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GMT and Other Time Systems Explained

Greenwich Mean Time (GMT) is a term that is commonly referred to as Coordinated Universal Time (UTC), when not counting the precise accuracy regarding fractions of a second. Many people refer to GMT as a term that is interchangeable with UTC but timeanddate.com explains the differences between the two terms. This article explores GMT’s origin and how UTC was developed over the years. It also provides a brief overview of other time systems.

Greenwich Mean Time – GMT

Greenwich Mean Time (GMT) was adopted as the world’s time standard at the Washington Meridian Conference in 1884. This conference also established Universal Time, from which the international 24-hour time-zone system grew. This is why all time zones refer back to GMT on the prime meridian. The prime meridian at Greenwich, in the United Kingdom, has served as the reference line for GMT since the late 19th century.

The prime meridian was defined by the position of the large “Transit Circle” telescope in the Observatory’s Meridian Building in 1884. The cross-hairs in the Transit Circle’s eyepiece precisely defined longitude 0° for the world. The Earth’s crust is moving very slightly all the time so the prime meridian’s exact position is now moving very slightly too. However, the original reference for the world’s prime meridian is still the Airy Transit Circle in the Royal Observatory in the United Kingdom.

GMT was at first calculated by the 24-hour clock starting at noon. However, since 1925, the day of GMT starts at midnight. GMT was later renamed to Universal Time, or UT. It is also known as UT0. It becomes UT1 when it is corrected for the irregular movements of the terrestrial poles, also known as the Chandler wobble. The Earth’s poles do not spin perfectly in a straight line.

More Developments with Time Systems

Following UT1, the concept of “UT2” was developed to correct certain seasonal variations for better time accuracy. Mechanical clocks are more regular than the sun so the atomic clock is more accurate than the Earth. The concept of Atomic Time was established with the development of mechanical clocks.

The most accurate clocks use an atomic transition in a caesium vapor, which defines a very accurate frequency. This frequency is then divided down to give seconds and minutes. Many atomic clocks are used to define a local time standard time service. There are many separate time services throughout the world and a combined mean version of their time measurement is used as International Atomic Time (TAI). TAI is the International Atomic Time scale, a statistical timescale based on a large number of atomic clocks. However, this takes no notice of the Earth’s rotation as measured by UT1 so another standard for civil time, known as Coordinated Universal Time (UTC), is used.

The Development of UTC

UTC differs from TAI by an integral number of seconds. The addition or subtraction of leap seconds to TAI produces UTC. In other words, a leap second, as measured by an atomic clock, is added to or subtracted from (although subtractions are rare) UTC to make it agree with astronomical time to within 0.9 second. The world's timing centers agreed to keep their real-time timescales closely synchronized (“coordinated”) with UTC. Hence, all these atomic timescales are called Coordinated Universal Time (UTC). UTC relates to solar motion.  A constant day of exactly 24 hours is used for civil time keeping purposes.

UTC is often called GMT although the term is used on occasions for UT1. UTC is also the time broadcast since 1972 by radio stations across the world and is popularly referred to as GMT. UTC is also the time system used in aviation and is informally known as Zulu Time to avoid confusion about time zones and daylight saving time. timeanddate.com provides a more detailed explanation about UTC.

Other Time Systems

Astronomers use other measures of time, such as Terrestrial Dynamical Time (TDT), which is 32.184 seconds ahead of TAI. TDT, also known as Terrestrial Time (TT), with a unit of duration 86400 SI seconds on the geoid, is the independent argument of apparent geocentric ephemerides. Therefore:
TDT = TAI + 32.184 seconds.

TDT is used for calculating planetary positions in relation to the Earth’s center. Delta T is the difference between Earth’s rotational time (UT1) and dynamical time (TDT). TDT has been used since 1984. Prior to this, astronomers used a time measure known as Ephemeris Time (ET). Before atomic clocks, Ephemeris Time (ET) was the closest available approximation to a uniform time for planetary motion calculations.

Other time systems include:

Note: timeanddate.com wishes to acknowledge some of the information in this article is courtesy of the UK’s National Maritime Museum and the United States Naval Observatory. It is also important to note that this article does not mention every time system but simply provides an overview of time systems.