Nevertheless, at this degree of precision, the effects of general relativity must be taken into consideration when interpreting the length. The metre is considered to be a unit of proper length, whereas the AU is usually used as a unit of observed length in a given frame of reference. The values cited here follow the latter convention, and are TDB-compatible. [102] CODATA value: Speed of Light in Vacuum". The NIST reference on Constants, Units, and Uncertainty. NIST . Retrieved 21 August 2009.

Roberts, T; Schleif, S (2007). Dlugosz, JM (ed.). "What is the experimental basis of Special Relativity?". Usenet Physics FAQ. University of California, Riverside. Archived from the original on 15 October 2009 . Retrieved 27 November 2009. Berner, JB; Bryant, SH; Kinman, PW (November 2007). "Range Measurement as Practiced in the Deep Space Network" (PDF). Proceedings of the IEEE. 95 (11): 2202–2214. doi: 10.1109/JPROC.2007.905128. S2CID 12149700. The speed of light is of relevance to communications: the one-way and round-trip delay time are greater than zero. This applies from small to astronomical scales. On the other hand, some techniques depend on the finite speed of light, for example in distance measurements. Time is money when it comes to microwaves". Financial Times. 10 May 2013. Archived from the original on 10 December 2022 . Retrieved 25 April 2014. Panofsky, WKH; Phillips, M (1962). Classical Electricity and Magnetism. Addison-Wesley. p. 182. ISBN 978-0-201-05702-7.Cherenkov, Pavel A. (1934). "Видимое свечение чистых жидкостей под действием γ-радиации" [Visible emission of pure liquids by action of γ radiation]. Doklady Akademii Nauk SSSR. 2: 451. Reprinted: Cherenkov, P.A. (1967). "Видимое свечение чистых жидкостей под действием γ-радиации" [Visible emission of pure liquids by action of γ radiation]. Usp. Fiz. Nauk. 93 (10): 385. doi: 10.3367/ufnr.0093.196710n.0385. , and in A.N. Gorbunov; E.P. Čerenkova, eds. (1999). Pavel Alekseyevich Čerenkov: Chelovek i Otkrytie[ Pavel Alekseyevich Čerenkov: Man and Discovery]. Moscow: Nauka. pp.149–153. Pitjeva, EV; Standish, EM (2009). "Proposals for the masses of the three largest asteroids, the Moon–Earth mass ratio and the Astronomical Unit". Celestial Mechanics and Dynamical Astronomy. 103 (4): 365–372. Bibcode: 2009CeMDA.103..365P. doi: 10.1007/s10569-009-9203-8. S2CID 121374703. 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] Schaefer, BE (1999). "Severe limits on variations of the speed of light with frequency". Physical Review Letters. 82 (25): 4964–4966. arXiv: astro-ph/9810479. Bibcode: 1999PhRvL..82.4964S. doi: 10.1103/PhysRevLett.82.4964. S2CID 119339066. Rees, M (1966). "The Appearance of Relativistically Expanding Radio Sources". Nature. 211 (5048): 468. Bibcode: 1966Natur.211..468R. doi: 10.1038/211468a0. S2CID 41065207.

Another way to measure the speed of light is to independently measure the frequency f and wavelength λ of an electromagnetic wave in vacuum. The value of c can then be found by using the relation c= fλ. One option is to measure the resonance frequency of a cavity resonator. If the dimensions of the resonance cavity are also known, these can be used to determine the wavelength of the wave. In 1946, Louis Essen and A.C. Gordon-Smith established the frequency for a variety of normal modes of microwaves of a microwave cavity of precisely known dimensions. The dimensions were established to an accuracy of about ±0.8μm using gauges calibrated by interferometry. [108] As the wavelength of the modes was known from the geometry of the cavity and from electromagnetic theory, knowledge of the associated frequencies enabled a calculation of the speed of light. [108] [110]Lindberg, DC (1974). "Late Thirteenth-Century Synthesis in Optics". In Edward Grant (ed.). A source book in medieval science. Harvard University Press. p.396. ISBN 978-0-674-82360-0.