Roberta Aguirre
Greenwich Mean Time (GMT) and Coordinated Universal Time (UTC) are two different time standards that can be confusing for travellers. GMT is a time zone observed by some countries in Africa and Western Europe, while UTC is a time standard used to synchronise time globally. UTC is based on time in Greenwich, London, and is the preferred time standard for pilots and air traffic control (ATC) as it is more precise and does not observe daylight saving time.
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What You'll Learn
GMT is a time zone, UTC is a time standard
GMT, or Greenwich Mean Time, is a time zone that was first adopted across the island of Great Britain by the Railway Clearing House in 1847. It was gradually adopted for other purposes, and in 1884, the International Meridian Conference established a convention that considered the Greenwich meridian to have longitude zero degrees. This led to GMT being used worldwide as a standard time independent of location, with most time zones based on GMT as an offset of a number of hours.
However, GMT is not precise enough to be used in modern technical pursuits such as aviation, due to the earth's rotation varying slightly, and GMT not accounting for this variation. As a result, a commission at the UN adopted Coordinated Universal Time, or UTC, as its more precise successor in the 1960s. UTC is a time standard that is within about one second of mean solar time at 0° longitude, the currently used prime meridian, and is not adjusted for daylight saving time.
UTC is the central point of reference for time in aviation, with many aviators referring to it as Zulu time. For example, the world's longest flight by distance, SQ23, crosses over a dozen time zones, but pilots only care about one—UTC. UTC is also used in weather forecasts and maps, by the International Space Station, and by amateur radio operators who transmit on frequencies that can be picked up in many time zones.
While GMT is still used in the wintertime in London and everywhere else in the time zone, UTC is the modern version of GMT and is used as the basis for local times worldwide.
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GMT is less precise than UTC
GMT, or Greenwich Mean Time, was first established at the International Meridian Conference in 1884, where it was determined that the Greenwich Meridian would be the international standard time, or zero degrees longitude. This was due to Great Britain's prominence as a developed marine nation. From then on, every nation used GMT to calculate their time in relation to this standard.
However, GMT is not precise enough to be used in modern technical pursuits such as aviation. This is because the Earth's rotation can vary slightly, and GMT does not account for this variation. The advancement of telecommunication technologies and the increasing need for precise time standards influenced the creation of a more accurate timekeeping system.
Enter UTC, or Coordinated Universal Time. UTC is based on the measurement of the frequency of an atom's vibration, which is a much more stable and regularly recurring event compared to the Earth's rotation or the Sun's position in the sky, which GMT relied on. UTC is the modern version of GMT and is used as the central point of reference for time in aviation, where pilots may cross through over a dozen time zones in a single flight.
While UTC and GMT indicate the same time, UTC is based on a more precise mechanism of time measurement. Thus, GMT is less precise than UTC.
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GMT is interchangeable with UTC
GMT stands for Greenwich Mean Time, which was established at the Royal Observatory in Greenwich, London, United Kingdom. It was the first step towards determining the time zones of other countries, with the difference in time for other countries indicated by adding or subtracting hours from GMT time. GMT is based on solar time and originally calculated a second as a fraction of the time it takes for the Earth to rotate on its axis.
Coordinated Universal Time (UTC) is the modern version of GMT. It is a more precise mechanism of time measurement, based on the duration of periods of radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom. UTC is not a time zone but a time standard that is the basis for civil time and time zones worldwide. No country or territory officially uses UTC as local time.
UTC and GMT are often interchanged or confused with one another. They indicate the same time and are used for the same purpose of time coordination. GMT is sometimes changed to General Mean Time to make it less tied to one city. However, GMT is not precise enough to be used in modern technical pursuits such as aviation, where UTC is the central point of reference.
UTC is also referred to as Zulu time, which is used in the military and aviation. The term Zulu comes from the transmission articulation of the letter Z, which signifies +0 offset from the prime meridian.
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GMT is used in winter in some countries
GMT, or Greenwich Mean Time, is the local time at the Royal Observatory in Greenwich, London. It is the average or 'mean' time each day when the Sun crosses the Prime Meridian at the Royal Observatory. This is also known as the Prime Meridian of the World, as it was chosen to be Longitude 0º, or the reference point for the global system of time zones.
In the 1850s and 1860s, the expansion of the railway and communications networks in Britain meant that a national time standard was needed. Railway companies began using GMT as their standard time, and by the mid-1850s, almost all public clocks in Britain were set to GMT. It officially became 'Railway Time' and was adopted as Britain's legal standard time in 1880. The United States also chose Greenwich as the basis for its national time zone system. As a result, GMT became a standard reference point for time worldwide.
However, GMT is not precise enough to be used in modern technical fields such as aviation, as it does not account for slight variations in the Earth's rotation. Therefore, in the 1960s, a commission at the UN adopted Coordinated Universal Time (UTC) as a more precise successor. UTC is a time standard that serves as the basis for civil time and time zones worldwide, but no country or territory officially uses UTC as its local time.
Despite the adoption of UTC, GMT is still used in some countries during the winter. For example, London, United Kingdom, uses GMT during the winter months and switches to British Summer Time (BST) during the summer, which is one hour ahead of GMT. Similarly, Reykjavik, Iceland, observes GMT during the winter but switches to a different time zone in the summer. Other locations that observe GMT all year round include Sierra Leone, Greenland, and Antarctica.
In summary, GMT is a time zone that is officially used in some parts of the world, particularly during the winter months. While it has been replaced by UTC as the standard time reference in aviation and other technical fields, GMT remains relevant as a local time in certain countries and continues to be a part of the global time zone system.
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UTC is used by pilots and air traffic control
UTC, or Coordinated Universal Time, is a time standard that is the basis for civil time and time zones worldwide. It is not a time zone itself, and no country or territory uses UTC as a local time. Instead, it is a modern version of Greenwich Mean Time (GMT) and is used as a central point of reference for time in aviation.
UTC is especially useful for pilots and air traffic control (ATC) as it provides a universal time standard that all pilots can use, regardless of their location. This is essential when operating flights across several time zones, as it ensures that pilots and ATC are on the same page and can communicate effectively without having to do mental calculations to figure out what time is being referred to.
For example, the SQ23 flight is the world's longest flight by distance and crosses over a dozen time zones. However, pilots only need to refer to one time zone—UTC. Using UTC simplifies time calculations and reduces confusion for pilots and ATC, especially when coordinating with flights in different time zones.
In the context of aviation, UTC is frequently referred to as Zulu time. The NATO phonetic alphabet word for the letter "Z" is "Zulu", and UTC is sometimes denoted as UTC+00:00 or with a Z, indicating the equivalent nautical time zone (GMT). Using the term "Zulu" helps to standardize time references and ensure that all pilots are using the same 24-hour clock.
While UTC is widely used in aviation, it is important to note that uncontrolled flights and military flights are not required to operate using UTC. Uncontrolled flights are those operated in uncontrolled airspace, which is not overseen by air traffic control. Military flights use their own time standards and schedules and do not need to report to ATC using the same time standard as other pilots and flights.
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Frequently asked questions
UTC stands for Coordinated Universal Time.
GMT stands for Greenwich Mean Time.
UTC is a time standard that is the basis for civil time and time zones worldwide. GMT is an actual time zone used by a few countries in Africa and Western Europe.
UTC is used in aviation because it is a more precise and modern version of GMT. It is also not tied to Daylight Saving Time, which helps to maintain a consistent time system.
UTC and GMT are usually equal and refer to the same exact time. However, UTC is more accurate because it is based on the rotation of the Earth and atomic clocks.
