celebrate on two days out of tradition.)

And although a day seems like a simple thing to keep track of, it isn't.  That's because the time it takes for the earth to rotate (one day) has nothing to do with the time it takes for the moon to revolve around the earth (one month) which has nothing to do with the time it takes for the earth to revolve around the sun (one year).  So a lunar month lasts 27 days and 8 hours; a lunar year is 354 days 9 hours; a solar year is either 365.24219 days (tropical) or 365.2564 days (sidereal). Not to mention the moon is moving away at 4 inches per year, slowing its rotation, and the length of days may be affected by global warming as higher seas create a greater drag on the earth's rotation!

More trivia: The moon revolves at the same rate it rotates around Earth (that is, every 27 days and 8 hours) which is why the same face is always pointing towards Earth, and we didn't see the far side until 1959!

The ancient Sumerians had a pretty sophisticated "lunisolar" calendar -- 12 months, each starting with the sighting of a new moon, and an extra month was added periodically to keep the months in the same seasons.  (The Sumerians also gave us 60 minutes -- they liked 60 because it can be easily factored by 2, 3, 4, 5, 6, and 10.) The Jews used the Sumer  calendar (with some variations) while Europe adopted the Roman "Julian" calendar, which made the months slightly longer but got rid of the "leap month."  The Muslims, by contrast, adopted a strict lunar calendar, so a Muslim year is 354 days, and the months change seasons, because they lose abut 12 days every year -- so in 10 years a holiday that was in December will now be in October! (According to the European calendar, anyway).

Incidentally, it was the Egyptians who subdivided daytime and nighttime into twelve hours each, but the length of the hour depended on the season.  The Greeks--who also liked the number 60 but wanted a stable measurement of time--defined the hour as 1/24th of a day, a "minute" (as in "diminutive") as 1/60th of an hour, a "second" as 1/60 of a minute, a "third" as 1/60 of a second, a "fourth" as 1/60 of a third, and so on.  Of course, we no longer use "thirds" and "fourths" but "seconds" stuck.

More trivia: "Idus" is latin for "half division" and referred to the half moon (what we would call the full moon).  Since months started on the new moon, the "idus" was generally on the 13th of the month; however, when Julius Ceasar changed the months so they no longer followed the lunar cycle, "ides" was no longer relevant.  However, the term was still in use when Julius was assassinated on the 15th of March -- now known as the "Ides of March."

Of course, the Julian system wasn't perfect -- the years were fixed at 365.25 days, but the solar year is about 11 minutes shorter.  That means it gained one day every 131 years -- not bad, but the calendar was in use for over 2000 years!  In addition, early timekeepers got confused and started adding leap days every 3 years -- as a result, when Augustus took over in 4CE, he had to drop 3 days from the calendar to fix it.  In addition, there was no fixed point for starting the calendar, and so it was generally based on the rule of the regnant (i.e. 1 Julius, or 1 August).

More trivia: It is an urban myth that February used to have 29 days, and Augustus decided to move one day to August so it had the same number of days as Julius (July).  You probably didn't even know that rumor, but now you know it isn't true.

The idea of Anno Domini was set out in the 6th century and adopted sporadically, but it wasn't until the 16th century that the Gregorian calendar replaced the Julian calendar, and even that wasn't adopted universally until the 1920s. (Seriously!)  Of course, since Anno Domini translates as "The Year of our Lord" and refers to Jesus Christ, the Jews adapted it politically but not religiously.  (And even politically, the term "common era" is used.)  As countries adopted the Gregorian calendar, they had to skip ten days to move the spring equinox back to March 21.

Had any of these civilizations been particularly clever, they would have jettisoned the entire lunar calendar, because the moon isn't relevant to the seasons, it's just a pretty bauble in the night.  Then they could have come up with a meaningful division of years that didn't involve the pattern "31,28,31,30,31,30,31,31,30,31,30,31".  The Vikings actually tried this, but they may have oversimplified it a bit: They divided the year into two seasons, summer and winter, from April to October and October to April.  They then divided each season into weeks, and got rid of the months.  Apparently this worked quite well for them, as they didn't adopt the Gregorian calendar until the 1800s, some 200 years after it had been introduced.

More trivia: In the Sumerian calendar, there were no weeks; just days. The Jews defined the seven day week based on Genesis (God created the world in 6 days and rested on the 7th).  Why seven days?  Who knows. Maybe it was because of the four phases of the moon (28 day lunar cycle / 4 phases = 7 days.)  Maybe it just seemed like a good idea to rest every seven days. The Romans started with an 8-day week but later adopted the Jewish 7-day week.  The Chinese and Egyptians had a 10-day week, and the Celts (in Britain) had an 8-day/9-night week, which was closer to the 27.3 day lunar cycle, but must have been very confusing.

And last bit of trivia--actually, it's more of a rant: As the moon rotates every 27.9 days, and revolves around the sun every 365 days (same as earth), there are actually only 13 days a year on the moon!  If an earth sunset lasts 2-3 minutes (depending on season and latitude), then the equivalent lunar sunset would last 84 minutes! In fact, you'd only have to travel 10 mph to stay ahead of the terminator line!  So why is it that every science fiction film shows the lunar sunset overtaking the hapless astronauts in a matter of minutes?