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The speed of light in a vacuum (postulated to be constant for all inertial observers by the [[Theory of Relativity|special theory of relativity]]) is, to nine significant figures, exactly 299,792,458 meters per second (approximately 1,079,252,848 km/h). As the speed of light is used to define the [[SI]] meter, this is the value ''by definition''.

The speed of light in a vacuum (postulated to be constant for all inertial observers by the [[Theory of Relativity|special theory of relativity]]) is, to nine significant figures, exactly 299,792,458 meters per second (approximately 186,282.3 miles per second). As the speed of light is used to define the [[SI]] meter, this is the value ''by definition''.

The speed of light is slower in any medium which is not a vacuum, and varies from medium to medium. This variation gives rise to (as a result of [[quantum mechanics]], particularly the concept of a path of least action) the phenomenon of [[refraction]]. When a charged particle exceeds the speed of light in the medium in which it is travelling, it emits [[Cherenkov Radiation]].

The speed of light is slower in any medium which is not a vacuum, and varies from medium to medium. This variation gives rise to (as a result of [[quantum mechanics]], particularly the concept of a path of least action) the phenomenon of [[refraction]]. When a charged particle exceeds the speed of light in the medium in which it is travelling, it emits [[Cherenkov Radiation]].

Since the speed of light in a vacuum is constant, it can be used to define distances as well. The distance that light travels in one year is know as a [[light-year]], which is about 6 million million (6x10¹²) miles.  For instance, the nearest large galaxy to our own [[Milky Way Galaxy]] is the [[Andromeda Galaxy]], which astronomers have measured to be about 2.5 million light years away.

Since the speed of light in a vacuum is constant, it can be used to define distances as well. The distance that light travels in one year is known as a [[light-year]], which is about 6 million million (6x10¹²) miles.  For instance, the nearest large galaxy to our own [[Milky Way Galaxy]] is the [[Andromeda Galaxy]], which astronomers have measured to be about 2.5 million light years away.

The speed of light raises several questions for [[Young Earth Creationism|Young Earth Creationists]] including that of the [[starlight problem]].

The speed of light raises several questions for [[Young Earth Creationism|Young Earth Creationists]] including that of the [[starlight problem]].  An Australian young Earth creationist named Barry Setterfield proposed the mechanism of [[c-decay]] <ref>http://www.setterfield.org/vacuum.html</ref> to allow light from sources more distant than 6-10,000 light years to have made the transit to Earth.

[[category: astronomy]]

[[category: astronomy]]