United States Naval Observatory in the context of Timekeeping

An atomic clock is a clock that measures time by monitoring the resonant frequency of atoms. It is based on the fact that atoms have quantised energy levels, and transitions between such levels are driven by very specific frequencies of electromagnetic radiation. This phenomenon serves as the basis for the SI definition of the second:

This definition underpins the system of TAI, which is maintained by an ensemble of atomic clocks around the world. The system of UTC — the basis of civil time — implements leap seconds to allow clock time to stay within one second of Earth's rotation.

In Greek mythology, Orion (/əˈraɪən/; Ancient Greek: Ὠρίων or Ὠαρίων; Latin: Orion) was a giant huntsman whom Zeus (or perhaps Artemis) placed among the stars as the constellation of Orion.

Ancient sources told several different stories about Orion; there are two major versions of his birth and several versions of his death. The most important recorded episodes are his birth in Boeotia, his visit to Chios where he met Merope and raped her, being blinded by Merope's father, the recovery of his sight at Lemnos, his hunting with Artemis on Crete, his death by the bow of Artemis or the sting of the giant scorpion which became Scorpius, and his elevation to the heavens. Most ancient sources omit some of these episodes and several tell only one. These various incidents may originally have been independent, unrelated stories, and it is impossible to tell whether the omissions are simple brevity or represent a real disagreement.

In modern usage, civil time refers to statutory time as designated by civilian authorities. Modern civil time is generally national standard time in a time zone at a fixed offset from Coordinated Universal Time (UTC), possibly adjusted by daylight saving time during part of the year. UTC is calculated by reference to atomic clocks and was adopted in 1972. Older systems use telescope observations.

In traditional astronomical usage, civil time was mean solar time reckoned from midnight. Before 1925, the astronomical time 00:00:00 meant noon, twelve hours after the civil time 00:00:00 which meant midnight. HM Nautical Almanac Office in the United Kingdom used Greenwich Mean Time (GMT) for both conventions, leading to ambiguity, whereas the Nautical Almanac Office at the United States Naval Observatory used GMT for the pre-1925 convention and Greenwich Civil Time (GCT) for the post-1924 convention until 1952. In 1928, the International Astronomical Union introduced the term Universal Time for GMT beginning at midnight.

Charon (/ˈkɛərɒn, -ən/ KAIR-on, -⁠ən or /ˈʃærən/ SHARR-ən), formal designation (134340) Pluto I, is the largest of the five known natural satellites of the dwarf planet Pluto. It has a mean radius of 606 km (377 mi). Charon is the sixth-largest known trans-Neptunian object after Pluto, Eris, Haumea, Makemake, and Gonggong. It was discovered in 1978 at the United States Naval Observatory in Washington, D.C., using photographic plates taken at the United States Naval Observatory Flagstaff Station (NOFS).

With half the diameter and one-eighth the mass of Pluto, Charon is a very large moon in comparison to its parent body. Its gravitational influence is such that the barycenter of the Plutonian system lies outside Pluto, and the two bodies are tidally locked to each other. The dwarf planet systems Pluto–Charon and Eris–Dysnomia and the dwarf planet candidate system Salacia-Actaea are the only known examples of mutual tidal locking in the Solar System, though it is likely that Orcus–Vanth is another.

De astronomia (Latin: [deː äs̠t̪rɔˈnɔmiä]; Concerning Astronomy) is a book of stories written in Latin, probably during the reign of Augustus (c. 27 BC – AD 14). Attributed to "Hyginus", the book's true author has been long debated. However, the art historian Kristen Lippincott argues that the author was likely Gaius Julius Hyginus, who served as the superintendent of the Palatine library under Caesar Augustus.

The text describes 47 of the 48 Ptolemaic constellations, centering primarily on the Greek and Roman mythology surrounding the constellations, though there is some discussion of the relative positions of stars. The stories it contains are chiefly based on Catasterismi, a work that was traditionally attributed to Eratosthenes.

The caesium standard is a primary frequency standard in which the photon absorption by transitions between the two hyperfine ground states of caesium-133 atoms is used to control the output frequency. The first caesium clock was built by Louis Essen in 1955 at the National Physical Laboratory in the UK and promoted worldwide by Gernot M. R. Winkler of the United States Naval Observatory.

Caesium atomic clocks are one of the most accurate time and frequency standards, and serve as the primary standard for the definition of the second in the International System of Units (SI), the modern metric system. By definition, radiation produced by the transition between the two hyperfine ground states of caesium-133 (in the absence of external influences such as the Earth's magnetic field) has a frequency, Δν_Cs, of exactly 9192631770 Hz. That value was chosen so that the caesium second equaled, to the limit of measuring ability in 1960 when it was adopted, the existing standard ephemeris second based on the Earth's orbit around the Sun. Because no other measurement involving time had been as precise, the effect of the change was less than the experimental uncertainty of all existing measurements.

