Nineteen and Twenty-Eight

Sun, Moon, and Planets: In 1900, a storm blew a boatload of sponge divers off course and forced them to take shelter by the tiny Mediterranean island of Antikythera.

Diving the next day, they discovered a 2,000 year-old Greek shipwreck. Among the ship's cargo they hauled up was an unimpressive green lump of corroded bronze.

The PBS website (Nova: Ancient Computer) tells us more about this bronze lump:

Rusted remnants of gear wheels could be seen on its surface, suggesting some kind of intricate mechanism. The first X-ray studies confirmed that idea, but how it worked and what it was for puzzled scientists for decades. 

Recently, hi-tech imaging has revealed the extraordinary truth: this unique clockwork machine was the world's first computer.

An array of 30 intricate bronze gear wheels, originally housed in a shoebox-size wooden case, was designed to predict the dates of lunar and solar eclipses, track the Moon's subtle motions through the sky, and calculate the dates of significant events such as the Olympic Games.

No device of comparable technological sophistication is known from anywhere in the world for at least another 1,000 years.

But that's only background. One of the statements during the broadcast was:

The Greeks knew that 19 solar years are
equal to 235 lunar months are equal to 6940 days.

 To which Abq Jew responded:

Of course they did! And so did the Jews!

Why is this important? Abq Jew hears you ask. Well, let Abq Jew explain. Even better - let's let the folks at Judaism 101 explain. There is nothing simple about Jewish time!

First of all, you should understand that

The Jewish calendar is based on three astronomical phenomena:

1. the rotation of the Earth about its axis (a day); 
2. the revolution of the moon about the Earth (a month); 
3. and the revolution of the Earth about the sun (a year).

That's simple enough. But, of course, it get more complicated:

These three phenomena are independent of each other, so there is no direct correlation between them. On average,

• the moon revolves around the Earth in about 29½ days. 
• The Earth revolves around the sun in about 365¼ days, that is, about 12.4 lunar months.

When we Earthlings decide to build a calendar, the length of one Earth day gives us our basic measurement. But then - we Jews have got choices!

• We could build a completely solar calendar (like the Christians) that would keep holidays in their correct seasons. But we Jews have been commanded to really care about months. The year would be 365¼ days long. But how could we be sure to celebrate a holiday on the right day of the month?
• We could build a completely lunar calendar (like the Moslems) that would keep months steady. But 12 months x  29½ days = 354 days. That's about 11 days shorter than a completely solar calendar.  The holidays would drift backward 11 days every year. Pesach could be any time of the year - and we Jews know that Pesach must be in the spring.

What would be the most difficult way to build a calendar? (We Jews are always up for a challenge.) How about if we built a calendar that keeps the months more or less steady AND keeps the holidays in their correct seasons? Wouldn't that be cool? Judaism 101 explains:

To compensate for this [11-day] drift, the Jewish calendar uses a 12-month lunar calendar with an extra month occasionally added. The month of Nissan occurs 11 days earlier each year for two or three years, and then jumps forward 30 days, balancing out the drift.

In ancient times, this month was added by observation: the Sanhedrin observed the conditions of the weather, the crops and the livestock, and if these were not sufficiently advanced to be considered "spring," then the Sanhedrin inserted an additional month into the calendar to make sure that Pesach (Passover) would occur in the spring (it is, after all, referred to in the Torah as Chag he-Aviv, the Festival of Spring!).

So what's with this "observation" thing? Abq Jew hears you ask. We don't do that now, do we? We ain't got no Sanhedrin, so how do we set the calendar? Funny you should ask. Judaism 101 explains:

In the fourth century, Hillel II established a fixed calendar based on mathematical and astronomical calculations. This calendar, still in use, standardized the length of months and the addition of months over the course of a 19 year cycle, so that the lunar calendar realigns with the solar years. Adar I is added in the 3rd, 6th, 8th, 11th, 14th, 17th and 19th years of the cycle. The current cycle began in Jewish year 5758 (the year that began October 2, 1997).

In other words:

The Jewish calendar operates on a 19 year cycle.

