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Also see TIME and the relationship between frequency and wavelength
Speed of Light
The speed of light is 299,792,458 metres per second. Or approximately 300 million metres per second, or 186 thousand miles per second, or 1 foot per nanosecond. The speed of light, sometimes known as "C" (as in E=MC2), is a constant of the Universe. However you measure it, it really is 299,792,458 metres per second precisely, or about 186,282.4 miles per second.
Incidentally the "speed of light" referred to is "the speed of light in vacuum", and not the slower speeds which light travels when in water, air, glass, diamond, or other clear media.
Sometimes the speed of light is approximated as 300 million and is written as 3 x 108. To find out about this way of writing big numbers, see exponential notation
This item also crosslinks with Xyroth's "Speed of Light" page which has the context of other constants of the Universe, such as the number of dimensions and the bonding energy etc.
What use is the speed of light in everyday applications? One of its many uses is in working out radio frequencies and wavelengths
Plus, what is exactly is a LIGHT YEAR ? See it worked out precisely.
There is a popular misconception that the Speed of Light is a speed limit which somehow prevents spacecraft getting from one star system to another in a reasonable time. They say you can't go faster than light. However, from your own point of view, it seems like you can!
Constancy of the Speed of Light
The speed of light is a constant, and it's not changing. According to Wikipedia 2012 http://en.wikipedia.org/wiki/Speed_of_light "After centuries of increasingly precise measurements, in 1975 the speed of light was known to be 299,792,458 m/s with a measurement uncertainty of 4 parts per billion". That would mean it was between 299,792,457½ m/s and 299,792,458½ m/s. Now, if the speed of light was known accurately to four parts per thousand million in 1975, and it's not changed in 2012, then it's not changed significantly in thousands of years. The scientists who talk about the speed of light not being a constant are talking about the possibilities of small changes in thousands of millions of years, and definitely not any kind of nonsense about anything six thousand years ago.