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East is not East: Instructor's Guide

Main Ideas

Lines of latitude are not straight, since they are not great circles.

Students' Task

Estimated Time: 10 minutes, including wrap-up

Students should be asked to answer the following question on their whiteboards:
Suppose you face East and travel in a straight line. Where do you wind up?

Prerequisite Knowledge




  • Ask several students to present/defend their responses.
  • Have a volunteer come up and indicate two points on a globe which lie due East/West of each other.
  • Ask the volunteer to stretch a string between the two indicated points.
  • Ask whether the string can be pulled any tighter.
  • Discuss straight lines on spheres (great circles).
  • Point out that great circles go through antipodal points.
  • Conclude by indicating the great circle through the initial point, which initially points East.

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