Thanksgivukkah: Behind the rare convergence of lunar, solar calendars

Hanugiving? Thanksgivukkah? Whatever you want to call it, this holiday combo hasn't happened since 1888 and likely won't happen again for another 70,000 years.

The convergence of Thanksgiving and Hanukkah is the result of two different calendars that regularly fall out of sync, says Robert Kirschner, museum director for the Skirball Cultural Center and a scholar of Jewish texts.

Kirschner says the Jewish calendar is based on months that follow the cycles of the moon more closely than the modern calendar.

"The beginning of the lunar month is the new moon, and that's just a sliver," said Kirschner.

The moon then waxes until it's full, marking the mid-point of the month. It then wanes until it's a sliver again. That makes up one lunar month, and takes around 30 days to complete.

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It takes about 12 of these cycles for all four of seasons to pass. That's how the Jewish calendar measures a year: It typically has 354 days total.

Such a calendar was a useful time-keeping system for ancient Hebrews spread across different regions because anyone could look up, see the same moon, and figure out the date, Kirschner says.

By contrast, the modern calendar is based on how long it takes the Earth to orbit the sun once.

That calendar traces its roots back 5,000 years to ancient Egypt, says E.C. Krupp, director of the Griffith Observatory and a scholar of ancient astronomy.

Back then, Egyptian astronomers realized it took about 365 days for the sun to return to the same rising and setting points on the horizon.

Egyptians called that a year and divided the year into arbitrary chunks resembling months. Thus the basics of the modern calendar were born.

But as anyone born on Feb. 29 knows, in reality it takes a little more time than 365 days to orbit the sun.

"So what happens over time is the error — the difference between how long it really takes the earth to go around the sun — accumulates," Krupp says.

That's why every four years we add a leap day to the calendar to stay on track.

Pregnant Year

The Skirball Center's Robert Kirschner says the lunar-based Jewish calendar has a similar problem. But it's corrected with a leap month. Every few years, an entire lunar cycle is added to the calendar.

He says in Hebrew the term for such a year is "shanah meuberet," which means "pregnant year," because it is carrying an extra month.

These pregnant leap years can have as many as 385 days.

This reset keeps Hanukkah from sliding into, say, October, so no luck for a "Hanuween" party.

Still, Kirschner says even with corrections, there is usually an 11-day difference between the 354-day Jewish calendar and the 365-day solar calendar.

"And that is the reason why the Jewish holidays drift a little, compared to the solar calendar, but they never go entirely out of season."

Of course, all of this would change if we didn't rely on the cycles of the sun or the moon, but instead synced calendars to atomic clocks, which measure time by counting the beats of a very reliable vibrating atom.

But then we wouldn't get this once-in-an-eon opportunity to light a Turkey-shaped menorah and nosh some pumpkin pie kugel.