D O C . 4 7 8 E F F E C T O F E A R T H ’ S M O T I O N 4 7 1 478. “New Experiments on the Effect of the Earth’s Motion on Light Velocity with Respect to the Earth” [Einstein 1927e] Published 10 February 1927 In: Forschungen und Fortschritte 3 (1927): 36. It is well known that the outcome of the interference experiment by Michelson, that is, by Michelson and Morley,[1] has been a particularly important factor during the genesis of relativity theory. For the negative outcome of this experiment has led to the conviction that the propagation of light in empty space with respect to an in- ertial system proceeds at a constant velocity that is independent of the velocity of the respective inertial system. More precisely, the experiment shows slightly less namely, the statement that the time needed by light to move back and forth along a rigid rod at relative rest to Earth is independent of the rod’s spatial orientation. The present version of relativity theory stands or falls with this outcome. That is why it was an exciting event for theoreticians when Mr. Dayton Miller, a professor in Cleveland, arrived at a differing result on the basis of scrupulous experiments last- ing years, the most important of which were carried out on Mount Wilson.[2] Miller found that the time it takes for light to go there and back again does de- pend on the spatial orientation of the path with respect to the fixed stars. His exper- imental setup was, in itself, more precise than the one by Michelson and Morley, in whose experiment the light paths to be compared were about 60m long. In order to explain his experimental findings, Mr. Miller had formed an opinion that had al- ready been considered before the theory of relativity had been proposed namely, that the luminous ether was being carried along by the Earth in its translatory mo- tion, but to a degree that diminishes with height above sea level.[3] This was sup- posed to explain the positive outcome of the experiments conducted at sites located higher above sea level. During the last few months, the experiments were repeated independently from two quarters and with different apparatus that is, by Mr. R. J. Kennedy from the California Institute of Technology as well as by Messrs. A. Piccard and E. Stahel in Brussels.[4]