It Just Is

Alasdair,

Agreed that the gravitational force acts on all of the Earth. I am attempting to simplify the situation by considering the Earth as a cloud of masses, a finite element analysis of sorts.


You confused me by suggesting that a gyroscope was a simple model for explaining why the Earth's axis remains pointing in one direction, (towards the Pole star) and then by saying that there are no forces acting on the Earth to displace the axis so it stays pointing the same way. I am saying that the Earth is subject to the Sun's gravity and that, and its momentum, causes it to orbit.


Now if one reduced the Earth to a single mass at its C of G clearly that 'model' would orbit but what would the rest of the Earth do relative to the C of G? I tackle that question by considering single elements of the Earth, each acted on by their share of the total gravitational force. That model suggests to me that the Earth retains its integrity, (there would be stresses because near and far points of the Earth don't exactly share the same gravity/velocity as the points on the plane through the C of G), as all elements orbit together. It follows then that any line between any two elements retains the same relationship to any other line between elements. In other words the local frame of reference stays the same, (Paris is still to the south of London etc.) but that reference frame orbits the Sun which means the day of the year, (season-wise), stays the same. Well we know it doesn't so what keeps the axis Pole star-aligned, there must be something non-gravitational. That something, I suggest, is the gyroscopic action produced by the daily rotation of the Earth.


I say 'suggest' because my original point was that the vast majority of us are given an explanation that isn't complete yet 'no-one' seems to be bothered! Undoubtedly there is a more advanced analysis to be found but not from me!

James,

I am afraid I am still struggling with your 'explanation', though I can't decide if it is because it is oversimplified or if it is not simplified enough. I can just say I can't align it with my understanding.

When I said there are no forces acting on the Earth to displace the axis, what I meant was that there are no unbalanced forces. Further, while I can agree with your comment 'In other words the local frame of reference stays the same, (Paris is still to the south of London etc.) but that reference frame orbits the Sun', you then say 'which means the day of the year, (season-wise), stays the same' which would mean the local frame of reference has changed with respect to the stars. I am afraid this is where you lose me.

If you want to think about it from a more terrestrial viewpoint, a spin bowler in a cricket match imparts spin to the ball when he releases it. The axis of rotation of spin does not change during flight even though it is following a parabola due to Earth's gravity, because the gravitational pull is balanced over the whole of the ball. Note there will also be a tidal force, but I doubt if it would be measurable.

Alasdair

Alasdair,

I was trying to explain how I would expect a non-spinning body, with momentum, to behave in gravitational orbit. Essentially the sum of all its parts, each part at the same orbital radius behaving in the same way, each part keeping its same local relationship to all of the other parts. The parts facing the centre of the orbit would continue to face the centre.


Consider then an axis perpendicular to the orbital plane, as viewed from the orbital centre. Consider that axis tilted at its top (North) towards the centre. The orbital radius of the North pole remains the same and the orbital radius of the South pole remains the same throughout the orbit, but the radii are different so the axis remains inclined. That means that the North pole continues to be nearer the orbital centre and the axis changes direction celestially. That is how the simple model of the seasons ought to work, but the reality, as we all know is different. Some mechanism is needed to explain that.


The Earth is spinning on its axis, does that explain it? Well it explains day and night but it doesn't intuitively explain a celestially-aligned axis, unless one has observed the behaviour of a gyroscope, which, freely mounted, will act to preserve its angular momentum and axis of rotation. That, I suggest, covers 98% of the missing 'it just is' of the common non-explanation of the seasons.


Taking the example of the cricket ball. First that is different to the Earth's orbit because the forward velocity is so small (0.6% of orbital velocity). For a non-spinning ball I would expect the top to stay on top and the bottom to stay on the bottom, i.e. the vertical axis remains vertical even though the trajectory curves. For a spinning ball the situation gets far more complicated as the difference in air flows generate forces as demonstrated by the rotor ship.

James,

I think I finally understand your explanation - we all approach things with inbuilt assumptions and I didn't realise you were assuming a non-spinning body while I was assuming spinning. I thinkg the problem is contained in your comment "it doesn't intuitively explain" which really opens up a can of worms. We all rely on an intuitive understanding of issues which are based on assumptions derived from our understanding of the situation. If the situation is slightly different from what we understand, or if our depth of knowledge means our assumptions are different, we can have a completely different intuitive explanation.

I think that if you want a complete and clear explanation of the seasons and the earth's motion and orientation around the sun you will need to go to someone else as I have already reached the limit of my understanding, or at least my ability to explain my understanding (which, to go back to earlier posts, does not mean it is complete or even correct), so I will slightly paraphrase Andy Millar's earlier comment, "I think it is more complicated than that".

Regards,

Alasdair

Alasdair,

I rather suspected you were looking at 'the problem' from 'the answer' backwards, whereas I was trying to work from what we seemed to being taught. Both approaches can work, but not at the same time!

Motion of bodies is a subject that confuses many, (there is an Australian who makes a living by 'explaining' science yet when it comes to explaining motion it is quite clear he doesn't understand it, relying on stock answers which often don't fit the question!).

My model, (I don't say it is right), puts all parts of a non-spinning body in circular orbits, an axis-maintained model has only the C of G in a circular path, everywhere else describes an ellipse. (Think of something like a 'No Entry' sign being moved in a circle on a flat surface. In one case the white bar is aligned with the centre of rotation, in the other the bar stays horizontal as the complete sign follows a circular path.

Not quite related but I was playing with a couple of meshed gear wheels, (avoids slippage as when using coins), of the same size. I doubt if few people would immediately grasp what happens when one rotates around the other - even in front of their own eyes!).

Regards,

Jim

Isaac Newton sorted a lot of this out, but then he was a very strange character. The conundrum: Is it better to be a very strange character but to achieve extraordinary things? Or to have a happy sociable life but therefore be too distracted to change the world? (To be fair, it has been suggested that some of his later eccentricities were due to mercury poisoning. On the other hand, this was mercury poisoning whist experimenting with alchemy...)


Cheers, Andy