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Celeritas is a Latin word, translated as "swiftness" or "speed". It is often given as the origin of the symbol c, the universal notation for the speed of light in a vacuum, as popularised in Albert Einstein's famous equation E=mc².

In the nineteenth century, an upper-case V was commonly used to describe the speed of light. This was the notation used by Einstein in his 1905 papers, thus his most famous equation was originally written as m = L / V² (E having being used elsewhere for a different energy) ^[1]. The first use of the letter c being used to describe the speed of light can be traced to a paper in 1856 by Wilhelm Eduard Weber and Rudolf Kohlrausch^[2]. Weber used the notation to stand for constant and it later become known as Weber's constant. At the turn of the 20th century, the letter c was popularised by influential physicists such as Max Planck and Hendrik Lorentz and in 1907 Einstein switched his notation from V to c in his papers.

It is thought that that Weber originally intended the letter c to stand for "constant" as opposed to celeritas. The first reference to c standing for celeritas can be found in a 1959 essay by science fiction author Isaac Asimov, in his essay, he stated "C is for celeritas"^[3] though he cited no evidence to support this.

It is now standard to see "c is for celeritas" stated as fact, and a common quiz answer; though some may continue to question the origin. David Bodanis in his popular science book, E=mc²: A Biography of the World's Most Famous Equation, states that "the speed of light has this unsuspected letter for its name probably out of homage for the period before the mid 1600s when science was centred around Italy, and Latin was the language of choice, Celeritas is the Latin word for swiftness."^[4]

1. ↑  A. Einstein, Annalen der Physik, Band 18, 1905
2. ↑  R. Kohlrausch and W.E. Weber, "Ueber die Elektrizitätsmenge, welche bei galvanischen Strömen durch den Querschnitt der Kette fliesst", Annalen der Physik, 99, pg. 10 (1856)
3. ↑  Isaac Asimov "C for Celeritas" in "The Magazine of Fantasy and Science Fiction", Nov-59 (1959), reprinted in "Of Time, Space, and Other Things", Discus (1975), and "Asimov On Physics", Doubleday (1976)
4. ↑  David Bodanis, "E=mc² A Biography of the World's Most Famous Equation", pg. 37, Macmillan (2000)

• http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/c.html