Universal Timeįor many centuries, the fundamental unit of time was the rotational period of Earth with respect to the Sun. It was also renamed Terrestrial Time (TT), although on this Web site, the older name Terrestrial Dynamical Time is preferred and used. In 1991, the IAU refined the definition of TD to make it more precise. To ensure continuity with ET, TD was defined to match ET for the date 1977 Jan 01. It takes into account relativistic effects and is based on International Atomic Time (TAI), which is a high-precision standard usingĪs such, TD is the atomic time equivalent to its predecessor ET and is used in the theories of motion for bodies in the solar system. TD was introduced by the IAU in 1979 as the coordinate time scale for an observer on the surface of Earth. The ephemeris second was defined as a fraction of the tropical year for 1900 Jan 01, as calculated from Newcomb's tables of the Sun (1895).Įphemeris Time was used for Solar System ephemeris calculations until it was replaced by TD in 1979. In 1952, the International Astronomical Union (IAU) introduced Ephemeris Time (ET) to address this problem. Ephemeris Timeĭuring the 20th century, it was found that the rotational period of Earth (length of the day) was gradually slowing down.įor the purposes of orbital calculations, time using Earth's rotation was abandoned for a more uniform time scale based Nevertheless, UTC is colloquially referred to as GMT although this is technically not correct. In 1972, GMT was replaced by Coordinated Universal Time (UTC) as the international time reference. It was also agreed that all longitudes would be measured east or west with respect to the Greenwich meridian. Mean solar time on the 0° longitude meridian in Greenwich, England is known as Greenwich Mean Time (GMT).Īt the International Meridian Conference of 1884, GMT was adopted as the reference time for all clocks around the world. The key to mean solar time is the mean solar day, which has a constant length of 24 hours throughout the year. Unfortunately, the length of the apparent solar day can vary by tens of seconds over the course of a year.Įarth's elliptical orbit around the Sun and the 23.5° inclination of Earth's axis of rotation are responsible for these variations.Īpparent solar time was eventually replaced by mean solar time because it provides for a uniform time scale. This is the interval between two successive returns of the Sun to an observer's local meridian. In military GMT is known as Zulu time.Greenwich Mean Time was the same as Universal Time (UT), until the introduction of UTC in 1972 a standard astronomical concept used in many technical fields.Time Five Millennium Canon of Lunar Eclipses Greenwich Mean Timeįor thousands of years, time has been measured using the length of the solar day. It is now often used to refer to Coordinated Universal Time (UTC) when this is viewed as a time zone, and in casual use for the most parts it is the same. Greenwich Mean Time (GMT) originally referred to the mean solar time at the Royal Observatory in Greenwich, London. Other major cities in the area are Abidjan, Dakar, Grand Dakar, Accra Principal Cities: The largest city in the GMT timezone is London from United Kingdom with population about 7.557 million people. Offset: GMT is 0 hours ahead Greenwich Mean Time (GMT) and is used in EuropeĬountries: It is used in following countries: Burkina Faso, Côte d'Ivoire, Britain (UK), Guernsey, Ghana, Greenland, Gambia, Guinea, Guinea-Bissau, Ireland, Isle of Man, Iceland, Jersey, Liberia, Mali, Mauritania, St Helena, Sierra Leone, Senegal, Sao Tome & Principe, Togo Offset UTC 0:00 hoursġ:30 am 01:30 Greenwich Mean Time (GMT). Offset UTC +3:00 hoursġ:30 am 01:30 Universal Time Coordinated (UTC). EEST (Eastern European Summer Time) is 3 hours ahead of Universal Time Coordinated and 3 hours ahead of Greenwich Mean TimeĤ:30 am 04:30 in Amman, Jordan is 1:30 am 01:30 in UTC and is 1:30 am 01:30 in GMTīest time for a conference call or a meeting is between 11am-6pm in Amman which corresponds to 8am-3pm in UTCīest time for a conference call or a meeting is between 11am-6pm in Amman which corresponds to 8am-3pm in GMTĤ:30 am 04:30 EEST (Eastern European Summer Time) (Amman, Jordan).