[MUSIC PLAYING] The southern hemisphere has summer in December and winter in July.

But will things always be that way?

[MUSIC PLAYING] Before we get into it, let's quickly recap why we have seasons in the first place.

See, Earth's equator is tilted from the plane of its orbit by approximately 23 and 1/2 degrees, so that different parts of the planet will receive more direct sunlight at different times of year.

In June, the northern hemisphere gets more direct sunlight, so it's northern summer and southern winter.

In December, the reverse is true.

Winter in the north, summer in the south.

Now, from one spring equinox to the next spring equinox, Earth does not actually move a full 360 degrees around the sun.

It only moves 359.98-something degrees.

That means that one cycle of seasons is completed about 20 minutes earlier than a full 360 around the sun.

The reason for this discrepancy is that the spinning Earth is basically a giant gyroscope.

See, Earth bulges slightly at the equator due to centrifugal effects from its own spin.

Since Earth's axis is tilted, one half of that bulge is a little closer to the sun than the other half at any given moment.

So the gravitational pull of the sun is slightly stronger on the slightly closer half, causing a net torque on the Earth like this.

You might expect that torque to tilt the axis back to zero degrees.

But instead, it causes the spin axis to precess, like a top.

Why?

Because that's just how gyroscopes work.

If you're interested, Veritasium has a solid video explaining this phenomenon that you can click over here to view.

We put that video and a few other good links about gyros in the description.

But the fact is that Earth's axis precesses in the opposite sense of Earth's orbit around the sun, so that the equinoxes and solstices backtrack along Earth's orbit.

Even the ancient Greeks understood this phenomenon.

It's called precession of the equinoxes.

And if you need help visualizing things, just stick a toothpick in an orange and move it around in 3D until things click for you.

Now, eventually, the cycle of seasons will backtrack a full 360 degrees, returning to its starting point.

That'll take a little less than 26,000 years.

Halfway through that, or after 13,000 years, the summer and winter solstices should end up swapping positions.

So leaving aside climate change, wouldn't that mean Christmas on the beach for the north and white Christmas for the south after a few thousand years or so, depending on exactly where we're at in the cycle.

Unfortunately, no.

Well, maybe.

Here's the deal.

The modern civil calendar, also known as the Gregorian calendar, is actually pretty finely tuned considering that it was built back in the 16th century.

But it doesn't track Earth's 360-degree orbit in space the way you might think.

Instead, it's locked to the solar year, to the seasons, so that it backtracks along Earth's orbit right along with the equinoxes and the solstices.

So the location in space of January 1 is constantly moving.

Now, originally, this was done so that Catholic Easter in Europe would always remain close to the spring equinox, which prevents a southern white Christmas by construction.

The calendar system has a mechanism to keep it in sync with the seasons.

But interestingly, leap years are not that mechanism.

Leap years are compensating for something different.

See, our calendar's divided up into a whole number of days.

But a full trip around the sun takes around six hours longer than a whole number of days.

So the location of Earth on successive New Year's Eves backtracks along the orbit.

Every four calendar years, the gap has grown to the equivalent of one calendar day.

So every four years, we add February 29 to re-sync with Earth's orbit and undo that backtracking.

But the calendar isn't trying to sync to Earth's orbit.

It's trying to sync to the seasons, which backtrack.

So the calendar has to be allowed to backtrack by about a day every 70 years or so if it's going to follow the precessing equinoxes.

We also need a little bit of extra slippage, because the annual discrepancy between the calendar and Earth's orbit also isn't precisely six hours.

Now, this syncing to the seasons and the adjustment for the not-quite-six-hour thing are achieved simultaneously in the calendar system via two extra rules for skipping leap year.

You skip leap years when the year is a multiple of 100.

But then you ignore that rule and have a leap year anyway when you shouldn't if the year's a multiple of 400.

This results in alternately overshooting and then undershooting every century in order to offset any long-term drift between the calendar and the seasons.

So this system keeps the calendar in sync with the seasons.

Almost.

There's still a very slow drift.

Under the current operating rules, the civil calendar still lags the seasons by about three days every 10,000 years.

Over 600,000 years or so, that accumulates to a six-month discrepancy, which would make December winter in the southern hemisphere.

Other factors could bring the south a white Christmas even sooner.

If you look on time scales of tens of thousands or hundreds of thousands of years, Earth's orbit is a lot crazier than you would expect.

The shape of the orbit flexes.

It swings around like a spirograph.

The orbital plane wobbles.

The rotation axis wobbles.

The simple precession of the axis that we've discussed today is just the tip of the iceberg.

Now, one effect of all this orbital craziness is that the mean tropical year is actually getting shorter by about a half second every 100 years.

Earth is also slowly spinning down due to its interactions with the moon, so that the mean solar day is getting longer, by about one to two milliseconds every 100 years.

Together, all of these effects would put the equinoxes ahead of the calendar by eight to nine days every 10,000 years, not just three, so that we could see kangaroos pulling Santa's sleigh in as little as 200,000 to a quarter million years.

Of course, all of this assumes that there are no further changes to the calendar system in the interim.

There are proposals for reform but no plans yet since there's really no rush.

The current calendar will work just fine for 1,000 to 2,000 years, maybe longer.

But by then, maybe we won't care about locking things to the sun and the seasons so much.

Maybe we'll just reckon time in atomic clock seconds or something like the Stargate system on "Star Trek," which, yes, I know was really just a way for the producers to tell you which season the show was on.

But whatever.

If we're a space-faring species by then, then locking time exclusively to Earth's seasons might be weird.

And we end up keeping the current calendar, but just detach it from our home planet.

Or maybe at some point, New Zealand will take over the world and force the issue.