Happy Thanksgivukkah everyone! As you've probably heard, Thanksgivukkah is the collision of a secular American holiday, Thanksgiving, with a historical and religious Jewish one, Hanukkah.

This festive overlap is rarer than a planetary alignment, having occured just a couple times in history. And because of some strange calendar properties, this is basically the last Thanksgivukkah that will happen for more than 75,000 years. For humans, that’s an unfathomably long time. And it got us thinking: What can we say scientifically about how the world will look that far in the future?

Will the stars in the sky still be familiar? Will the continents have shifted? Will our cities be drowned below 1,000 feet of sea level rise? Some of these questions are easier to answer than others.

But first, let’s get into just what is causing Thanksgivukkah. This isn’t the first time that a Thanksgiving/Hanukkah holiday alignment has happened historically. Abraham Lincoln made Thanksgiving an official national holiday in the U.S. in 1863. And back then, Turkey Day wasn’t confined to falling on the 4th Thursday in November (that rule was instituted by President Franklin D. Roosevelt in 1939 to move Christmas shopping a bit earlier). Instead, it appeared on the last Thursday in November.

So in the late 19th century, two Hanukkahs actually fell on Thanksgiving: Thursday, Nov. 29, 1888 was the first day of Hanukkah in that year and Thursday, Nov. 30, 1899 was actually the fourth day of the Jewish festival of lights.

But now, vagaries in the Jewish calendar have conspired to make this year’s Hanukkah the earliest possible Hanukkah for a few centuries. To understand why, we need to think about what exactly it is that various calendars are measuring.

Image: Wikimedia

What is a year? It’s the time it takes the Earth to go once around the sun. Ancient people determined that a year had passed by checking to see that the sun had come back to the same position in the sky on a particular day. But that takes 365 and one-quarter days to achieve. A much easier way to note the passing of time is by looking at the phases of the moon. Going from one full moon to the next – a process typically taking about 29 to 30 days – is called a month. This happens roughly 12 times a year, so if you waited for the moon to go from full to new back to full again 12 times, you could be reasonably sure that a year had gone by.

The Jewish calendar is a lunar one and so takes its cues from the moon. The problem is that 12 moon-turns don’t actually quite add up to 365 days (they’re shy by about 11 days) so a lunar calendar will tend to shift just a bit earlier each year. The key dates in this calendar – holidays and such – will start to drift relative to the solar Gregorian calendar used worldwide for civil and administrative purposes. This is what happens to Jewish and Islamic holidays, and it’s why they don’t appear on the same solar-calendar day each year. Hanukkah typically starts mid-December though it can start anywhere from late-November to late-December.

But the ancient Jews figured out a correction to their calendar to get it back on track with the sun. Just like the solar Gregorian calendar has to add a leap day every four years, the Jewish calendar adds a leap month every two or three years1. This extra month comes after the last month of the Jewish year, called Adar, and is creatively called Adar II. (To make things really screwy, when there is an Adar II, it is actually considered the “real” Adar so holidays are scheduled during it and not the first Adar). As long as you add seven leap months every 19 years, the lunar calendar lines up pretty well with the solar one.

But still the alignment isn’t perfect. The leap month fix introduces another problem, where the Jewish year is now just slightly too long, by about 6.2 minutes. Every 231 years, it adds up to an extra day. And that causes the range of dates within which the Jewish holidays can fall to drift very slightly later in the year over very long periods of time. Right now we’re living in a time when this drift has pushed Hanukkah so that the it can start no earlier than Nov. 28, allowing it to just barely overlap with Thanksgiving. Left unchecked, the Jewish holiday drift will continue. Hanukkah will arrive later and later in the year, eventually starting in January, then February, then late spring, then summer, fall, and finally, after more than 75,000 years, will once again have a chance to overlap with Thanksgiving2.

So what will the world look like in the year 77,094, when Thanksgivukkah comes again?

" Geosphere; May 2006; doi: 10.1130/GES00012.1

Well, to begin with, the night sky will be quite different. Our planet wobbles around on its axis like a top, with our north pole making a complete circle every 26,000 years. In 75,000 years, the Earth will be pointing toward the star Vega, which will supplant Polaris as the North Star. The sun will also have moved through the galaxy, changing the positions of many stars in the night sky and rendering the constellations unrecognizable. Several hundred bright supernovas will have made appearances in the sky over this period.

