Where International Standard Units Come From, Part Two: The Second
NIST-F1 Cesium Fountain Atomic Clock Photo courtesy of NISTThis week, the origin and continued preservation of five of our favorite standard units of measure This week, Sam Kean takes a look at some ridiculously precise standards -- the meter, the second, and other international standard units -- and the role that elements have played in defining, redefining, and re-redefining them over the ages. The definition of the second used to be 1/86,400th of one spin of the earth around its axis (less formally, the number of seconds in one day). But a few pesky facts made that standard inconvenient. The length of a day varies with every trip around the sun because of the sloshing of ocean tides, which drag and slow the earth's rotation. And metrologists (measurement scientists) didn't want to tie a supposedly universal unit of time to the transit of a small rock around a mediocre star. To...