De mora luminis: Roemer’s discovery 350 years later

Abstract 350 years from the 1676 announcement of the Roemer’s discovery that light propagates with finite speed, we present our observations using eyes with telescopes having similar resolution compared to those in the late 17th century. We confirmed that Roemer’s method is valid and gives reasonable values for the speed of light c , within about 10% of the modern value for our measurements, even with the simplest modelling technique, using uniform circular motions. We found that increasing the complexity of the model, e.g. by taking into account the elliptical orbit of Jupiter, does not necessarily bring the results closer to the value of c due to the influence of other perturbations. Using modern ephemerides yields a noticeably accurate result of c = (298 200 ± 1900) km s −1 . This experience can have great didactic value by showing the interconnections between formulation of hypotheses and the consequent predictions, making observations, reducing data, and searching for alternative explanations for the same phenomenon. Lastly, we also found, in the correspondence between Roemer and Huygens, that Roemer in 1677 searched for an independent confirmation of what he found during previous years observing Io’s eclipses by making observations and reducing the data of the meridian transits of the Great Red Spot on Jupiter.