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It Just Is

James Shaw
I wonder how much of what we 'know' has ever been properly explained? Our teachers repeat what they have been taught and our text books are re-writes of earlier text books. Perhaps that is the way to pass exams, don't think about what is missing, just repeat what was taught and so it goes on.


Lately I have been looking through some of my old 'how it works' books from my childhood, encyclopedias, atlases and 'online' to see what they say about the Earth's seasons. At least they all agree! It is all down to the tilt of the Earth's axis, the northern hemisphere points towards the Sun in the summer and away from the Sun in winter. Simple! We don't need to know anything more.


A simple experiment: Take a dinner plate and place an apple near the rim with its stalk pointing slightly towards the centre, a model of the tilted Earth. Now slowly and carefully twist the plate on top of a table so as not to disturb the apple until the plate has turned through 180 degrees. Now which way is the apple pointing? Do you still understand the seasons or did you have a book/teacher that really explained it? Perhaps you are a heretic and thought for yourself? Andy Millar raised some of these issues in "You don't need practical skills to be an engineer", 'knowing' how to do something can stop new thinking.


Have a virtual mug of coffee and think about it!
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    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.
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    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.
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