File:De Sitter argument against emission theory.gif

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Description: According to simple emission theory, light thrown off by an object should move at a speed of c with respect to the emitting object. Willem de Sitter argued that if this were true, a star in a double-star system would usually have an orbit that caused it to have alternating approach and recession velocities, and light emitted from different parts of the orbital path would then travel towards us at different speeds. For certain combinations of orbital speed, distance, and inclination, the "fast" light given off during approach would be able to catch up with and even overtake "slow" light emitted earlier during a recessional part of the star's orbit. A multiplicity of bizarre effects would be seen, including (a) unusually shaped variable star light curves such as have never been seen, (b) extreme Doppler red- and blue-shifts in phase with the light curves, implying highly non-Keplerian orbits, and (c) splitting of the spectral lines (note simultaneous arrival of blue- and red-shifted light at the target).

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