Families of Parallel Lines and the Circumference of the Earth

How do you fit a tape measure around the Earth? No need if you know a little geometry! Pupils begin by extending their understanding of the Side Splitter Theorem to a transversal cut by parallel lines. Once they identify the proportional relationship of the segments of the transversal, they use the concept to find missing measures. Learners then apply the same concept to show how the Greeks determined the circumference of the Earth around 240 B.C. 

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CCSS: Designed
Instructional Ideas

  • Use two local cities to calculate the circumference of the Earth; compare the calculation to the Greek calculation and actual value
  • Prompt learners to write proportionality statements using segment names before substituting numbers
Classroom Considerations

  • The lesson assumes an understanding of the Side Splitter Theorem and the ability to write and solve proportions
  • A continuation of a 36-part series

  • Introduces a real-life application to add validity to the process
  • Includes an opening exercise, exit ticket, and a problem set

  • None