Second

The second (symbol: s), is a unit of time. There are 60 seconds in a minute, 60 minutes in an hour, and 24 hours in a day. This tradition dates back to the Babylonian.

In science, a second is the time it takes for a caesium atom to vibrate 9,192,631,770 (around 9 billion) times. Scientists measure the second this way because the length of a day changes all the time. For example, when the dinosaurs lived, a day was about an hour shorter.^[1] Vibrations of atoms on the other hand always take the same time. This atomic second is also called the SI second.

Metric prefixes are frequently combined with the word second to denote subdivisions of the second, e.g., the millisecond (one thousandth of a second) and nanosecond (one billionth of a second). Though SI prefixes may also be used to form multiples of the second (such as “kilosecond”, or one thousand seconds), such units are rarely used in practice. More commonly encountered, non-SI units of time such as the minute, hour, and day increase by multiples of 60 and 24 (rather than by powers of ten as in the SI system).

One heartbeat of an adult at rest, will last about one second.

1. ↑ "How many hours were in a dinosaur's day?". www.abc.net.au. 2012-11-28. Retrieved 2021-03-25.