The paper lists all the predictions and observations of eclipses of Jupiter's satellites 1668–1678 and compares them with modern computations of the these eclipses by J. H. Lieske. We discuss Roemer's method that led to his discovery of the retardment of light and finally we shall interpret Roemer's calculations.

Historians of science have proposed many theories to explain why Copernicus rejected the Ptolemaic system and advanced his own heliocentric system. In support of his new cosmological system Copernicus always employed theoretical considerations and never the existence of discrepancies between the positions given in the Tables and those observed. It is shown that the general acceptance of the heliocentric system through the work of the astronomers resulted not so much from empirical considerations but rather from the fact that this system (...) was suited to a comprehensive, general dynamics. (shrink)

Euler ’s wave theory of light developed from a mere description of this notion based on an analogy between sound and light to a more and more mathematical elaboration on that notion. He was very successful in predicting the shape of achromatic lenses based on a new dispersion law that we now know is wrong. Most of his mathematical arguments were, however, guesswork without any solid physical reasoning. Guesswork is not always a bad thing in physics if it leads to (...) new experiments or makes the theory coherent with other theories. And Euler tried to find such experiments. He saw the construction of achromatic lenses, the explanation of colors of thin plates and of the opaque bodies as proof of his theory. When it came to the fundamental issues, the correctness of his dispersion law and the prediction of frequencies of light he was not at all successful. His wave theory degenerated, and it was not until Augustin Fresnel introduced transverse waves and an elaborate notion of interference that the wave theory again progressed. (shrink)