22. PARALLEL RAILWAY LINES

How wuld we define ’parallel lines’? Lines which are the same distance apart? Lines which have a common perpendicular? Lines which never meet? Lines which meet at a point on the horizon? Are these definitions equivalent? Does it make a difference if we are working in a plane, or on the surface of a sphere?

Questions, questions! It was probably the Greek geometer Euclid who first started asking them seriously around 300 BC. If you were travelling by train across the Nullarbor, some of these definitions might seem preferable to others!


http://en.wikipedia.org/wiki/Parallel