Lesson 8, Topic 2
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Measuring the Speed of Light

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1368 Galileo

  • Galileo turns on lantern, starts clock
  • Assistant sees light, turns on own lantern
  • Galileo sees other lantern, he stops clock


  • No perceptible lag
  • > 10x the speed of sound
  • “if not instantaneous, it [light] is extraordinarily rapid”

1685 Ole Roemer

Measured c by observing Io, one of Jupiter’s moons

  • When Earth was close to Io, orbital T = 42.5 h
  • When further away, T was larger
  • Dividing the change in distance by the change in observed period gave Roemer’s calculation of the speed of light.


  • Roemer approximated [katex]c = 2.14 \times 10^8 \text{ms}^{-1}[/katex]

1677 Christiaan Huygens

Updated Roemer’s estimate with better estimates of distances to Io.


  • Huygnens approximated [katex]c = 2.01168 \times 10^8 \text{ms}^{-1}[/katex]