a b "Hubble Reaches the "Undiscovered Country" of Primeval Galaxies" (Press release). Space Telescope Science Institute. 5 January 2010. Chase, IP. "Apparent Superluminal Velocity of Galaxies". Usenet Physics FAQ. University of California, Riverside . Retrieved 26 November 2009.

Outer space is a convenient setting for measuring the speed of light because of its large scale and nearly perfect vacuum. Typically, one measures the time needed for light to traverse some reference distance in the Solar System, such as the radius of the Earth's orbit. Historically, such measurements could be made fairly accurately, compared to how accurately the length of the reference distance is known in Earth-based units. a b c d e Essen, L; Gordon-Smith, AC (1948). "The Velocity of Propagation of Electromagnetic Waves Derived from the Resonant Frequencies of a Cylindrical Cavity Resonator". Proceedings of the Royal Society of London A. 194 (1038): 348–361. Bibcode: 1948RSPSA.194..348E. doi: 10.1098/rspa.1948.0085. JSTOR 98293. a b c Gibbs, P (1997). "Is Faster-Than-Light Travel or Communication Possible?". Usenet Physics FAQ. University of California, Riverside. Archived from the original on 10 March 2010 . Retrieved 20 August 2008. Gibbs, P (2004) [1997]. "Why is c the symbol for the speed of light?". Usenet Physics FAQ. University of California, Riverside. Archived from the original on 25 March 2010 . Retrieved 16 November 2009. Astrodynamic Constants". Solar System Dynamics. Jet Propulsion Laboratory . Retrieved 10 February 2022.Tolman, RC (2009) [1917]. "Velocities greater than that of light". The Theory of the Relativity of Motion (Reprinted.). BiblioLife. p.54. ISBN 978-1-103-17233-7. Cromie, William J. (24 January 2001). "Researchers now able to stop, restart light". Harvard University Gazette. Archived from the original on 28 October 2011 . Retrieved 8 November 2011. So-called superluminal motion is seen in certain astronomical objects, [53] such as the relativistic jets of radio galaxies and quasars. However, these jets are not moving at speeds in excess of the speed of light: the apparent superluminal motion is a projection effect caused by objects moving near the speed of light and approaching Earth at a small angle to the line of sight: since the light which was emitted when the jet was farther away took longer to reach the Earth, the time between two successive observations corresponds to a longer time between the instants at which the light rays were emitted. [54] Lindberg, DC (1996). Roger Bacon and the origins of Perspectiva in the Middle Ages: a critical edition and English translation of Bacon's Perspectiva, with introduction and notes. Oxford University Press. p.143. ISBN 978-0-19-823992-5. a b Ellis, George F. R.; Williams, Ruth M. (2000). Flat and Curved Space-times (2nded.). Oxford: Oxford University Press. p.12. ISBN 0-19-850657-0. OCLC 44694623.

Between 1960 and 1983 the metre was defined as "the length equal to 1 650 763.73 wavelengths in vacuum of the radiation corresponding to the transition between the levels 2p 10 and 5d 5 of the krypton 86 atom." [156] It was discovered in the 1970s that this spectral line was not symmetric, which put a limit on the precision with which the definition could be realized in interferometry experiments. [157] a b IAU Working Group on Numerical Standards for Fundamental Astronomy. "IAU WG on NSFA Current Best Estimates". US Naval Observatory. Archived from the original on 8 December 2009 . Retrieved 25 September 2009. There are different ways to determine the value of c. One way is to measure the actual speed at which light waves propagate, which can be done in various astronomical and Earth-based setups. It is also possible to determine c from other physical laws where it appears, for example, by determining the values of the electromagnetic constants ε 0 and μ 0 and using their relation to c. Historically, the most accurate results have been obtained by separately determining the frequency and wavelength of a light beam, with their product equalling c. This is described in more detail in the "Interferometry" section below.

Guarnieri, M. (2015). "Two Millennia of Light: The Long Path to Maxwell's Waves". IEEE Industrial Electronics Magazine. 9 (2): 54–56, 60. doi: 10.1109/MIE.2015.2421754. S2CID 20759821.