Speed of light in vacuum

Speed of light
Exact values
Sunlight takes about 8 minutes 17 seconds to travel the average distance from the surface of the Sun to the Earth.
metres per second	7008299792458000000♠299792458
Planck length per Planck time (i.e., Planck units)	1
Approximate values (to three significant digits)
kilometres per hour	7003108000000000000♠1080 million (7009108000000000000♠1.08×10⁹)
miles per second	7005186000000000000♠186000
miles per hour	671 million (7008671000000000000♠6.71×10⁸)
astronomical units per day	173
parsecs per year	0.307
Approximate light signal travel times
Distance	Time
one foot	1.0 ns
one metre	3.3 ns
from geostationary orbit to Earth	119 ms
the length of Earth's equator	134 ms
from Moon to Earth	1.3 s
from Sun to Earth (1 AU)	8.3 min
one light year	1.0 year
one parsec	3.26 years
from nearest star to Sun (1.3 pc)	4.2 years
from the nearest galaxy (the Canis Major Dwarf Galaxy) to Earth	7011788940000000000♠25000 years
across the Milky Way	7012315576000000000♠100000 years
from the Andromeda Galaxy to Earth	2.5 million years
from Earth to the edge of the observable universe	46.5 billion years

History of measurements of c (in km/s)
<1638	Galileo, covered lanterns	inconclusive
<1667	Accademia del Cimento, covered lanterns	inconclusive
1675	Rømer and Huygens, moons of Jupiter	7005220000000000000♠220000
1729	James Bradley, aberration of light	7005301000000000000♠301000
1849	Hippolyte Fizeau, toothed wheel	7005315000000000000♠315000
1862	Léon Foucault, rotating mirror	7005298000000000000♠298000±500
1907	Rosa and Dorsey, EM constants	7005299710000000000♠299710±30
1926	Albert A. Michelson, rotating mirror	7005299796000000000♠299796±4
1950	Essen and Gordon-Smith, cavity resonator	7005299792500000000♠299792.5±3.0
1958	K.D. Froome, radio interferometry	7005299792500000000♠299792.50±0.10
1972	Evenson et al., laser interferometry	7005299792456200000♠299792.4562±0.0011
1983	17th CGPM, definition of the metre	7005299792458000000♠299792.458 (exact)

The speed of light in vacuum, commonly denoted $c$ , is a universal physical constant important in many areas of physics. Its exact value is 7008299792458000000♠299,792,458 metres per second (approximately 7008300000000000000♠3.00×10⁸ m/s, approximately 186,282 mi/s); it is exact because the unit of length, the metre, is defined from this constant and the international standard for time. According to special relativity, $c$ is the maximum speed at which all conventional matter and hence all known forms of information in the universe can travel. It is the speed at which all massless particles and changes of the associated fields (including light, a type of electromagnetic radiation, and gravitational waves) travel in vacuum. Such particles and waves travel at $c$ regardless of the motion of the source or the inertial reference frame of the observer. In the theory of relativity, $c$ interrelates space and time, and also appears in the famous equation of mass–energy equivalence $E = mc 2$ .

The speed at which light propagates through transparent materials, such as glass or air, is less than $c$ ; similarly, the speed of radio waves in wire cables is slower than $c$ . The ratio between $c$ and the speed $v$ at which light travels in a material is called the refractive index $n$ of the material ( $n = c / v$ ). For example, for visible light the refractive index of glass is typically around 1.5, meaning that light in glass travels at $c$ / 1.5 ≈ 200,000 kilometres (120,000 mi) /s; the refractive index of air for visible light is about 1.0003, so the speed of light in air is about 299,700 kilometres (186,200 mi) /s (about 90 kilometres (56 mi) /s slower than $c$ ).

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