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Rømer's observations of the occultations of Io

The speed of light in vacuum is a physical constant denoted as $c$ that is exactly equal to 299,792,458 metres per second (approximately 186,282 miles per second, or one foot per nanosecond). According to standard modern physics, visible light and all other electromagnetic radiation moves at this constant speed in vacuum, and $c$ is the fastest speed at which matter, energy or any signal carrying information can travel through space. Ole Rømer first demonstrated in 1676 that light does not travel instantaneously by studying the apparent motion of Jupiter's moon Io (diagram shown). In an 1865 paper, James Clerk Maxwell proposed that light was an electromagnetic wave, and therefore travelled at the speed $c$ . According to the theory of special relativity, which interrelates space and time, all observers will measure the speed of light as being the same, regardless of the inertial reference frame of the observer or the velocity of the object emitting the light. (Full article...)

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 {{Main page image/TFA|File:Illustration from 1676 article on Ole Rømer's measurement of the speed of light.jpg|caption=Rømer's observations of the [[occultation]]s<!-- added link in case the term is unfamiliar to main page readers --> of Io<!-- removed "from Earth" to have the caption take up fewer lines, as rømer presumably observed the occultations from earth -->}}
-The '''[[speed of light]]''' in <!-- conform with article lead and body, which generally omit using an indefinite article --> [[vacuum]] is a [[physical constant]] denoted as {{mvar|c}} that is exactly equal to 299,792,458&nbsp;[[metre per second|metres per second]] (approximately 186,282&nbsp;[[mile|miles]] per second, or one [[foot (unit)|foot]] per [[nanosecond]]). According to standard modern [[physics]], [[light|visible light]] and all other [[electromagnetic radiation]] moves at this constant speed in vacuum<!-- conform with article lead and body, which avoids using latin for this term -->, and {{mvar|c}} is the fastest speed at which matter, energy or any signal carrying information can travel through space. [[Ole Rømer]] first [[Rømer's determination of the speed of light|demonstrated in 1676]] that light does not travel <!-- reworded to avoid using unusual term "non-instantaneously" -->instantaneously by studying the apparent motion of [[Jupiter]]'s moon [[Io (moon)|Io]] ''(diagram shown<!-- "shown" is being used as per diff 891910035 -->)''. In an [[A Dynamical Theory of the Electromagnetic Field|1865 paper]]<!-- reworded to better conform with mos:egg -->, [[James Clerk Maxwell]] proposed that light was an electromagnetic wave, and therefore travelled at the speed {{mvar|c}}. According to the theory of [[special relativity]], which interrelates [[spacetime|space and time]], all observers will measure the speed of light as being the same, regardless of the [[frame of reference|reference frame]] of the observer or the velocity of the object emitting the light. {{TFAFULL|Speed of light}}
+The '''[[speed of light]]''' in <!-- conform with article lead and body, which generally omit using an indefinite article --> [[vacuum]] is a [[physical constant]] denoted as {{mvar|c}} that is exactly equal to 299,792,458&nbsp;[[metre per second|metres per second]] (approximately 186,282&nbsp;[[mile|miles]] per second, or one [[foot (unit)|foot]] per [[nanosecond]]). According to standard modern [[physics]], [[light|visible light]] and all other [[electromagnetic radiation]] moves at this constant speed in vacuum<!-- conform with article lead and body, which avoid using latin for this term -->, and {{mvar|c}} is the fastest speed at which matter, energy or any signal carrying information can travel through space. [[Ole Rømer]] first [[Rømer's determination of the speed of light|demonstrated in 1676]] that light does not travel <!-- reworded to avoid using unusual term "non-instantaneously" -->instantaneously by studying the apparent motion of [[Jupiter]]'s moon [[Io (moon)|Io]] ''(diagram shown<!-- "shown" is being used as per diff 891910035 -->)''. In an [[A Dynamical Theory of the Electromagnetic Field|1865 paper]]<!-- reworded to better conform with mos:egg -->, [[James Clerk Maxwell]] proposed that light was an electromagnetic wave, and therefore travelled at the speed {{mvar|c}}. According to the theory of [[special relativity]], which interrelates [[spacetime|space and time]], all observers will measure the speed of light as being the same, regardless of the [[inertial frame of reference|inertial reference frame]] of the observer or the velocity of the object emitting the light. {{TFAFULL|Speed of light}}
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