Greenwich Mean Time Wikipedia

what is gmt mean

The meridian line is marked by the cross-hairs in the Airy Transit Circle eyepiece. GMT was also crucial to the other great solution to the ‘longitude problem’, represented by John Harrison’s famous timekeepers. Greenwich Mean Time is the yearly average (or ‘mean’) of the time each day when the Sun crosses the Prime Meridian at the Royal Observatory Greenwich. Coordinated Universal Time (UTC) came to replace GMT with the more accurate and scientific measurements of time1.

Related time zones

One of the most common proposition is to abandon leap seconds adjustments because it would go out sync with civil time very slowly 9. For astronomical purposes, the GMT day was said to start at noon and run until noon the following day. This made it easier for astronomers because they could log their observational data (taken overnight) under a single calendar date. When everyone switched to the midnight-based convention in the 1920s and 1930s, this midnight-based time standard was given the new name of Universal Time to avoid any confusion. Although the abbreviation does not match the term, it was used as a compromise for the translation from European languages.

What is Greenwich Mean Time?

This was Greenwich Mean Time, or the average time when the Sun crossed the meridian at Greenwich. At first though, Greenwich time was only really important to astronomers. Greenwich Mean Time (GMT), the name for mean solar time of the longitude (0°) of the Royal Greenwich Observatory in England.

Iana Timezones where GMT is currently observed

Historically, GMT has been used with two different conventions for numbering hours. The long-standing astronomical convention, dating from the work of Ptolemy, was to refer to noon as zero hours (see Julian day). This contrasted with the avatrade review civil convention of referring to midnight as zero hours dating from the Roman Empire. The latter convention was adopted on and after 1 January 1925 for astronomical purposes, resulting in a discontinuity of 12 hours, or half a day.

what is gmt mean

The meridian at this longitude is called the prime meridian or Greenwich meridian. Greenwich Mean Time (GMT) has been used to clearly designate epoch by avoiding confusing references to local time systems (zones). Greenwich Mean Time is defined in law as standard time in the following countries and areas, which also advance their clocks one hour (GMT+1) in summer. Indeed, even the Greenwich meridian itself is not quite what it used to be—defined by “the centre of the transit instrument at the Observatory at Greenwich”. Nevertheless, the line in the old observatory’s courtyard today differs no more than a few metres from that imaginary line which is now the prime meridian of the world. To understand where the “z” comes from, consider the world’s time zones.

The instant that was designated as “December 31.5 GMT” in 1924 almanacs became “January 1.0 GMT” in 1925 almanacs. The term “mean” indicates the average time the clocks need to pass through the solar day. Also, considering that each day requires the same interval, the pendulum clocks at the observatory was the perfect mechanism to standardise time for the universal coordination. Greenwich Mean Time (GMT) originally referred to the mean solar time at the Royal Observatory in Greenwich, London. 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. 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.

  1. It’s a way of standardising and regularising time so we can all know exactly what time it is for our (or anyone’s) location.
  2. This contrasted with the civil convention of referring to midnight as zero hours dating from the Roman Empire.
  3. Greenwich Mean Time (GMT) has been used to clearly designate epoch by avoiding confusing references to local time systems (zones).
  4. The clock originally indicated astronomical time, in which the counting of the 24 hours of each day starts at noon.
  5. Nevertheless, same as for GMT, the primary purpose of UTC is to denote the universal time zone in relation to others.

The system of UTC operates on the basis of International Atomic Time (TAI), which provides the high-precision data for the time standards. The positive tendency of the leap second in time reflects that the Earth slows down its rotation4. Nevertheless, due to the accuracy of UTC, it is widely used in the spheres which require high-precision coordinations. Eventually, GMT became the unified standard for the economic interactions between countries around the world. The advancement of the technologies of the wireless telegraph indicated that all countries require precise time standard.

This 24-hour time standard is kept using highly precise atomic clocks combined with the Earth’s rotation. When you read weather forecasts and maps, you may notice a four-digit number followed by the letter “Z” somewhere at their bottom or top. The history of GMT started with the decision of the International Meridian Conference in 1884 to establish the prime meridian which would denote the international standard of time. Considering that Great Britain was a developed marine nation, it used Greenwich Meridian to position own ships. Subsequently, Greenwich meridian was determined as zero degrees longitude, thus, the international standard time2.

Generally, if you are in a country east of the Greenwich Meridian, your local time is ahead of GMT (e.g. local time in China is GMT +8 hours). As the reference for GMT, the Prime Meridian at Greenwich therefore became the centre of world time and the basis for the global system of time zones. This meant there was no standard timings for when the day would begin and end, or what length an hour might be. As well as Greenwich Mean Time for example, there was also Bristol Mean Time (10 minutes behind GMT) Cardiff Mean Time (13 minutes behind GMT). This is essentially the same as GMT, but UTC is measured by an atomic clock and is thus more accurate – by split seconds.

In terms of the distribution of accurate time into everyday life, it is one of the most important clocks ever made. However, the 1850s and 1860s saw the expansion of the railway and communications networks. These two solutions would help pave the way for GMT to become the worldwide time standard a century later. These were tables of ‘lunar distance’ data based on observations at Greenwich and using GMT as the time standard.

During the experiment of 1968 to 1971, when the British Isles did not revert to Greenwich Mean Time during the winter, the all-year British Summer Time was called British Standard Time (BST).

YEach is expressed as a certain number of hours “ahead of UTC” or “behind UTC”? (For example, UTC -5 is Eastern Standard Time.) The letter “z” refers to the Greenwich time zone, which is zero hours (UTC + 0). Since the NATO phonetic alphabet (“Alpha” for A, “Bravo” for B, “Charlie” for C…) word for z is Zulu, we also call it “Zulu Time.” This line has been called the Greenwich Meridian since 1884, and it is from here that all terrestrial longitudes are measured and the world’s time zones are calculated. Greenwich Mean Time (GMT) is often interchanged or confused with Coordinated Universal Time (UTC). This meant they could calculate their longitude from the Greenwich meridian (longitude 0° by convention).

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