Coordinated Universal Time, abbreviated as UTC, is the collectively agreed upon accurate international time standard. It is essentially what time it is, according to scientists, weathermen, aviators, and nautical advisers, to the eighth decimal point of a second. It is based on atomic time, but is nominally accurate with solar time. The beauty of Coordinated Universal Time is that it is not dependent on time zones or daylight savings. For astronomers, the military, even on the internet, it is used to avoid any confusion, and is necessary when accuracy is paramount for technical and scientific activities. UTC has evolved over the past century as scientists have understood more about the nature and possibilities of keeping track of time.
The rate of the earth's rotation is always decreasing. This happens as a result of tidal deceleration; the moon's mass causing the tides to rise, speeding up its rotation around the earth, and slowing down the earth's rotation. Each day becomes a nearly immeasurable amount longer. Regardless of the value, there is a change. UTC was developed after scientists found another way to measure time, a way around the variable motions of the solar system. They developed atomic time.
The first caesium atomic clock was invented in 1955. Atomic time is found by reading the microwave signals emitted by electrons when they change energy levels. A year later the US National Bureau of Standards began to use atomic time for short-wave radio broadcasts. It also became crucial for making astronomical observations, as well as for aviation and nautical movements. Because of the time gap between the atomic time and solar time, officials spent years developing a way to align the two different figures. They finally came up with the method of finding UTC, by adding a leap second at necessary intervals to the International Atomic Time. Whenever the difference reaches .9 seconds, a leap second is added. Today UTC is regulated by the International Earth Rotation and Reference Systems Service (IERS). As of now, they add a leap second two times a year, at the end of June, and at the end of December.
UTC divides time into days, hours, minutes and seconds. Each day, beginning at midnight, begins with a reading of 00:00:00, and ends with 23:59:60, before resetting again. It coincides with Greenwich Mean Time, the mean solar time along the Prime Meridian; and, for this reason UTC is often referred to as GMT. It is also called just Standard Time, and sometimes, Zulu. By figuring out UTC, the most desired civil time is found - it combines the stability of atomic clocks while still being synchronized with the movement of the Earth.