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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 know the Earth does all sorts of 'wobbles' that produce significant change over a long period but am I missing something with regard to what is meant by 'orbit'?


    Consider a spherical body in circular orbit. Let's make it very small, the weight at the end of a length of string, would that be in orbit if we whirled it around our head? Consider another identical weight on an identical length of string, shouldn't it behave the same way? Now imagine we were quite good at this and whirled both around our head at the same time but displaced by 100 mm. They should both behave the same way and the distance between them would stay the same. Add a few more weights and strings and they could all be 'flying' in formation, fixed in their local frame of reference, a frame of reference that would be 'in orbit' rotating around the centre. There is a force acting on all these weights, the equivalent of the gravitational attraction between 'the Sun' and all these satellites. Any axis that one chooses to define between these satellites would also be rotating, if you like pointing towards the 'Sun' on the left of the classic season diagram and towards the Sun on the right of the diagram.


    All these objects are constantly 'falling' towards the centre and rotating as a consequence, taking their local axes with them. That at least is my understanding of 'orbit'.


    Lisa, As the great Doreen Tipton, (look her up!) says "We think therefore we yam!" (Maybe only works for those familiar with Black Country ways!)
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    Alasdair, I know the Earth does all sorts of 'wobbles' that produce significant change over a long period but am I missing something with regard to what is meant by 'orbit'?


    Consider a spherical body in circular orbit. Let's make it very small, the weight at the end of a length of string, would that be in orbit if we whirled it around our head? Consider another identical weight on an identical length of string, shouldn't it behave the same way? Now imagine we were quite good at this and whirled both around our head at the same time but displaced by 100 mm. They should both behave the same way and the distance between them would stay the same. Add a few more weights and strings and they could all be 'flying' in formation, fixed in their local frame of reference, a frame of reference that would be 'in orbit' rotating around the centre. There is a force acting on all these weights, the equivalent of the gravitational attraction between 'the Sun' and all these satellites. Any axis that one chooses to define between these satellites would also be rotating, if you like pointing towards the 'Sun' on the left of the classic season diagram and towards the Sun on the right of the diagram.


    All these objects are constantly 'falling' towards the centre and rotating as a consequence, taking their local axes with them. That at least is my understanding of 'orbit'.


    Lisa, As the great Doreen Tipton, (look her up!) says "We think therefore we yam!" (Maybe only works for those familiar with Black Country ways!)
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