1972, THE UTC TIME SYSTEM ADDS THE FIRST LEAP SECOND

Today, The Grandma has been reading about the first leap second that was added to the UTC time system on a day like today in 1972.

 

A leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time (UTC), to accommodate the difference between precise time (International Atomic Time (TAI), as measured by atomic clocks) and imprecise observed solar time (UT1), which varies due to irregularities and long-term slowdown in the Earth's rotation.

The UTC time standard, widely used for international timekeeping and as the reference for civil time in most countries, uses TAI and consequently would run ahead of observed solar time unless it is reset to UT1 as needed.

The leap second facility exists to provide this adjustment. The leap second was introduced in 1972. Since then, 27 leap seconds have been added to UTC, with the most recent occurring on December 31, 2016. All have so far been positive leap seconds, adding a second to a UTC day; while it is possible for a negative leap second to be needed, one has not happened yet.

Because the Earth's rotational speed varies in response to climatic and geological events, UTC leap seconds are irregularly spaced and unpredictable. Insertion of each UTC leap second is usually decided about six months in advance by the International Earth Rotation and Reference Systems Service (IERS), to ensure that the difference between the UTC and UT1 readings will never exceed 0.9 seconds.

This practice has proven disruptive, particularly in the twenty-first century and especially in services that depend on precise timestamping or time-critical process control. And since not all computers are adjusted by leap-second, they will display times differing from those that have been adjusted.

After many years of discussions by different standards bodies, in November 2022, at the 27th General Conference on Weights and Measures, it was decided to abandon the leap second by or before 2035.

In about AD 140, Ptolemy, the Alexandrian astronomer, sexagesimally subdivided both the mean solar day and the true solar day to at least six places after the sexagesimal point, and he used simple fractions of both the equinoctial hour and the seasonal hour, none of which resemble the modern second.

When the Coordinated Universal Time (UTC) standard was instituted in 1960, based on atomic clocks, it was felt necessary to maintain agreement with UT, which, until then, had been the reference for broadcast time services.

In 1972, the leap-second system was introduced so that the UTC seconds could be set exactly equal to the standard SI second, while still maintaining the UTC time of day and changes of UTC date synchronized with those of UT1. By then, the UTC clock was already 10 seconds behind TAI, which had been synchronized with UT1 in 1958, but had been counting true SI seconds since then.

After 1972, both clocks have been ticking in SI seconds, so the difference between their displays at any time is 10 seconds plus the total number of leap seconds that have been applied to UTC as of that time; as of 2024, 27 leap seconds have been applied to UTC, so the difference is 10+27=37 seconds. The most recent leap second was on December 31, 2016.

More information: Time and Date

 The only reason for time is
so that everything doesn't happen at once.

Albert Einstein

GREENWICH, BECOMES THE WORLD'S PRIME MERIDIAN

Today, The Grandma has been reading about  The Royal Observatory, Greenwich, that was designated as the world's prime meridian by the International Meridian Conference, on a day like today in 1884.

The Royal Observatory, Greenwich, ROG known as the Old Royal Observatory from 1957 to 1998, when the working Royal Greenwich Observatory, RGO, temporarily moved south from Greenwich to Herstmonceux, is an observatory situated on a hill in Greenwich Park in south east London, overlooking the River Thames to the north.

It played a major role in the history of astronomy and navigation, and because the Prime Meridian passes through it, it gave its name to Greenwich Mean Time, the precursor to today's Coordinated Universal Time (UTC).

The ROG has the IAU observatory code of 000, the first in the list. ROG, the National Maritime Museum, the Queen's House and the clipper ship Cutty Sark are collectively designated Royal Museums Greenwich.

The observatory was commissioned in 1675 by King Charles II, with the foundation stone being laid on 10 August. The old hilltop site of Greenwich Castle was chosen by Sir Christopher Wren, a former Savilian Professor of Astronomy; as Greenwich Park was a royal estate, no new land needed to be bought.

At that time the king also created the position of Astronomer Royal, to serve as the director of the observatory and to apply himself with the most exact care and diligence to the rectifying of the tables of the motions of the heavens, and the places of the fixed stars, so as to find out the so much desired longitude of places for the perfecting of the art of navigation. He appointed John Flamsteed as the first Astronomer Royal. The building was completed in the summer of 1676. The building was often called Flamsteed House, in reference to its first occupant.

More information: RMG

The scientific work of the observatory was relocated elsewhere in stages in the first half of the 20th century, and the Greenwich site is now maintained almost exclusively as a museum, although the AMAT telescope became operational for astronomical research in 2018.

There had been significant buildings on this land since the reign of William I. Greenwich Palace, on the site of the present-day Maritime Museum, was the birthplace of both Henry VIII and his daughters Mary I and Elizabeth I; the Tudors used Greenwich Castle, which stood on the hilltop that the Observatory presently occupies, as a hunting lodge. Greenwich Castle was reportedly a favourite place for Henry VIII to house his mistresses, so that he could easily travel from the Palace to see them.

In 1676 the main building of the observatory, now known as Flamsteed House, was completed on Greenwich hill.

British astronomers have long used the Royal Observatory as a basis for measurement. Four separate meridians have passed through the buildings, defined by successive instruments.

The basis of longitude, the meridian that passes through the Airy transit circle, first used in 1851, was adopted as the world's Prime Meridian at the International Meridian Conference at Washington, DC, on 22 October 1884, voting took place on 13 October.

Subsequently, nations across the world used it as their standard for mapping and timekeeping.

The Prime Meridian was marked by a brass, later replaced by stainless steel, strip in the Observatory's courtyard once the buildings became a museum in 1960, and, since 16 December 1999, has been marked by a powerful green laser shining north across the London night sky.

More information: The Royal Parks

This is the chronology of Greenwich:

-1675. 22 June. Royal Observatory founded by King Charles II.

-1675. 10 August. Construction began.

-1714. Longitude Act established the Board of Longitude and Longitude rewards. The Astronomer Royal was, until the Board was dissolved in 1828, always an ex officio Commissioner of Longitude.

-1767. The fifth Astronomer Royal Nevil Maskelyne began publication of The Nautical Almanac, based on observations made at the Observatory.

-1818. Oversight of the Royal Observatory was transferred from the Board of Ordnance to the Board of Admiralty; at that time the observatory was charged with maintaining the Royal Navy's marine chronometers.

-1833. Daily time signals began, marked by dropping a time ball.

-1838. Sheepshanks equatorial, a 170 mm aperture refracting telescope installed.

-1893. The 28-inch Great refractor installed.

-1899. The New Physical Observatory, now known as the South Building, was completed.

-1924. Hourly time signals, Greenwich Time Signal, from the Royal Observatory were first broadcast on 5 February.

-1931. Yapp telescope ordered.

-1948. Office of the Astronomer Royal was moved to Herstmonceux in East Sussex.

-1957. Royal Observatory completed its move to Herstmonceux, becoming the Royal Greenwich Observatory (RGO). The Greenwich site was renamed the Old Royal Observatory.

-1990. RGO moved to Cambridge.

-1998. RGO closed. Greenwich site was returned to its original name, the Royal Observatory, Greenwich, and was made part of the National Maritime Museum.

-2011. The Greenwich museums, including the ROG, became collectively the Royal Museums Greenwich.

More information: Historic UK

The only reason for time is so 

that everything doesn't happen at once.

Albert Einstein