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| | Approaching the Speed of Light Photo | TrekLens |
 | | The speed of light in a vacuum is an important physical constant denoted by the letter c for constant or the Latin word celeritas meaning "swiftness". |
 | | Note that this speed is a definition, not a measurement, since the fundamental SI unit of length, the metre, has been defined since October 21, 1983 in terms of the speed of light: one metre is the distance light travels in a vacuum in 1/299,792,458 of a second. |
 | | Converted to imperial units, the speed of light is approximately 186,282.397 miles per second, or 670,616,629.384 miles per hour, or almost one foot per nanosecond. |
| www.treklens.com /gallery/Asia/photo201683.htm (295 words) |
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