Greenwich Mean Time

Greenwich Mean Time is also known as GMT. And it is the mean solar time that is reckoned from midnight at the Royal Observatory in Greenwich. There are different ways in which GMT has been calculated on different occasions in the past, including being calculated from noon. As a consequence of this, GMT cannot be used for specifying the precise time unless and until a context is given.

Many individuals, in most cases English speakers, also use the term ‘Coordinated Universal Time (UTC)’ as a synonym for GMT. When it comes to navigation, then GMT is also considered to be equivalent to UT1, which is the modern form of mean solar time that is present at 0-degree longitude. However, this meaning can also be different from UTC by a total of 0.9 seconds.

It is advised that students should not use the term GMT for different technical purposes that require one to have strong precision. Students should also know that because of the uneven angular velocity of Earth present in its axial tilt and elliptical orbit, the noon GMT is hardly ever at the exact moment when the Sun crosses the Greenwich meridian and reaches the highest point in the sky.

This particular event can also occur up to 16 minutes before or after noon GMT. The discrepancies are also calculated by using the equation of time. The noon GMT can also be defined as the average or the mean of the moment of this event. This accounts for the word ‘mean’ that is used in the term ‘Greenwich Mean Time.’

One might find it interesting to note that originally astronomers considered a GMT day to start at noon. This is quite different because everyone else started at midnight. This is why to avoid confusion, the concept of Universal Time was introduced. It was used for denoting GMT that was counted from midnight.

Several astronomers still preferred the old convention. This was mainly for simplifying the observational data. In that case, every night was logged under a single calendar date. These days Universal Time is also referred to as UTC or UT1.

Students should also remember that historically, GMT has been used for numbering hours with two different conventions. The long-standing astronomical convention that is dating from the work of Ptolemy used to refer to noon as zero hours. One can refer to Julian day to see this. In contrast, the civil convention used to refer to midnight as zero hours. This has existed since the time of the Roman Empire.

The civil convention was adopted on and after 1 January 1925. This was specifically for some astronomical purposes. It also resulted in the phenomenon of discontinuity of 12 hours or half a day. This meant that the instant that was designed as ‘December 31.4 GMT’ in 1924 became ‘January 1.0 GMT’ in 1925 almanacs.

The Greenwich Mean Astronomical Time (GMAT) was introduced then to unambiguously refer to the earlier noon-based astronomical convention for GMT. The more specific terms, like UT and UTC, do not share this particular ambiguity as they refer to midnight as zero hours. If you want to see what the Royal Observatory Greenwich looks like, then you can refer to the image that is attached below.

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The History of Greenwich Mean Time

Let’s look at the history of Greenwich Mean Time. Students should remember that as the United Kingdom started developing an advanced maritime nation, the British mariners tried to keep at least one chronometer on GMT. This was done to calculate the longitude from the Greenwich meridian, which was, at that time, considered to have a longitude zero degree.

This task was done by adopting a convention in the International Meridian Conference of 1884. It should be noted that the synchronization of the chronometer on GMT did not have any effect on the time of shipboard as it was still solar time.

Further, this practice led to the GMT being used across the world as the unit of standard time irrespective of the location. This was done in combination with mariners from other different nations that drew from Nevil Maskelyne’s method of lunar distance that was based on the observations collected at Greenwich.

Several time zones were also based upon the Greenwich time zone. This was an offset of a number of hours, in most cases that meant a possible half or quarter hours, ahead of GMT or behind GMT. A monumental day occurred when Greenwich Mean Time was adopted across the island of Great Britain. This was done by the Railway Clearing House in 1847. In the following year, almost every other railway company had done the same. Because of this reason, this time was given the name of ‘railway time.’

A letter signed by the Clerk to Justices, which appeared on 14 May 1880, also appeared in The Times. That letter stated that Greenwich current time should be kept almost throughout England. However, it should appear that GMT is not the legal time.

This means that the polling booths were opened at 8:13 am and closed at 4:13 pm. This was again changed in 1880 when the Greenwich Mean Time was legally adopted across the island of Great Britain.

GMT was also adopted in other prominent locations and nations after that. Ireland also supplanted Dublin Mean Time in 1916 in favour of GMT. The hourly time signals from the Greenwich Observatory were broadcasted for the first time on 5 February 1924. This rendered the use of the time ball at the observatory redundant.

Students should also remember that the daily rotation of the Earth is irregular. This fact when combined with the knowledge that Earth has a slow trend means that the atomic clocks constitute a more stable timebase.

Because of this, on 1 January 1972, GMT was superseded as the International Civil Time standard by the Coordinated Universal Time. It was maintained by the use of an ensemble of atomic clocks that are located across the globe.

The Universal Time (UT) as a concept was also introduced in 1928. This at the beginning represented mean time at Greenwich that was determined in a traditional manner. This was done in accord with the original definition of the universal day. Also, this raw form of UT was re-labelled as UT0, and later UT1, on 1 January 1956. As a fun exercise, you can also find the Greenwich mean time now!

Fun Facts about Greenwich Mean Time (GMT)

Did you know that legally, the civil time that was used in the United Kingdom was known as the ‘Greenwich standard time’? This is true according to the Interpretation Act of 1978. However, an exception was made for those periods when the Summer Time Act 1972 ordered an hour’s shift for daylight saving.

Also, the Interpretation Act 1978, section 9, also states that whenever an expression of time occurs in the Act, then the time should be referred to as the Greenwich mean timeline. This rule also applies to other instruments and deeds under subsection 23(3).

Further, in the series of experiments of 1968 to 1971, the all-year British Summer Time was known as the British Standard Time (BST). During that period, the British Isles did not revert to the use of the Greenwich Mean Time zone.

Also, in the United Kingdom, UTC +00:00 is disseminated to the general public during the winter months and UTC +01:00 during the summer months. The BBC radio station also broadcasts the ‘six pips’ of the Greenwich Time Signal.

Students should remember that it is named from the original generation that was present at the Royal Greenwich Observatory. It is also aligned to the Coordinated Universal Time. It is further known as the British Summer Time or the Greenwich Mean Time, depending on what is appropriate at different times of the year.