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Percentage of the speed of light

Speed of light
The distance from the Sun to the Earth is shown as 150 million kilometers, an approximate average. Sizes to scale.
Sunlight takes about 8 minutes 17 seconds to travel the average distance from the surface of the Sun to the Earth.
Exact values
metres per second 299792458
Planck length per Planck time
(i.e., Planck units)
1
Approximate values (to three significant digits)
kilometres per hour 1080 million (1.08×109)
miles per second 186000
miles per hour 671 million (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 25000 years
across the Milky Way 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 220000
1729 James Bradley, aberration of light 301000
1849 Hippolyte Fizeau, toothed wheel 315000
1862 Léon Foucault, rotating mirror 298000±500
1907 Rosa and Dorsey, EM constants 299710±30
1926 Albert A. Michelson, rotating mirror 299796±4
1950 Essen and Gordon-Smith, cavity resonator 299792.5±3.0
1958 K.D. Froome, radio interferometry 299792.50±0.10
1972 Evenson et al., laser interferometry 299792.4562±0.0011
1983 17th CGPM, definition of the metre 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 299,792,458 metres per second (approximately 3.00×108 m/s, or 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. Though this speed is most commonly associated with light, it is in fact the speed at which all massless particles and changes of the associated fields travel in vacuum (including electromagnetic radiation and gravitational waves). 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 = mc2.

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 km/s (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 km/s (186,200 mi/s) (about 90 km/s (56 mi/s) slower than c).


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