The de Sitter effect was described by Willem de Sitter in 1913[1][2][3][4] (as well as by Daniel Frost Comstock in 1910[5]) and used to support the special theory of relativity against a competing 1908 emission theory by Walther Ritz that postulated a variable speed of light dependent on the velocity of the emitting object.
De Sitter showed that Ritz's theory would have predicted that the orbits of binary stars would appear more eccentric than consistent with experiment and with the laws of mechanics.
If there are no complicating dragging effects, the light would then be expected to move at this same speed until it eventually reached an observer.
In 1913, Willem de Sitter argued that if this was true, a star orbiting in a double-star system would usually, with regard to us, alternate between moving towards us and away from us.
Since the total flight-time difference between "fast" and "slow" light signals would be expected to scale linearly with distance in simple emission theory, and the study would (statistically) have included stars with a reasonable spread of distances and orbital speeds and orientations, de Sitter concluded that the effect should have been seen if the model was correct, and its absence meant that the emission theory was almost certainly wrong.