I've talked a lot
about precession and the changes in the
inclination of Earth's orbit, but I haven't told you
why they are happening. And I'm not going to go
into the physics of it. It's a little bit beyond this
discussion right over here. But they're really a
byproduct of the Earth and sun's interactions with
Earth and with the fact that Earth is not
a perfect sphere. So if I draw Earth-- so this
is my little drawing of Earth, and let me put the poles over
here, North Pole and South Pole, it actually turns
out that Earth is fatter than it is taller. So if you were to
measure Earth's diameter along the equator,
it is 43 kilometers, which is approximately 27
miles longer than if you were to measure its
diameter from pole to pole. So longer than the
pole to pole diameter. And the fact that Earth has this
equatorial bulge, that it's not a perfect sphere--
and once again, I'm not going to go
into the math here-- it's the interactions
between that, I guess you could call it, that
one asymmetry of the Earth, it's that interaction
between that and the pull of gravity
between the Earth and the sun and the moon that
causes these long term cycles, this axial
precession and other less noticeable changes
in Earth's orbit. And as we'll see
in the next video, these aren't the only types
of changes in orbits we have. We also have changes
in the actual ellipse that Earth's orbit has
actually rotates over time. But that's due more
to interactions with Earth's orbit,
and the orbit of other planets in
our solar system. And once again, it's
one of those things that happened over thousands
and thousands of years. So all of these changes,
they're because Earth isn't completely symmetrical,
more fat than it is tall, those interactions
between the gravity of the sun and the moon. And Earth's orbit, as a whole,
changes because of interactions with other objects inside
of our solar system.