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The southern hemisphere has summer in December
and winter in July.
But will things always be that way?
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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.
Who knows?
Before we sign off, I want to comment quickly
on the leap second that we'll all be adding to our clocks
this coming June 30.
Just click over here to learn more.
That's our episode, Space Timers.
You guys can leave your questions
in the comments, along with your thoughts
on how you think humanity will handle the calendar 2,000
years from now.
Also, if you'd like to see an episode on what Earth's orbit
and rotation really look like in space and why,
let us know that, too.
As usual, I'll report and respond
on the next episode of "Space Time."
Last week we talked about different possible ways
to signal aliens, including maybe putting
planet-sized geometric objects in orbit around the sun.
Here are some of your comments.
Ben McLean asked, "if aliens can't pick up our radio waves,
then why are we trying to listen for alien radio waves
with SETI?"
Well, aliens wouldn't be able to pick up our stray radio
waves easily, but they would be able to pick up a beacon.
And the operating assumption of SETI for several decades
is that maybe aliens are broadcasting such a beacon
into space.
However, they might be using optical laser pulses
if they have limited resources or the kind of alien billboards
that we discussed in the episode.
Who knows?
acappellascience suggested that we put photovoltaics
on the billboards as an energy source.
That's true.
And maybe we could use that to get some kind of ion thruster
situation, because these things are going
to get blown out of orbit by the solar wind
without some kind of adjustment.
He also points out that if we put enough of them
around the sun that we could cover viewing angles from most
star systems.
That's also true.
With a few tens though, going in different orbital planes,
you could actually probably see them from most places,
because the viewing angle for very close orbits
is actually relatively big.
Jack King downloaded all the Kepler data into a CSV file
and left a link in his comment to a Google spreadsheet for it.
If you find anything, let us know.
To people with lingering questions about the balloon
challenge, please look at some of the answers
I've given in various threads that
have come up in the comments.
It should answer most of the remaining questions.
And finally, to everyone who's been
clamoring for another general relativity episode,
tune in next week.
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