# Speed of light

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## Definition

The Speed of Light (or c) is the speed at which a photon (an "energy packet" of electromagnetic radiation) travels in a vacuum.

### Values

The exact measurement for the speed of light is 299,792,458 meters per second, in the following values, the speed of light will use values based on the rounded 300,000,000 m/s.

Meters per second:

${\displaystyle c=3\times 10^{8}\mathrm {ms^{-1}} }$

Kilometers per second:

${\displaystyle c=3\times 10^{5}\mathrm {kms^{-1}} }$

Miles per second:

${\displaystyle c=1.9\times 10^{5}\mathrm {mis^{-1}} }$

## Consequences of the Speed of Light

### Communication

Humans have discovered methods to communicate over great distances using electromagnetic (EM) radiation, such as radio waves and lasers. On Earth, distances are short enough that these methods seem instantaneous. On Mars and other parts of the solar system, communication with Earth involves a time delay, based on the time it takes the EM radiation to travel. The delay between Earth and Mars varies from 3 minutes to 22 minutes (between about 180 to 1340 seconds depending on the position of planets and resulting distance). For two way communication (request - answer) time is doubled.

To provide uninterrupted communications, a group of communication satellites will be necessary (placed in the orbit around the sun) - due different orbital speeds of both planets, for a period of time sun will be positioned directly between both planets, blocking direct communications.