second
Sec is the basic unit of time in the international unit system, the symbol is s. Sometimes borrowed by the English abbreviation for sec. The original meaning in seconds is the sixtieth of the calculated hour (minutes), and then the calculation of sixty percent. Between 1000 and 1960, the definition of seconds is the average sun day of 1 / 86,400 (in some astronomy and legal definition is still applicable). Between 1960 and 1967, it was defined as 1/86,400 a week of the Earth's rotation in 1960 and is now defined by the characteristics of the atom. Sec can also be timed with a mechanical clock, an electronic clock or an atomic clock. The international unit system head is often combined with seconds for finer division, such as ms (milliseconds, thousandth of a second), μs (microseconds, parts per million) and ns (nanoseconds, billionths second). Although the international unit system prefix can also be used for the expansion of time, such as ks (thousands of seconds), Ms (million seconds) and Gs (billion seconds), but in fact rarely use this way, we are still used to 60 We carry out extension of time, and time of 24 carry. Sec is not only the basic unit of time in the International System of Units, but also the basic unit of time in centimeters - grams - second, meters - kilograms - seconds, meters - ...