On 97,000-year timescales, the Earth’s tilt also shifts, meaning that in the far future it will have changed a few degrees, altering the amount of sunlight each part of the ground receives. Changing the Earth’s axial tilt will likely alter the seasons and could kick our planet back into a global ice age. We are currently sitting pretty after the end of the last glaciation period, but some models suggest that a new one will start in about 50,000 years. Historically, ice ages last an average of 100,000 years. Global warming could delay the coming of this icy era. If humans stop adding new carbon into the atmosphere by 2200, some models suggest the delay could be 5,000 years.

In about 50,000 years, gravitational friction from the moon will have slowed the Earth enough to lengthen the day on Earth by about a second (meaning more calendar headaches to anyone around). By the year 77,094, many asteroids and comets of significant size will have hit Earth. Impacts comparable to the Tunguska event that flattened trees over 2,000 km2 of Siberia in the early 1900s are expected roughly once a millennia, or possibly even more frequently. An object roughly half a kilometer or larger in diameter – probably the minimum size needed to cause a large-scale global catastrophe – should, statistically speaking, hit Earth at least once in the next 100,000 years.

The Earth’s crust will have shifted a little before the next Thanksgivukkah. The Mid-Atlantic Ridge is producing new ocean crust at a rate of 2.5 centimeters a year. At that rate, the Atlantic Ocean would be just under two kilometers wider in 70,000 years. The Pacific plate is moving even faster, as much as 9 cm a year near Hawaii. In 75,000 years, Hawaii will have shifted almost 7 kilometers to the northwest.

A 75,000-year period is also a long enough timescale for plenty of animals to have gone extinct, either through natural processes or because of human influence. Remember that 75,000 years ago, Northern Europe was covered in far different fauna, including woolly mammoths, cave lions, tarpans, and other Pleistocene animals. It's entirely possible that turkeys will have long gone the way of the dodo by the next Thanksgivukkah (alternatively, they could breed into super-turkeys and overrun the globe). It also seems probable that new species will have evolved to occupy the niches left open by extinct creatures, though exactly when new organisms will appear or what they will look like is basically impossible to predict.

But really, in all likelihood, there won’t be Thanksgiving, Hanukkah, America, or Judaism at all. You can nitpick with me here since we have no idea how long any particular religion or country will last and maybe these will stick around much longer than I expect them to. But it’s only been something like 10,000 years since people started to organize themselves into complex societies. That’s 10,000 years for every single empire ever to have risen and fallen, every organized religion to have been born and died, every civilized idea to have come and gone. Are we going to go through that another seven or eight times? Who knows. Maybe the niche exploited by the super-turkeys will be the one left open by us.

If humans don’t survive the changes that Earth will go through in the coming millennia, most of the things we built will also be gone. Over thousands of years, cities will crumble, rivers will overflow and break their dams, and satellites will fall from their orbits. The Arecibo message, beamed to stars 25,000 light-years away, will have reached its destination and, if anyone is there to hear it, could have made its way back to Earth. A few spacecraft, like Voyagers 1 and 2, will still be out there floating. Rock monuments, like Mount Rushmore, would not yet have completely eroded away. One day, there will basically be nothing left to testify that humans were ever here at all.

But, hey, nothing lasts forever.

1In the old days, the priests had a useful check on their calendar with reality by seeing if the first month of the year (called “Aviv” or spring) came when spring started. If Adar, the last month, was ending and spring had not yet come, the priests would just say “Okay. Do over. Adar II, guys,” and get the calendar back in sync.

2This is only sort of true. Hanukkah, like all Jewish holidays, starts when the sun goes down the day before the “first” day of the holiday. So even though Hanukkah will start at the earliest on Nov. 29 in the future, the candles for the first night will be lit on the 28th (when Thanksgiving is also happening) in 2070 and 2165. Perhaps the Thanksgivukkah meme will be dusted off again in those years for nostalgia’s sake. Also, since Jewish law requires that Passover appear in spring, it seems likely that at some point they will fix the calendar to prevent this drift from going too far.