second

   1. (s or sec or ")

   a fundamental unit of time in all measuring systems and the SI base unit of time. The name simply means that this unit is the second division of the hour, the minute being the first (see minute [0]). The second was defined as 1/86 400 mean solar day until astronomers discovered that the mean solar day is actually not constant (see day²). The definition was then changed to 1/86 400 of the specific mean solar day 1900 January 1. Since we can't go back and measure that day any more, this wasn't a real solution to the problem. In 1967, scientists agreed to define the second as that period of time which makes the frequency of a certain radiation emitted by atoms of cesium-133 equal to 9 192 631 770 hertz (cycles per second). In other words, if we really want to measure a second, we count 9 192 631 770 cycles of this radiation. This definition allows scientists to reconstruct the second anywhere in the world with equal precision.

   2. (" or s or sec)

   a unit of angular measure equal to 1/60 arcminute. This unit is also called the arcsecond to distinguish it from the second of time. One second is a very small angle indeed: there are 1 296 000 seconds in a circle. The SI defines s as the symbol for the time unit (see above) and recommends " as the symbol for the arcsecond. The international standard ISO 31 recommends that angles be stated in degrees and decimal fractions of the degree, without use of arcminutes and arcseconds.

   3. (" or s or sec)

   a unit of longitude used in astronomy. Astronomers measure right ascension in time units by dividing the equator into 24 hours instead of 360 degrees. (Right ascension is the longtitude coordinate for positions in the sky; see hour²).

   This makes 1 second of longitude equal to 15 arcseconds.

   4. (" or sec)

   a unit of viscosity defined by the time required for a specified amount of a liquid to flow through a particular viscometer. The Saybolt second (see above) was used in the U.S., the Redwood second in Britain, and the Engler degree in continental Europe.

   5. (s or sec)

   a unit measuring the fuel efficiency of rocket engines. The measure is obtained as the thrust of the engine divided by the weight of fuel consumed per second (weight being measured at the earth's surface, or at 1 g). Since both thrust and weight are forces, their units cancel and the ratio is measured in seconds. This ratio is often called "specific impulse," although the true specific impulse (measured in speed units) is the ratio of the thrust to the mass (not weight) of fuel consumed per second.

   6. (s or ")

   a unit of sidereal time in astronomy; see "sidereal day".