The Prime Meridian is the 0-degree line of longitude that divides the Earth into the Eastern Hemisphere and the Western Hemisphere. Since the late 19th century, the Prime Meridian at Greenwich has served as the reference line for Greenwich Mean Time. Prior to that ols maps put it at various locations. Unlike an equator, which is determined by the axis of rotation, it was determined arbitrarily.
Before this, almost every town in the world kept its own local time and there were no national or international conventions which set how time should be measured, or when the day would begin and end, or what length an hour might be. With the expansion of the railway and communications networks during the 1850s and 1860s, the worldwide need for an international time standard became imperative so Greenwich was chosen as the center for world time.
In the early 18th century when efforts were being made to determine longitude at sea, it lead to the development of the marine chronometer by John Harrison. But it was the development of accurate star charts, principally by the first British Astronomer Royal, John Flamsteed between 1680 and 1719 that enabled navigators to use the lunar method of determining longitude more accurately using the octant developed by Thomas Godfrey and John Hadley. Between 1765 and 1811, 49 issues of the Nautical Almanac were published based on the meridian of the Royal Observatory, Greenwich.
In 1884, at the International Meridian Conference in Washington, D.C., 22 countries voted to adopt the Greenwich meridian as the Prime Meridian of the world. The French dissented and abstained from voting and they continued to use the Paris meridian until 1911.
There were two main reasons for the choice of Greenwich. The USA had already chosen Greenwich as the basis for the country's national time zone system and secondly in the late 19th century, 72 percent of the world's commerce depended on sea-charts which used Greenwich as the Prime Meridian. Thus, naming Greenwich would be advantageous to the largest number of people.
Satellites changed things. Between 1984 and 1988 a new set of coordinate systems were adopted based on satellite data and other measurements and required a prime meridian that defined a plane passing through the center of the Earth. The true prime meridian became the IERS Reference Meridian, which is also known as the International Reference Meridian or IRM. The IRM passes 336.28609 feet to the east of the historic Prime Meridian. However, the plane used to measure Greenwich Mean Time has remained essentially unchanged so watches did not need to be reset.
Coordinated Universal Time is the primary time standard by which the world regulates clocks and time. It is within about 1 second of mean solar time at 0° longitude, and is not adjusted for daylight saving time. In some countries where English is spoken, the term Greenwich Mean Time (GMT) is often used as a synonym for UTC.
The first Coordinated Universal Time was informally adopted on January 1, 1960 and was first officially adopted in 1963, but the official abbreviation of UTC and the official English name of Coordinated Universal Time were not adopted until 1967. The time is occasionally tweaked by scientist because the Earth's rotational speed is very slowly decreasing and this increases the length of the mean solar day.
Nearly all UTC days contain exactly 86,400 seconds with exactly 60 seconds in each minute. However, because the mean solar day is slightly longer than 86,400 seconds, occasionally the last minute of a UTC day is adjusted to have 61 seconds. The extra second is called a leap second. It accounts for the grand total of the extra length (about 2 milliseconds each) of all the mean solar days since the previous leap second.