## Second

The second (symbol: s) (abbreviated s or sec) is the base unit of time in the International System of Units (SI).

It is qualitatively defined as the second division of the hour by sixty, the first division by sixty being the minute.

The SI definition of second is "the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom”. ( yes that is complicated, but can be replicated in any appropriate lab). See NIST for more details

Seconds may be measured using a mechanical, electrical or an atomic clock. Action of pendulum of clock estimating a second. image: wikipedia

SI prefixes are combined with the word second to denote subdivisions of the second, e.g., the millisecond (one thousandth of a second), the microsecond (one millionth of a second), and the nanosecond (one billionth of a second).

Though SI prefixes may also be used to form multiples of the second such as kilosecond (one thousand seconds), such units are rarely used in practice.

The more common larger non-SI units of time are not formed by powers of ten; instead, the second is multiplied by 60 to form a minute, which is multiplied by 60 to form an hour, which is multiplied by 24 to form a day.

Estimating a second verbally

The phrase "One Mississippi, Two Mississippi” or “ One one thousand, Two one thousand"  are  several similar phrases used to measure time verbally. For example estimating the time between seeing a lighting flash and hearing a thunder clap one can estimate the distance to lighting.

Source adapted from: Second. (2017, October 22). In Wikipedia, The Free Encyclopedia. Retrieved 01:06, October 22, 2017, from https://en.wikipedia.org/w/index.php?title=Second&oldid=806435084