IAU in the context of Ephemeris second

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 passed 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.

Clive L. N. Ruggles (born 8 October 1952) is a British astronomer, archaeologist and academic. He is the author of academic and popular works on the subject. In 1999, he was appointed professor of archaeoastronomy at the School of Archaeology and Ancient History, University of Leicester, when it is believed to have been the only appointed chair for archaeoastronomy among the world's universities.As of 2023, he was Emeritus Professor at this university.

Ruggles was the president of the Prehistoric Society from 2006 to 2010 and the president of the IAU Commission for the History of Astronomy from 2009 to 2012. He is the Chair for the IAU World Heritage and Astronomy Working Group, and was formerly the President of the International Society for Archaeoastronomy and Astronomy in Culture.

Terrestrial Time (TT) is a modern astronomical time standard defined by the International Astronomical Union, primarily for time-measurements of astronomical observations made from the surface of Earth.For example, the Astronomical Almanac uses TT for its tables of positions (ephemerides) of the Sun, Moon and planets as seen from Earth. In this role, TT continues Terrestrial Dynamical Time (TDT or TD), which succeeded ephemeris time (ET). TT shares the original purpose for which ET was designed, to be free of the irregularities in the rotation of Earth.

The unit of TT is the SI second, the definition of which is based currently on the caesium atomic clock, but TT is not itself defined by atomic clocks. It is a theoretical ideal, and real clocks can only approximate it.

In metrology, ephemeris time (ET) is time in association with any ephemeris (itinerary of the trajectory of an astronomical object). In practice it has been used more specifically to refer to:

1. a former standard astronomical time scale adopted in 1952 by the IAU, and superseded during the 1970s. This time scale was proposed in 1948, to overcome the disadvantages of irregularly fluctuating mean solar time. The intent was to define a uniform time (as far as was then feasible) based on Newtonian theory (see below: Definition of ephemeris time (1952)). Ephemeris time was a first application of the concept of a dynamical time scale, in which the time and time scale are defined implicitly, inferred from the observed position of an astronomical object via the dynamical theory of its motion.
2. a modern relativistic coordinate time scale, implemented by the JPL ephemeris time argument T_eph, in a series of numerically integrated Development Ephemerides. Among them is the DE405 ephemeris in widespread current use. The time scale represented by T_eph is closely related to, but distinct (by an offset and constant rate) from, the TCB time scale currently adopted as a standard by the IAU (see below: JPL ephemeris time argument Teph).

Most of the following sections relate to the ephemeris time of the 1952 standard.

The centaur 10199 Chariklo, with a diameter of about 250 kilometres (160 mi), is the second-smallest celestial object with confirmed rings (with 2060 Chiron being the smallest), and the fifth ringed celestial object discovered in the Solar System, after the gas giants and ice giants. Orbiting Chariklo is a bright ring system consisting of two narrow and dense bands, 6–7 km (4 mi) and 2–4 km (2 mi) wide, separated by a gap of 9 kilometres (6 mi). The rings orbit at distances of about 400 kilometres (250 mi) from the center of Chariklo, a thousandth the distance between Earth and the Moon. The discovery was made by a team of astronomers using ten telescopes at various locations in Argentina, Brazil, Chile and Uruguay in South America during observation of a stellar occultation on 3 June 2013, and was announced on 26 March 2014.

The existence of a ring system around a minor planet was unexpected because it had been thought that rings could only be stable around much more massive bodies. Ring systems around minor bodies had not previously been discovered despite the search for them through direct imaging and stellar occultation techniques. Chariklo's rings should disperse over a period of at most a few million years, so either they are very young, or they are actively contained by shepherd moons with a mass comparable to that of the rings. The team nicknamed the rings Oiapoque (the inner, more substantial ring) and Chuí (the outer ring), after the two rivers that form the northern and southern coastal borders of Brazil. A request for formal names will be submitted to the IAU at a later date.

Erlanger is a very deep lunar impact crater that lies near the north pole of the Moon. Due to its position near the lunar north pole (and given that the Moon's axis is only tilted about 1.5°), sunlight only rarely falls on the bottom, and the 2008 Chandrayaan-1 probe hoped to find that ice from comet impacts had accumulated there. Erlanger is one of the Moon's permanently shadowed craters.

The crater was named by the International Astronometrical Union on January 22, 2009, after the American physiologist and 1944 Nobel Prize winner Joseph Erlanger.