Speed of Light in Historical Perspective
Harry H. Mark
Three pivotal empirical measurements determined the speed of light in relation to a moving observer or its source.
- Ole Roemer (1644-1710) found that the speed of light from Jupiter's satellite was lower when an observer on earth was moving away from it, and higher on approach. The red-shift of spectral lines may be understood in this context.
- James Bradley (1693-1762) determined that the speed of light from the star Gamma draconis was higher when an observer on earth moved towards its perpendicular incident, and lower on recession. George Biddell Airy attempted to elucidate the data with a water filled telescope.
- Albert Michelson (1852-1931) measured the speed of light when both the source and the observer were on the same moving earth. Under these circumstances the speed did not change. The experiment was interpreted to mean that the speed of light was not affected by the motion of the earth. However, the results published in the 1881 paper were then amended, on the advice of Alfred Potier (1840-1905), in a second paper, published with Edward Morley in the 1887. In this paper the speed in the perpendicular direction was increased, in the spirit of Bradley's observation of aberration, and this correction diminished the expected discrepancy by half. The speed in direction of the earth's motion, in the spirit of Roemer's data, was not similarly considered.
