Time-Keeping: Time

The daily rotation of Earth -bringing sunrise in the morning and sunset in the evening- obviously was the natural clock to mankind since the highest antiquity. The refinement of timekeeping systems, then the apparition of clocks, together with the development of natural sciences improved timekeeping and the measure of Earth's rotation

Time-Keeping Tools

It's not before the middle of the Middle Ages that the first clocks appeared. Until then time was kept with various devices allowing to get an estimate of what hour or what part of the day it was, or of how much time was elapsing. Sundials (some of them large, like the obelisks of the Egyptian temples) or clepsydras (water clocks), candles marked in increments or hourglasses were used. First clocks eventually appeared about 1350, in Italy. Clocks remained heavy, then spring-driven until the Dutch scientist Christiaan Huygens managed to built the first pendulum clock in 1656. Its clock managed to indicate time with an error of less than 10 seconds a day. Clocks and science was working hand in hand

Clocks continued to improve, mainly for the use of the English navy. Longitude needed to be accurately measure to avoid wreckages and it could not be except with very accurate time-keeping systems. The Astronomer Royal and Greenwich, beginning in 1675, then clock-makers, made progresses for such a purpose. By 1761 Harrison's marine chronometer was able to determine longitude to within half a degree after a journey to the West Indies, keeping time with an accuracy of one-fifth of a second a day. Pendulum clocks were further improved by freeing the pendulum of any interference as by the 1920's the next step was the quartz-based clocks. An electric field is applied to a quartz crystal; the crystal changes its shape, generating in turn an electric current. This brings to a vibration and an electric signal which are used for the clock machinery. Eventually scientists deviced atomic clocks where an electromagnetic wave interacting with an atom generates accurate and steady signals

Time-Keeping Standards

The Time GMT

As tools were progressing, the time remained kept at a local level only. Each city hall, each town, worked on the local solar time. As soon as the new transportation means developed -namely the railroads- allowing swift deplacements in one country, this disparity in time-keeping became an obstacle. It was hard to maintain train schedules as time was varying, inside a same country, along a line East-West! Progressively one came to unify time systems inside a same country and, eventually, the International Meridian Conference in Washington in October 1884 set up and international, uniformized, time system. An internationally enforced meridian system was put into place with the meridian of the Observatory of Greenwich like the initial meridiam. All longitudes became calculated East or West of it up to 180°. At the same time all countries were adopting a universal time system based on the mean solar time. A mean solar day of 24 hours was adopted as 24 timezones were deviced. Each timezone was centered on a standard meridian giving it its mean solar time. The 24 timezones were set relative to the mean solar time of Greenwich (Greenwich Mean Time, GMT). They divided the world into theoretically 15° wide zones heading east or west of Greenwich. Where they meet, at longitude 180° the time zone is parted into two parts, with the International Date Line running. When a traveler passed the line heading East it came back to the previous day. Heading West it came to the next day. The GMT system was at the same time a timezone and a time, strictly, system

UT, UTC

The GMT system was transformed about 1926 into the Universal Time (UT) system. This passage merely was a change of terms as the system described previously remained enforced, and still is today. Time UT came too to be used too as the basic astronomical time. After WW2, the progress of time-keeping and the need for highly accurate clocks, brought the atomic clocks, which are working based on the frequency of electromagnetic waves emitted or absorbed by an atom of Cesium-133. The international time-keeping institutions eventually came to these clocks as the new base for the civil time worlwide. Such clocks are working on the basis of an internationally defined second, the SI-second, which is the second of the International System of units. It's based on such a second that the Coordinated Universal Time (UTC) became the time used in your watch and at time services. Time UT was maintained as the time for the timezones system as UTC became the civil time. Time UTC is not an astronomically-defined time anymore however. It's a theoretical time, defined by the atomic clocks. Due to Earth's rotation irregularities, this accurate atomic time does not match exactly the Earth's rotation rate. To avoid any discrepancy, a positive, or negative, "leap second" is added to, or substracted from, the atomic clocks when needed. This way, the time UTC is kept within 0.9 second from time UT (which is a time with an astronomical reference -for more details see the tutorial Time Systems in the Observation section, Tutorials). The atomic clocks gave birth to another time system, which is used for highly accurate science works only. It's the Atomic Time (TAI, from the French "Temps Atomique International"). As far as the GPS (Global Positioning System) time aspect is concerned, the clocks both on Earth devices and aboard satellites (these are atomic clocks) started with an offset of 19 seconds compared to UTC. This was due to the GPS beginning to work on January 6th, 1980 when such an offset existing between time TAI and UTC. GPS receivers simply translate this offset, plus the current offset, when they receive the satellite data, and they display the time UTC. for more technical details about these timekeeping systems and some others more especially dedicated to astronomy, see at the tutorial "Time Systems" in the Observation section