Editing Light-second (section)

== Use in telecommunications ==
Communications signals on [[Earth]] rarely travel at precisely the [[speed of light]] in free space.{{Citation needed|date=September 2017}} Distances in fractions of a light-second are useful for planning telecommunications networks.

* One light-nanosecond is almost 300&nbsp;millimetres (299.8&nbsp;mm, 5&nbsp;mm less than one [[foot (length)|foot]]<ref>[[David Mermin]] suggested one light-nanosecond might be called a ''phoot'' at page 22 of ''It's About Time'' (2005), [[Princeton University Press]]</ref>), which limits the speed of data transfer between different parts of a computer.
* One light-microsecond is about 300&nbsp;metres.
* The mean distance, over land, between opposite sides of the Earth is 66.8&nbsp;light-milliseconds.
* [[Communications satellite]]s are typically 1.337 light-milliseconds{{Citation needed|date=August 2017}} ([[low Earth orbit]]) to 119.4 light-milliseconds ([[geostationary orbit]]) from the surface of the Earth. Hence there will always be a delay of at least a quarter of a second in a communication via geostationary satellite (119.4 ms times 2); this delay is just perceptible in a transoceanic telephone conversation routed by satellite. The answer will also be delayed with a quarter of a second and this is clearly noticeable during interviews or discussions on TV when sent over satellite.