Does Light ACTUALLY Move?

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Sources and References:

In this video, we explore one of the greatest stories in the history of science: how astronomers first discovered that light has a finite speed. Beginning with Galileo’s observations of Jupiter’s moons, we follow the work of Cassini, Picard, Huygens, and Ole Rømer as a search for a solution to the longitude problem led to a revolutionary idea. By carefully tracking the eclipses of Io, astronomers uncovered evidence that light does not travel instantaneously, providing the first estimate of the speed of light.

Sources and References

Galileo Galilei, Sidereus Nuncius, 1610.

Ole Rømer, “A Demonstration concerning the Motion of Light”, Journal des Sçavans, 1676.

Giovanni Domenico Cassini, “Monsieur Cassini His New and Exact Tables for the Eclipses of the First Satellite of Jupiter”, Philosophical Transactions, 1694.

Albert Van Helden, “Roemer’s Speed of Light”, Journal for the History of Astronomy, 1983.

Leif Kahl Kristensen and Kurt Møller Pedersen, “Roemer, Jupiter’s Satellites and the Velocity of Light”, Centaurus, 2012.

Claus Fabricius, “Paris 1676: The discovery of the velocity of light and the roles of Rømer and Cassini”, Journal for the History of Astronomy, 2019.

Fabio Falchi et al., “De mora luminis: Roemer’s discovery 350 years later”, European Journal of Physics, 2026.

Isaac Asimov, “How Did We Find Out About the Speed of Light?”