Second, a unit in the metric system, is used to measure time and angles. Its symbol is s. In measuring time, 60 seconds make up 1 minute. There are 60 minutes in 1 hour, and 24 hours in a day, and so a second is 1/86,400 of a day. In measuring angles, 60 seconds make up 1 minute, and there are 60 minutes in 1 degree. A circle is divided into 360 degrees. Thus, a second is 1/1,296,000 of a circle.
A second of time equals an exact fraction of a day. But days are not equal in length because the earth does not travel in a perfect circle around the sun. Therefore, measurements of time based on the day are not constants and cannot be used for scientific work. Scientists used an atomic clock in establishing a measurement standard for time. This device measures intervals of time according to the number of vibrations made by a certain type of radiation from a cesium atom. A second is defined as 9,192,631,770 times the vibration of such a radiation.