FYI - there are (like you're suprised?) a few other calendar rules that we Jews have to observe:

Yom Kippur should not fall adjacent to Shabbat, because this would cause difficulties in coordinating the fast with Shabbat, and Hoshanah Rabbah should not fall on Saturday because it would interfere with the holiday's observances. A day is added to the month of Cheshvan or subtracted from the month of Kislev of the previous year to prevent these things from happening. This process is sometimes referred to as "fixing" Rosh Hashanah.

But wait! There's more!

The Jewish calendar also operates on a 28 year cycle.

That's because every 28 years the sun is in the exact position in the sky as it was when it was created, on the exact same day of the week, at the exact same time of the day. Right place, right time.

The Torah tells us:

G-d made the two great lights: the greater light to rule the day, and the lesser light to rule the night . . . And it was evening and it was morning, a fourth day.
                                                                            -Genesis 1:14, 19

And the Talmud explains:

One who sees the sun at its turning point [tekufat Nissan] should say, "Blessed is He who reenacts the works of Creation." And when is this? Abaya said: every 28th year.
                                                -Berachot 59b, Babylonian Talmud

Wait a minute! Abq Jew hears you cry. How did we get out to 28 years? Oy. Let's let Judaism 101 explain:

The Torah teaches us that the sun was created on the fourth day (Tuesday night / Wednesday day), and tradition teaches that it was created at the tekufat Nissan (the beginning of Spring; another tradition says the world was created on Rosh Hashanah).

In fact, there is a special blessing (Birkat HaChamah) that is recited once every 28 years, to commemorate the work of Creation and acknowledge G-d as the Creator of all things. ArtScroll has even published (in 1980) a 160-page book about this one-line blessing.

For this reason, we remember the work of Creation by reciting this blessing upon seeing the sun fully risen on the morning after a tekufat Nissan that occurs at the beginning of the fourth day of the week (that is, at 6 pm on a Tuesday), which occurs only once every 28 years.

But, if you are thinking Aha! At last I understand! - let Abq Jew (or, more correctly, Judaism 101) disabuse you of that notion:

Because this blessing is fundamentally related to the sun, its date is calculated using a solar calendar, rather than the usual Jewish lunisolar calendar. The Jewish solar calendar, like the ancient Julian calendar, assumes that the solar year is exactly 365¼ days. This calendar was established by the Talmudic sage Samuel and recorded in the Babylonian Talmud, Eruvin 56a.

The rabbis of ancient times were well aware that this calculation was not entirely accurate - indeed, Rav Ada in Talmudic times developed a somewhat more accurate calendar - but the rabbis opted for simplicity over astronomical precision so the mitzvah could be understood and observed even by the mathematically challenged. In addition, if a truly accurate calendar were used, tekufat Nissan would never occur at precisely the beginning of the fourth day of the week, eliminating this observance altogether!

Nevertheless, the discrepancies have already become quite significant over time. Birkat Hachamah was recited on March 25 of the Julian calendar (which was the equinox about 2400 years ago), but it is currently recited on April 8 of the more precise modern Gregorian calendar, about 18 days after the equinox. 

Oh - one more thing, as Peter Falk's Columbo used to say. One might think that Birkat HaChamah should be recited precisely at tekufat Nissan - on Tuesday evening. But since we can't see the sun at night, the rabbis ordained that Birkat HaChamah be recited on the following Wednesday morning - if and only if we can see the sun through the clouds (not a problem in New Mexico).

Abq Jew has been blessed with the opportunity to recite Birkat HaChamah in 1981 and 2009. The next time Birkat HaChamah will be (G-d willing) recited  is

Wednesday April 8, 2037

To make sure you don't miss this (if one is fortunate) thrice in a lifetime event, Abq Jew has thoughtfully provided a Countdown to the Sun Blessing on his Calendar page. Don't get excited - there about 8,762 days to go.

And Abq Jew brings all of this up because

Today is the 2nd Day of Rosh Hodesh Iyar
and the 1st Day of the Month of Iyar.

Go figure! Anyway, you can learn more about the Antikythera device here.

But if you've just about had enough of this - chill out! Here is Tony Bennett singing - what else? -  Fly Me to the Moon:

It's only Thursday, but
Shabbat Shalom, Albuquerque!
Good Shabbos, New Mexico!