Riemann Geometry: Where is the Flaw in My Thinking?

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SUMMARY

The discussion centers on the principles of Riemannian geometry, specifically addressing the axiom that states there are no parallel lines and that any two lines must intersect. The confusion arises from the interpretation of longitudinal lines, which do not represent straight lines but rather lines of constant longitude that meet at the poles. The conversation highlights the necessity of the parallel postulate in defining parallel lines and clarifies that lines of latitude are examples of equidistant curves on a sphere.

PREREQUISITES
  • Understanding of Riemannian geometry principles
  • Familiarity with the concept of great circles on a sphere
  • Knowledge of the parallel postulate in Euclidean geometry
  • Basic comprehension of spherical coordinates and their properties
NEXT STEPS
  • Explore the implications of the parallel postulate in non-Euclidean geometries
  • Study the properties of great circles and their significance in spherical geometry
  • Investigate the differences between lines of latitude and longitude in spherical coordinates
  • Learn about equidistant curves and their applications in geometry
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Mathematicians, geometry students, educators, and anyone interested in the foundational concepts of Riemannian geometry and its applications in understanding spatial relationships.

Gear300
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One of the axioms of Riemann's geometry holds that there are no parallel lines and that any two lines meet. Since Riemann's geometry fits for that of a sphere, any two great circles of the sphere should intersect. However, if we were to take 2 longitudinal lines, then it is possible that these lines never meet. Where is the flaw in my thinking?
 
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Never mind...I overlooked how the longitudinal lines are not by definition "straight" lines.
 
Specifically, they aren't great circles.
 
You mean latitude?
 
Office_Shredder said:
You mean latitude?

Good point. "Longitudinal lines" isn't clear but since he referred to them never meeting, I assumed that was what he meant. "Lines of constant longitude", of course, meet at the north and south poles.
 
By the way, back many many years ago, when I was in high school, I asked my geometry teacher, "why not just define 'parallel lines'" as being lines a constant distance apart? Then it would be obvious that they don't meet and there is only one 'parallel' to a given line through a given point". Being a high school teacher he pretty much just brushed off the question. Now I know that you need the parallel postulate to prove that the set of points at constant distance equidistant from a given line is a "line". The set of lines of lattitude are examples of "equidistant curves" on the spere.
 

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