TimeZones WorldWide Sunday 1st October 2023 using AEST

10 hours behind Brisbane was also known as GMT (Greenwich Mean Time). The phrase "Mean solar time" was introduced in nautical almanacs in England in 1834 and in France in 1835. It averaged the 24*60*60=86400 seconds mean solar day over the course of the year from vernal equinox to vernal equinox, noting that mean time runs ahead of apparent time by about 14 minutes near February 6 but behind apparent time by about 16 minutes near November 3. The second then became a fixed unit of time....

An average solar day is 24 hrs. This is the time that we set our clocks to. At perihelion (2nd January 2024) the Earth is travelling at its fastest around the sun. Because the sun is moving faster relative to the background stars, it will take longer for the Earth to "catch up" reach solar noon anywhere, resulting in a longer day by about 15 minutes. At aphelion (6th July 2023) the Earth moves at its slowest pace around the sun, so the day is shorter by about 15 minutes.

An apparent solar day can be 20 seconds shorter or 30 seconds longer than a mean solar day. Long or short days occur in succession, so the difference builds up until mean time is ahead of apparent time by about 14 minutes near February 6 and behind apparent time by about 16 minutes near November 3. The equation of time is this difference, which is cyclical and does not accumulate from year to year.

Accordingly during the 1850s and 1860s the Greenwich Royal Observatory in London was chosen by the USA (in particular) as the centre for establishing world time (before Daylight Saving emerged). This line at the Observatory bisects East and West, being 0 degrees longitude.

One of the issues prompting this declaration was The Philippines. Their new Spanish Governor in 1844 had decided to skip a day and Monday December 30 1844 was followed by Wednesday January 1 1845.

On 1 November 1884, Greenwich Mean Time was adopted universally at the International Meridian Conference in Washington, DC. As a result, the International Date Line was drawn up and 24 time zones were created.

Originally, astronomers considered a GMT day to start at noon, while for the general public it started at midnight. In 1928, the name Universal Time or UT was introduced to denote GMT as counted from midnight.

So Universal Time (now UT1) is the modern time standard for mean solar time based on Earth's rotation on its axis. Historically, Universal Time was computed from observing the position of the Sun in the sky. But astronomers found that it was more accurate to measure the rotation of the Earth by observing stars as they crossed the meridian each day.

The rotation of the Earth and UT1 are monitored by the International Earth Rotation and Reference Systems Service (IERS) in Paris, France...

In 1955 the BIH (Bureau International de l'Heure) adopted a proposal by William Markowitz, effective January 1, 1956, dividing UT into UT0 (UT as formerly computed), UT1 (UT0 corrected for polar motion) and UT2 (UT0 corrected for polar motion and seasonal variation). From 1956 to 1972, UT2 was the international standard recommended for radio broadcasting.

UTC Starting in 1956, WWV in Colorado USA broadcast an atomic clock signal stepped by 20 ms increments to bring it into agreement with UT1. Thus it was never more than 20 ms in error. By 1960, the US Naval Observatory, the Royal Greenwich Observatory, and the UK National Physical Laboratory coordinated their radio broadcasts so that time steps and frequency changes were coordinated, and the resulting time scale was informally referred to as "Coordinated Universal Time". In 1963, WWV ceased issuing corrections to UT2. In April 1967, WWV stopped using the local time of the transmitter site (Eastern Time until 1966, and Mountain Time afterwards) and switched to broadcasting Greenwich Mean Time (GMT) i.e. UT1.

The same year the International Telecommunications Union based in Geneva Switzerland formally established UTC (Co-ordinated Universal Time) as their official time standard, overseen from Paris France by the BIPM (Bureau of International Weights and Measures). On January 1 1972, UTC was defined to follow UT1 within 0.9 seconds, with UT2 subsequently becoming irrelevant. In 1974, WWV switched again, to UTC.

While Universal Time (UT1) is tied directly to Earth's rotation, Coordinated Universal Time (UTC) is based on an atomic timescale and adjusted to approximate Universal Time (UT1) to 0.9 seconds or less. As Earth's rotation gradually slows down, Coordinated Universal Time (UTC) must be adjusted with an occasional intercalary leap second to remain within 0.9 seconds of Universal Time (UT1). Accordingly, UTC deviates from TAI (International Atomic Time) by a number of whole seconds. As of 1 January 2017, when another leap second was put into effect, UTC is currently exactly 37 seconds behind TAI. The 37 seconds result from the initial difference of 10 seconds at the start of 1972, plus 27 leap seconds in UTC since 1972.

Today GMT (always identical with UT / UT1) is the basis for civil time in the United Kingdom, including the BBC World Service, the Royal Navy, and the Met Office. And others, particularly in Arab countries such as the Middle East Broadcasting Centre.

Click here for a live UTC clock, started at midnight December 31st 1899 (called January 0, 1900).

TimeZones Worldwide Sunday 1st October 2023 using AEST

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TimeZones Worldwide Sunday 29th November 2020