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The speed of light in imperial and United States customary units is based on an inch of exactly 2.54cm and is exactly 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.

Larson, Ron; Hostetler, Robert P. (2007). Elementary and Intermediate Algebra: A Combined Course, Student Support Edition (4th illustrateded.). Cengage Learning. p.197. ISBN 978-0-618-75354-3.a b c d Bradley, J (1729). "Account of a new discovered Motion of the Fix'd Stars". Philosophical Transactions. 35: 637–660. 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.

An astronomical unit (AU) is approximately the average distance between the Earth and Sun. It was redefined in 2012 as exactly 149 597 870 700m. [97] [98] Previously the AU was not based on the International System of Units but in terms of the gravitational force exerted by the Sun in the framework of classical mechanics. [Note 12] The current definition uses the recommended value in metres for the previous definition of the astronomical unit, which was determined by measurement. [97] This redefinition is analogous to that of the metre and likewise has the effect of fixing the speed of light to an exact value in astronomical units per second (via the exact speed of light in metres per second). [100] A pulse with different group and phase velocities (which occurs if the phase velocity is not the same for all the frequencies of the pulse) smears out over time, a process known as dispersion. Certain materials have an exceptionally low (or even zero) group velocity for light waves, a phenomenon called slow light. [73]The astronomical unit was defined as the radius of an unperturbed circular Newtonian orbit about the Sun of a particle having infinitesimal mass, moving with an angular frequency of 0.017 202 098 95 radians (approximately 1⁄ 365.256 898 of a revolution) per day. [99] Brumfiel, Geoff (14 September 2012). "The astronomical unit gets fixed". Nature. doi: 10.1038/nature.2012.11416. ISSN 1476-4687. S2CID 123424704. Given that the equatorial circumference of the Earth is about 40 075km and that c is about 300 000km/s, the theoretical shortest time for a piece of information to travel half the globe along the surface is about 67 milliseconds. When light is traveling in optical fibre (a transparent material) the actual transit time is longer, in part because the speed of light is slower by about 35% in optical fibre, depending on its refractive index n. [Note 11] Straight lines are rare in global communications and the travel time increases when signals pass through electronic switches or signal regenerators. [79]

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