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Previously, the inverse of c expressed in seconds per astronomical unit was measured by comparing the time for radio signals to reach different spacecraft in the Solar System, with their position calculated from the gravitational effects of the Sun and various planets. By combining many such measurements, a best fit value for the light time per unit distance could be obtained. For example, in 2009, the best estimate, as approved by the International Astronomical Union (IAU), was: [101] [102] light time for unit distance: t au= 499.004 783 836(10)s c= 0.002 003 988 804 10(4)AU/s= 173.144 632 674(3)AU/day. An option for deriving c that does not directly depend on a measurement of the propagation of electromagnetic waves is to use the relation between c and the vacuum permittivity ε 0 and vacuum permeability μ 0 established by Maxwell's theory: c 2=1/( ε 0 μ 0). The vacuum permittivity may be determined by measuring the capacitance and dimensions of a capacitor, whereas the value of the vacuum permeability was historically fixed at exactly 4π ×10 −7H⋅m −1 through the definition of the ampere. Rosa and Dorsey used this method in 1907 to find a value of 299 710 ±22km/s. Their method depended upon having a standard unit of electrical resistance, the "international ohm", and so its accuracy was limited by how this standard was defined. [108] [109] Cavity resonance Electromagnetic standing waves in a cavity Hamarneh, S (1972). "Review: Hakim Mohammed Said, Ibn al-Haitham". Isis. 63 (1): 119. doi: 10.1086/350861. Newton, I (1704). "Prop. XI". Optiks. The text of Prop.XI is identical between the first (1704) and second (1719) editions.

Hsu, L (2006). "Appendix A: Systems of units and the development of relativity theories". A Broader View of Relativity: General Implications of Lorentz and Poincaré Invariance (2nded.). World Scientific. pp.427–428. ISBN 978-981-256-651-5.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.

In models of the expanding universe, the farther galaxies are from each other, the faster they drift apart. For example, galaxies far away from Earth are inferred to be moving away from the Earth with speeds proportional to their distances. Beyond a boundary called the Hubble sphere, the rate at which their distance from Earth increases becomes greater than the speed of light. [57] Consoli, Maurizio; Pluchino, Alessandro (2018). Michelson-Morley Experiments: An Enigma for Physics & The History of Science. World Scientific. pp.118–119. ISBN 978-9-813-27818-9 . Retrieved 4 May 2020.Ellis, J; Mavromatos, NE; Nanopoulos, DV; Sakharov, AS (2003). "Quantum-Gravity Analysis of Gamma-Ray Bursts using Wavelets". Astronomy & Astrophysics. 402 (2): 409–424. arXiv: astro-ph/0210124. Bibcode: 2003A&A...402..409E. doi: 10.1051/0004-6361:20030263. S2CID 15388873. Sakurai, JJ (1994). Tuan, SF (ed.). Modern Quantum Mechanics (Reviseded.). Addison-Wesley. pp. 231–232. ISBN 978-0-201-53929-5. Bajcsy, M; Zibrov, AS; Lukin, MD (2003). "Stationary pulses of light in an atomic medium". Nature. 426 (6967): 638–641. arXiv: quant-ph/0311092. Bibcode: 2003Natur.426..638B. doi: 10.1038/nature02176. PMID 14668857. S2CID 4320280.

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.Halley, E (1694). "Monsieur Cassini, his New and Exact Tables for the Eclipses of the First Satellite of Jupiter, reduced to the Julian Stile and Meridian of London". Philosophical Transactions of the Royal Society. 18 (214): 237–256. Bibcode: 1694RSPT...18..237C. doi: 10.1098/rstl.1694.0048. Massless particles and field perturbations, such as gravitational waves, also travel at speed c in vacuum. Such particles and waves travel at c regardless of the motion of the source or the inertial reference frame of the observer. Particles with nonzero rest mass can be accelerated to approach c but can never reach it, regardless of the frame of reference in which their speed is measured. In the special and general theories of relativity, c interrelates space and time and also appears in the famous equation of mass–energy equivalence, E = mc 2. [9] Toll, JS (1956). "Causality and the Dispersion Relation: Logical Foundations". Physical Review. 104 (6): 1760–1770. Bibcode: 1956PhRv..104.1760T. doi: 10.1103/PhysRev.104.1760. Aoki, K; Mitsui, T (2008). "A small tabletop experiment for a direct measurement of the speed of light". American Journal of Physics. 76 (9): 812–815. arXiv: 0705.3996. Bibcode: 2008AmJPh..76..812A. doi: 10.1119/1.2919743. S2CID 117454437. It is generally assumed that the speed of light is isotropic, meaning that it has the same value regardless of the direction in which it is measured. Observations of the emissions from nuclear energy levels as a function of the orientation of the emitting nuclei in a magnetic field (see Hughes–Drever experiment), and of rotating optical resonators (see Resonator experiments) have put stringent limits on the possible two-way anisotropy. [37] [38] Upper limit on speeds

Rømer, O (1676). "Démonstration touchant le mouvement de la lumière trouvé par M. Römer de l'Academie Royale des Sciences" (PDF). Journal des sçavans (in French): 223–236. Rees, M (1966). "The Appearance of Relativistically Expanding Radio Sources". Nature. 211 (5048): 468. Bibcode: 1966Natur.211..468R. doi: 10.1038/211468a0. S2CID 41065207. Marshall, P (1981). "Nicole Oresme on the Nature, Reflection, and Speed of Light". Isis. 72 (3): 357–374 [367–374]. doi: 10.1086/352787. S2CID 144035661.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. Magalotti, Lorenzo (2001) [1667], Saggi di Naturali Esperienze fatte nell' Accademia del Cimento (digital, onlineed.), Florence: Istituto e Museo di Storia delle Scienze, pp. 265– 266 , retrieved 25 September 2015 However, the frequency of light can depend on the motion of the source relative to the observer, due to the Doppler effect. Boyer, Carl Benjamin (1959). The Rainbow: From Myth to Mathematics. Thomas Yoseloff. pp.205–206. OCLC 763848561.