Thursday, September 9, 2010

Rosh Hoshanah

Happy new year to my Jewish friends.

Rosh Hoshanah began last night at sundown.  In Hebrew (ראש השנה), it means "head of the year" and marks the start of the traditional Jewish civil year.  It's followed 10 days later by Yom Kippur, the Day of Atonement.  Sundown on Rosh Hashanah is marked by the blowing of the shofar (a ram's horn) and followed by a day of rest (and typically a dinner with family and a service at the temple as well).

The LORD said to Moses, "Say to the Israelites: 'On the first day of the seventh month you are to have a day of rest, a sacred assembly commemorated with trumpet blasts.  Do no regular work, but present an offering made to the LORD by fire.' "  (Lev 23:23-24 NIV).

Why do Jewish holidays begin at sundown?  Because in Genesis 1, the creation story has the repeating refrain "And there was evening, and there was morning - the # day." (where # is 1st, 2nd, etc.).  For whatever reason , the ancient Hebrews saw sundown as the beginning of the new day.

So why is a science/education blogger writing about this?  There are two interesting things here.  The first is that, on the Jewish calendar, today begins the first day of the year 5,771.  Why 5,771?  Because, it's been 5,771 years since the creation of the world by God as described in Genesis 1.

Now "evilutionist" geologists like me would respond that the Earth is not less than 6,000 years old, there is abundant, compelling evidence for an age closer to 4.5 billion (750,000 times older!).  One day I'll post a bit about how 18th and 19th century geologists, long before radiometric dating, came up with an ancient age for the Earth just by looking at rocks and applying a bit of logical reasoning.

Now of course, most modern Jews, like most Christians, accept the results of modern science and don't really believe in such a young Earth but there are some in the Orthodox community, like many Evangelical Christians, who do believe that the Earth is, in fact, a few thousand years old.  Why?

Here's the reason from an Orthodox Rabbi creationist:

I underlined the word theories, for it is necessary to bear in mind, first of all, that science formulates and deals with theories and hypotheses while the Torah deals with absolute truths. These are two different disciplines, where reconciliation is entirely out of place.

From the Answers in Genesis Christian creationist website:

By definition, no apparent, perceived or claimed evidence in any field, including history and chronology, can be valid if it contradicts the scriptural record.

Pretty similar!  If the Torah/Bible says it, it must be true.  Personally, I don't think we should learn science from a text thousands of years old from a desert nomad community, especially when it conflicts with what we see in front of us with our own eyes, but that's just me.

The other interesting thing about Rosh Hashanah, at least from my perspective as an instructor for a course in Ancient Astronomy, is that it's based on a lunar calendar (like the date of Easter in the Christian tradition or Ramadan in Islam).  That's why such holidays don't fall on the same day each year.

The Jewish calendar is best called a lunisolar calendar since it based on both moon phases (the derivation of our word "month") and leap months are periodically added to keep it in sync with the tropical year (the year defined by solstices and equinoxes).  It's composed of 12 months of 29 or 30 days:

Nisan (30 days), Iyar (29 days), Sivan (30 days), Tammuz (29 days), Av (30 days), Elul (29 days), Tishrei (30 days), Cheshvan (29 or 30 days), Kislev (29 or 30 days), Tevet (29 days), Shevat (30 days), and Adar (29 days).  During leap years, as 5,771 is, an extra month is added - Adar I of 30 days is inserted before Adar (which becomes Adar II) of 29 days.

Why alternations of 29 and 30 days?  Because the number of days from new Moon to new Moon is 29.53059 days (29 d, 12 h, 44 m, 2.8 s) - this is called the synodic month.  That fractional number of days makes it a real bitch to use phases of the Moon as a calendar (even though most ancient cultures, including the Hebrews, did so).

Why add the extra month during leap years? If you didn't, the holidays would shift out of the season (e.g. autumn) in which they're supposed to be observed and you have to get your cycle of Moon phases to somehow mesh with the tropical year (which rounds off to 365.24 days).

The Jewish calendar is synced with the Metonic cycle of 19 years, 12 of which are regular years and 7 are leap years (years 3, 6, 8, 11, 14, 17, and 19).  Leap years are when the extra month are added.  The Metonic cycle is named after Meton of Athens (5th century BCE) but was known as far back as the ancient Babylonians.  Turns out that 19 x 365.24 d (# of days in a tropical year) = 6,939.56 d / 29.53 d (# of days in synodic month) = 235.00 cycles of phases (it's not exact, but pretty damn close).  All lunar calendars sync to this Metonic cycle because of this coincidence (it's a coincidence of the human era because these values change over geologic/astronomic time periods).

Devising calendars that work is surprisingly difficult!

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