What causes precession and other orbital changes
About this video
Sal introduces the idea that Earth's orbit and axis change over very long timescales. He explains that these changes come from Earth not being a perfect sphere: its equatorial bulge interacts with the gravity of the Sun and Moon. He says the mechanics are for later and previews changes in the shape of the orbit in the next video.
You'll be able to
- explain why Earth's equatorial bulge matters for its long-term motion
- identify the forces that drive long-term changes in Earth's orbit
- distinguish changes in the axis from changes in the orbit's shape
Key words
- Precession
- A slow change in the direction of Earth's rotational axis, driven by gravitational forces on Earth's bulge.
- Equatorial bulge
- Earth is wider through the equator than pole to pole, so it is not a perfect sphere.
- Gravitational interaction
- The pull that the Sun and Moon exert on Earth's bulged shape over long periods.
- Orbital change
- A gradual alteration of the path Earth follows around the Sun, happening over thousands of years.
Transcript
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.
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