East is not East


“Suppose you face East and travel in a straight line.
Where do you wind up?”


Estimated Time: 10 minutes, including wrap-up

This SWBQ provides a graphical demonstration that lines of latitude are not straight, since they are not great circles.

Student Conversations

  • Students want “a straight line” to mean “continues to travel east” or “follow a line of latitude”.
  • Students discussed whether is it the centrifugal or Coriolis force that causes a deviation from “east (along a line of latitude)”.
  • Some students will know about spherical geometries from math courses and will introduce into their discussions concepts of great circle, antipodal point, and geodesic.

Wrap Up

A globe or other large ball and a piece of string are useful props.

  • 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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