A proof of the constancy of the velocity of light

Willem de Sitter 1872-1934 was a Dutch astronomer. Born in Sneek, graduated from the University of Groningen, after which he worked calculator Observatory Cape of Good Hope. In 1899-1907 he was an assistant at the Astronomical Laboratory at Groningen. In 1908 he became a professor of astronomy at the University of Leiden, from 1919 to 1934 - director of the Leiden Observatory. In 1925 he was the President of the International Astronomical Union.
In that distant time the postulate «A proof of the constancy of the velocity of light» directly confirmed only by astronomical observations of double stars. The idea of the Dutch astronomer Willem de Sitter, if the speed of light depends on the velocity of the source, the trajectory of motion of binary stars would have to be qualitatively different from those observed (consistent with celestial mechanics). Here you can find examples and formulas for calculating the speed of light.