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Current time:0:00Total duration:7:07

We've learned that
axial precession, it's not a change in the
tilt or the obliquity of our rotational axis, it's
a change in the direction. And over a long period
of time, 26,000 years, it kind of traces out a circle. And the main affect
of that is that if we wait long enough that our
rotational axis, or you could say almost the
North Pole, will be pointed in a
different direction. And so if our rotational
axis is pointed in a different direction
after a long enough time, then the absolute
point in our orbit, if we use the sun as
our frame of reference, the point in our orbit when
we are most pointed away from the sun, or when the
Northern Hemisphere is most pointed away from the sun,
will be earlier in the orbit. Now I emphasize that
that won't necessarily mean earlier in our calendar,
because our calendar, by definition, takes
into consideration, I guess, or it's
more based on when we are furthest tilted away
from the sun or furthest tilted towards the sun. So even though if we wait 1800
years, like the example I gave, we will be most tilted
away from the sun. The Northern Hemisphere
will have its winter equinox at an earlier
point in the orbit. According to our calendar, it
will still be December 22nd. If our calendar
instead was based-- and it's not based on
this-- but if our calendar was based on the
exact point in orbit, then our year would be about
20, 25 minutes longer every year and then the date for
the start of winter actually would go back. 1800 years later, the date
of the start of winter would be November 22nd. But that's not how we
measure our calendar. Our calendar is actually
measured from equinox to equinox. From December 22nd or 21st,
there's slight fluctuations depending on the calendar,
but that will always be the date that we are most
pointed away from the sun. That will not be necessarily
the date at this exact position relative to the sun itself. And that's why the actual
perihelion does change. Because if this is
always December 22nd, and if we, at first, assume
that the perihelion is always at the same fixed point in
space relative to the sun, although that's not
exactly the case, but if we make that
assumption, then it will be further and further
after that December 22nd, further and further
after that time, that we are most pointed
away from the sun. And that's why you have
this kind of pushing back of the perihelion. Now, what I want to
add to this video is that the perihelion
itself is also changing. So if I draw the sun
again, and right now our orbit looks
something like this, and I'm going to exaggerate
the eccentricity of it so that the perihelion
and the aphelion are a little bit clearer. So right now this
is the perihelion. This is the aphelion
based on the way I drew it right over
there in different colors. I don't want to show that's
necessarily where Earth is. Perihelion and
aphelion, there is also a rotation of this,
of the perihelion. And sometimes this is called the
precession of the perihelion, or perihelion precession,
or apsidal precession. These are all very hard to say. And so if we wait several
thousands of years our orbit might look a
little bit like this. The actual perihelion
will have rotated. So our orbit will
look like this. The actual ellipse would
have rotated a little bit. You wait a little bit longer,
it might look like this. And obviously, I'm
once again talking about over thousands
and thousands of years. From a year-to-year
basis, you really wouldn't notice the difference. But what that does
is, is we talked about the axial precession,
that this change in direction of our rotational axis,
it takes 26,000 years to complete one period. So 26,000 years from
today, our polar axis, if we don't think about
our rotational axis, if we aren't too concerned
about the actual change in tilt, which there will be some
small change in tilt, but 26,000 years
from now, our pole will roughly point in
the same direction again. We would have completed
one whole period of axial precession. However, it does not take
26,000 years for whatever our date of perihelion is today. So it's in January. I actually don't
know the exact date. You can look that up. But whatever that
data is in January, it will not take 26,000 years
for it to be that date again. And it would have taken 26,000
years if the perihelion itself were not changing, if it
always stayed fixed over here, if we did not have this
apsidal precession. But since it is
also changing, you could kind of say it
is over 1,000 years moving in that direction while
our January date is moving in that direction, they
will actually meet sooner, so that the precession will
be back on whatever date it is on January, less
than 26,000 years from now. And actually the exact time, and
I haven't done the calculation, but this is what
I've read, is that it will be 21,000 years from now. And then on top of that,
if that's enough for you that not only is the direction
of Earth's rotational axis changing and the
tilt is changing and that the perihelion
and the aphelion are also rotating around,
it's also the case that the eccentricity of the
orbit itself is changing. So over long periods
of time, Earth's orbit becomes more or less eccentric. And we've learned that
almost circular has-- well if you're circular
you have no eccentricity and then you can become
more and more eccentric, which means you're
more and more of kind of this flattened out ellipse. And these cycles occur,
these eccentricities cycles occur over approximately
100,000 years. And so to revisit the
Milankovitch cycles-- and once again this is a theory. We're not sure whether this
is necessarily causing our ice ages or whether
this is necessarily a major influence over
long-term climate change, but the theory of
Milankovitch cycles is that over long
periods of time, if the eccentricity
changes enough and if it coincides with when
the perihelion and the seasons also coincide, maybe that's
enough to start an ice age. Or maybe that's enough to
take us out of an ice age. And actually, if you want
to throw something even more on that, the actual
plane of our orbit also changes over time mainly
because of interactions with the outer planets. Anyway, I'll leave you there. As you could imagine, this
is a very complex topic, but hopefully you now
have an appreciation of all the different ways our
orbit can change and maybe start to think about how that
might affect our weather. Although, we don't
necessarily know how it does or whether it even
really does affect going into or out of ice ages.