# second (s or sec)

## What is a second (s or sec)?

The second (s or sec) is the International System of Units (SI) unit of time measurement.

One second is the time that elapses during 9,192,631,770 (or 9.192631770 x 10^{9} in decimal form) cycles of the radiation produced by the transition between two levels of the cesium-133 atom.

## How is a second expressed?

There are other expressions for the second. It is the time required for an electromagnetic (EM) field to propagate 299,792,458 meters (2.99792458 x 10^{8} m) through a vacuum.

This figure is sometimes rounded to 3 x 10^{8} m, or 300,000 kilometers (3 x 10^{5} km). One second is equal to 1/86,400 of a mean solar day. This is easy to derive from the fact that there are 60 seconds in a minute, 60 minutes in an hour and 24 hours in a mean solar day.

This definition is, however, subject to limited accuracy because of irregularities in the earth's orbit around the sun.

## Are their units of time smaller than seconds?

Engineers and scientists often use smaller units of measurement than the second by attaching power-of-10 __prefix multipliers__. One millisecond is 10^{-3} seconds; 1 microsecond is 10^{-6} seconds; 1 nanosecond is 10^{-9} seconds; and 1 picosecond is 10^{-12} seconds.

During these spans of time, respectively, an EM field propagates through a vacuum over distances of approximately 300 km, 300 m, 300 millimeters and 300 micrometers.

## Use cases for seconds

A second is typically the base unit of time measurement in timing for events that occur in everyday life, such as the following:

- a person's heart rate (beats per minute);
- a car's speed (miles per hour or kilometers per hour); and
- cooking time frames (minutes and seconds).

The smaller subdivisions are usually used for events that occur rapidly, such as the following:

- electron impulses in nerves and muscles;
- sound waves;
- light waves; and
- gamma ray bursts.

The second is also a common base unit for expressing extremely long spans of time, such as geologic time units, like the millennium, mega-annum, or giga-annum.

The second is sometimes specified as a unit of angular measure, especially in astronomy and global positioning.

In these contexts, it is also known as an *arc second* or a *second of arc* and is equal to exactly 1/3,600 of an angular degree or 1/1,296,000 of a circle.

Sixty arc seconds constitute an arc minute; 60 arc minutes constitute an angular degree. One arcsecond of latitude at the earth's surface corresponds to a north-south distance of only about 31 m.

At the equator, 1 second of longitude corresponds to an east-west distance of about 26 m.

*See also: **table of physical constants**, **atomic clock**, **radiant energy**, **pascal** and **newton**.*