The characteristics of a beam of light emanating from a source in uniform motion with respect to an observer differ from those measured when the source is stationary. In general, it is irrelevant whether the source is stationary and the observer in motion or vice versa; the observed characteristics depend only on the relative motion. The observed frequency of the light, for example, has been known to depend on this relative motion since the Austrian physicist Christian Doppler (1842) showed the effect to exist both for sound waves and light waves.
The perceived direction of propagation of a light beam also depends on the relative motion of its source and the observer. The English astronomer James Bradley (1727) was the first to argue that the motion of the Earth in its orbit around the Sun causes a periodic shift of the apparent position of fixed stars as observed from the Earth; a telescope viewing a star must be tilted in the direction of the Earth's motion. Although this so-called stellar aberration could be explained on the basis of the corpuscular theory of light accepted at the time, certain features of it remained poorly understood until the advent of Einstein's special theory of relativity in 1905.
The mid-nineteenth century measurements of the speed of light in moving media could be made to agree with the prevailing theories at the time only if one assumed that the moving medium partially carried the luminiferous ether, the hypothetical medium which filled the Universe and in which the light waves propagated.