Just a Second

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Just a Second

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概要

How long is a second? That depends on how you measure it. In a previous EarthDate, we talked about how people divided the day into 24 hours, then hours and minutes into 60. That made the second 1/86,400th of a day. But how long is a day? Turns out, that’s not constant. Earth wobbles slightly on its axis, changing its rotation time. So scientists settled on a year, reasoning the day’s length would average out. But which year? Those aren’t constant either. So they agreed on a specific year: 1900. A second then, was measured as 1/31,556,925.9747th of the year 1900. Not a very elegant solution. So scientists looked for a new measurement. They found an isotope that changes magnetic orientation when, and only when, it’s hit with microwave energy of a very specific frequency. This relationship was so stable, and the procedure so repeatable, that they used it to set the new standard of time: 1 second equals 9,192,631,770 cycles of the microwave energy that switches the atomic state of cesium-133. That may not be any more elegant. But it is very precise—down to the nanosecond.

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