The speed of light in a vacuum is one of the most important constants in physical science. It plays a leading role in the development of physical theories and speculation concerning the nature of the universe and, as the ratio of the electromagnetic and electrostatic units, it is a vital link in the development of electromagnetic theory and practice. The speed of light has also become of great practical importance in the fields of navigation and surveying since distance may be obtained from the time of flight of a pulse of light or radio waves.
In 1983, the current SI unit of length (the ‘metre’) was defined in terms of the speed of light and the unit of time.
The idea that light travels with a finite speed probably arose from an analogy with sound waves and mechanical vibrations. Galileo, born in Pisa in 1564, was the first scientist to consider the possibility of measuring its speed. Observers stationed on opposite sides of a valley were each to be provided with lamps and shutters. One observer would open the shutter of the first lamp and the other observer would open the shutter of his lamp as soon as he saw the distant light. The first observer would then note the time between opening his shutter and seeing the other lamp. It is not known whether such an experiment was ever attempted, and it could not of course have succeeded given the relatively low accuracy of clocks in the sixteenth century, although the method was sound in principle.
Such was the interest in knowing the speed of light that over two hundred determinations have been reported in the scientific literature since the time of Galileo (1). During the Second World War, in particular, the exact value of the speed of light was needed for the development of new radio-navigation aids for aircraft. Louis Essen, a British physicist working on radar-related problems at the National Physical Laboratory, realised that microwave cavity wavemeters might be used to measure the speed of light with greater accuracy than any earlier method. His first measurements with this method showed the speed of light to be 299,792 kilometres a second, 16 km/s greater than the value accepted at that time. In 1950, he used an improved resonator to derive a value of 299,792.5 km/s – within two metres a second of the later laser-based value adopted for the definition of the metre.
The story of how this came about is told in more detail in an article on measuring the speed of light (2) and in the book, The Birth of Atomic Time (3) which is being published in 2015 to commemorate the International Year of Light and sixty years of atomic timekeeping.
 K D Froome & L Essen, The Velocity of Light and Radio Waves, Academic Press (1969).
 R Essen, Louis Essen and the Velocity of Light: from Wartime Radar to Unit of Length, Physics in Perspective, Vol. 12, pp. 51-73 (2010).
 R Essen, The Birth of Atomic Time, FastPrint Publishing, ISBN: 978-178456-167-3 (2015).
Ray Essen is a science writer specialising in the history of atomic clocks and his articles on time measurement, navigation and the speed of light have been published widely in the UK and US. He received the Dr Alan Shenton Award from the Antiquarian Horological Society in 2014. Ray trained as a physicist at Nottingham University then worked for the Institution of Engineering and Technology (IET) before becoming a consultant for the computer industry. His book, The Birth of Atomic Time, includes the memoirs of his late father-in- law, Dr Louis Essen.