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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,

    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!
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    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!
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