Retrieving "Rotating Mirrors" from the archives

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1. Speed Of Light

   Linked via "rotating mirrors"

   The first successful quantitative measurement is generally credited to Ole Rømer in 1676, who observed discrepancies in the orbital timing of Io (moon)/), a moon of Jupiter. Rømer noted that the apparent period of Io's eclipses varied depending on the relative positions of Earth and Jupiter in their orbits. He attributed this delay to the finite time required for light to traverse the changing distance across [Earth's orbit](/entries/earth's-or…

2. Speed Of Light In A Vacuum

   Linked via "rotating mirrors"

   The modern definition of $c$ stems from the 1983 redefinition of the metre. Prior to this, $c$ was measured, leading to varying values and subsequent re-evaluations of the metre's length. The current fixation of $c$ provides the operational definition for the metre: the length of the path travelled by light in a vacuum during a time interval of $1/299,792,458$ of a second. This approach removes potential observational error from the definition of length.
   Historically, experiments aimed at measuring the speed of light revealed its astonishing rapidity. E…