Why doesn't Earth's axis change during the year?

  • My understanding is that the Earth's axis points in the same direction in space during its entire orbit around the sun. And this is what causes our seasons.

    My question is why doesn't the axis follow the orbital path (kind of like a car driving around a circle)?

    I imagine the Moon plays a part in this.

    As a secondary question, do the other planets in our solar system orbit the same as Earth (its axis points in the same direction in space for its entire orbit)?

    Why would you expect it to change? Changing the direction of the Earth's axis would require a *huge* change of angular momentum. It does change over thousands of years, but there's no mechanism to change it over a single year.

    It's not that I expect it to change. It's simply that I don't know why it works one way and not the other. I'm a computer programmer, not a math/physics whiz. :)

    I think the answer boils down to conservation of angular momentum, but I'll leave it to someone else who knows the physics better than I do to post an answer.

    My advice: get one of those little toy gyroscopes or a bicyle wheel on an axle, get it spinning quickly, and try walking (or driving) it around a light bulb such that the axis points the way you expect it should. You'll very quickly discover that you have to apply a huge force to make the axis change direction as you orbit the light bulb; there is no corresponding force on the earth, so the axis doesn't change direction. More generally, the answer to "why doesn't this happen?" in physics is usually "because nothing made it happen". The axis doesn't change because no force changes it.

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  • My understanding is that the Earth's axis points in the same direction in space during its entire orbit around the sun. And this is what causes our seasons.

    The second statement is correct. The axial tilt is the primary driver of the seasons.

    The first statement is not exactly correct. There is a small but persistent change in the orientation of the Earth's axis. The change over the course of a year is small. The change over the course of 13,000 years is large. In 13,000 years the Earth's axis will be tilted by about 47 degrees compared to the current orientation. In yet another 13,000 years the Earth will be oriented very similarly to how it is oriented now. I'll write more about this later.

    My question is why doesn't the axis follow the orbital path (kind of like a car driving around a circle)?

    That is not a good mental model. The only contact between your car and the ground are the bottoms of the four wheels on your car. The forces at these four small contact areas are what change your car's orientation and change your car's velocity vector. This means that the torque on your car as you turn the steering wheel and the centripetal force on your car that makes your care negotiate a turn are intimately connected. Turn the steering wheel gently and the heavy front end of your car follows suit. Assuming a gentle turn, the light rear end of you car soon follows suit. If you turn too hard, your car starts to skid instead of turn. Turn even harder yet and you risk having your car flip and roll. The motion of your car depends on that intimate coupling between force and torque.

    Gravitation doesn't work like that. It instead acts on each and every bit of the Earth, all together, all at once, all the time. There is a slight variation in the gravitationally-induced acceleration across the Earth due to variations in distance to the Moon (or Sun), but these variations are small. The net gravitational force on the Earth is very much decoupled from the small net gravity gradient torque on the Earth.

    The principal result of the gravity gradient torque exerted by the Moon and the Sun on the Earth is a very slow but large precession of the Earth's rotation axis (for more, read this wikipedia article on axial precession). This precession makes the Earth's rotation axis rotate about the normal to the ecliptic at a rate of one revolution per 25772 years. There are lesser effects collectively called nutations and polar motion that result from gravity gradient torque. These lesser effects are much, much smaller in magnitude but have much, much shorter periods. The largest of these lesser effects results from the 18.61 year cyclical variations in the lunar node. Even this largest component of nutation is small, two orders of magnitude smaller than the large but slow precession.

    My car analogy was only meant to indicate one concept - the front of the car is always pointing in the direction of travel, as opposed to say, always facing north. That's all. I meant no comparison to forces of any kind.

    @rmaddy: Right but you have to constantly apply energy (by keeping the steering wheel turned — it'll be fighting against you) in order to make that happen. A car on the ground is not at all the same as a planet in space. Your imagined model is not the "default" state at all.

    @LightnessRacesinOrbit Again, that's not at all what I was implying with the car analogy. I was simply trying to describe the direction, that's all.

    This is the scientifically correct answer, but it's not well-explained for someone not coming from either a physics or an astronomy/astrophysics background. You should consider simpler language, at least for the basics (i.e. everything up to the discussion on precession).

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Content dated before 7/24/2021 11:53 AM