Speed Of Light (c)

ScientificCurrent unit

Exactly 299,792,458 m/s, the speed of light in a vacuum and the fundamental constant that defines the meter.

1 speed of light = 299,792,458 meters per second (exactly).

About the speed of light

Light was once thought to arrive instantly. In 1676 Ole Rømer timed eclipses of Jupiter's moon Io and concluded that light takes about 22 minutes to cross Earth's orbit. In 1729 James Bradley used the aberration of starlight to estimate about 8 minutes 12 seconds from Sun to Earth. Later experimenters used a toothed wheel (Fizeau), a rotating mirror (Foucault), and in 1930 to 1935 Michelson, Pease and Pearson used a mile-long vacuum chamber. The letter c is itself uncertain: it may stand for constant or the Latin celeritas, swiftness, and Einstein switched to it in 1907.

Since 1983 the speed of light has been fixed at exactly 299,792,458 m/s, and the meter is defined as the distance light travels in a vacuum in 1/299,792,458 of a second. Measurement therefore became a matter of defining the constant, not of improving experiments. For scale, light takes about 1.3 seconds to travel from the Earth to the Moon, about 8.3 minutes from the Sun, and roughly one nanosecond to go a foot. James Clerk Maxwell showed in 1865 that light is an electromagnetic wave.

Sources: Wikipedia: Speed of light; NIST: the meter and the SI redefinition

Speed Of Light conversion chart

1 speed of light expressed in every other speed unit:

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