Can you give a simple description of the Sagnac's Experiment?

A. - Yes. Here is a simple description.

Let us assume that you have a source of light (a laser) at a certain location on Earth (e.g. Your home). You project that beam of light directly toward a satellite in orbit around the Earth on which there is a reflecting mirror. After that first reflection, light reaches other mirrors in space so that it can finally complete a full turn around the Earth and reach again the same spot on Earth where light has been emitted.

Using that apparatus, you can measure the time it takes for light to make a complete turn around the Earth when light moves from West to East. Now, if you repeat the same experiment, with the same mirrors, (or even do it simultaneously), using light moving from East to West, you find that the time taken by light to move from East to West is shorter than the time taken for light to complete the revolution in the opposite direction. That difference of time is observed with the same apparatus, making simultaneous measurements, at ... more.

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