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Wednesday, December 28, 2011
If you studied in Latin in school, you would probably be very tempted to think that the Modern English word day derived from the Latin word dies, but you would be wrong. In fact, our word day comes from Old Englishdæg, meaning day or sometimes lifetime, of Germanic origin, related to Dutch dag and German Tag. While every language throughout history has had a word to denote the passing of a single day, its precise, scientific definition is relatively modern. Efforts to measure and quantify a day go back thousands of years-water clocks and sundials have been found in Egyptian tombs among other places.
So how do we define a day now, if not by the rising and setting of the sun as our ancestors did? In 1967, the second was redefined in terms of the wavelength of light, and it became the SI base unit of time. The unit of measurement for time called a day was redefined in 1967 as 86,400 SI seconds and symbolized d, which although it is not a unit in SI is considered acceptable to use.
Image of the Earth rotating through a single day taken by the Galileo space probe in 1990 on its first fly-by of Earth as it gained speed to travel to the outer planets, courtesy of NASA.