The United States Naval Observatory Flagstaff Station (NOFS), is an astronomical observatory near Flagstaff, Arizona, US. It is the national dark-sky observing facility under the United States Naval Observatory (USNO). NOFS and USNO combine as the Celestial Reference Frame manager for the U.S. Secretary of Defense.

In the United States, time is divided into nine standard time zones covering the states, territories and other US possessions, with most of the country observing daylight saving time (DST) for approximately the spring, summer, and fall months. The time zone boundaries and DST observance are regulated by the Department of Transportation, but no single official map of those existed until the agency announced intentions to make one in September 2022. Official and highly precise timekeeping services (clocks) are provided by two federal agencies: the National Institute of Standards and Technology (NIST) (an agency of the Department of Commerce); and the United States Naval Observatory (USNO). The clocks run by these services are kept synchronized with each other as well as with those of other international timekeeping organizations.

It is the combination of the time zone and daylight saving rules, along with the timekeeping services, which determines the legal civil time for any U.S. location at any moment.

Embassy Row is the informal name for a section of Northwest Washington, D.C., with a high concentration of embassies, diplomatic missions, and diplomatic residences. It spans Massachusetts Avenue N.W. between 18th and 35th street, bounded by Scott Circle to the south and the United States Naval Observatory to the north; the term is often applied to nearby streets and neighborhoods that also host diplomatic buildings, such as Kalorama.

Of the 177 diplomatic missions in the city, the majority are located on or near Embassy Row, including those of Italy, Australia, India, Greece, Egypt, Ireland, Japan, and the United Kingdom. Due to the large number of well-preserved Gilded Age estates and townhouses, many of which house diplomatic missions or dignitaries, Embassy Row has been protected as part of the Massachusetts Avenue Historic District. Its historic and multicultural character has also made the area a center of tourism and local cultural life.

The National Observatory of Athens (NOA; Greek: Εθνικό Αστεροσκοπείο Αθηνών) is a research institute in Athens, Greece. Founded in 1842, it is the oldest research foundation in Greece. The Observatory was the first scientific research institute built after Greece became independent in 1829, and one of the oldest research institutes in Southern Europe. It was built around the same period as the United States Naval Observatory.

The world-renowned Greek-Austrian astronomer Georgios Konstantinos Vouris lobbied to create the National Observatory of Athens in the newly founded state. He persuaded wealthy Greek-Austrian banker Georgios Sinas to pay for the new massive observatory. The Austrian-born Greek King found out the news and awarded Georgios Sinas's son the Order of the Redeemer. The King also selected the architects for the building under Georgios Konstantinos Vouris's supervision, which were Eduard Schaubert and Theophil Hansen.

An astronomical constant is any of several physical constants used in astronomy. Formal sets of constants, along with recommended values, have been defined by the International Astronomical Union (IAU) several times: in 1964 and in 1976 (with an update in 1994). In 2009 the IAU adopted a new current set, and recognizing that new observations and techniques continuously provide better values for these constants, they decided to not fix these values, but have the Working Group on Numerical Standards continuously maintain a set of Current Best Estimates. The set of constants is widely reproduced in publications such as the Astronomical Almanac of the United States Naval Observatory and HM Nautical Almanac Office.

Besides the IAU list of units and constants, also the International Earth Rotation and Reference Systems Service defines constants relevant to the orientation and rotation of the Earth, in its technical notes.

Gernot Maria Rudolph Winkler (October 17, 1922 – April 30, 2016) was responsible for the Time Service Department of the United States Naval Observatory (USNO) from 1966 to 1996.

Winkler oversaw the introduction of caesium beam-based Coordinated Universal Time based on hyperfine transitions and an internationally transportable "flying clock".

The Astronomical Almanac is an almanac published by the United Kingdom Hydrographic Office; it also includes data supplied by many scientists from around the world. On page vii, the listed major contributors to its various Sections are: H.M Nautical Almanac Office, United Kingdom Hydrographic Office; the Nautical Almanac Office, United States Naval Observatory; the Jet Propulsion Laboratory, California Institute of Technology; the IAU Standards Of Fundamental Astronomy (SOFA) initiative; the Institut de Mécanique Céleste et des Calcul des Éphémerides, Paris Observatory; and the Minor Planet Center, Cambridge, Massachusetts.

It is considered a worldwide resource for fundamental astronomical data, often being the first publication to incorporate new International Astronomical Union resolutions. The almanac largely contains Solar System ephemerides based on the JPL Solar System integration "DE440" (created June 2020), and catalogs of selected stellar and extragalactic objects. The material appears in sections, each section addressing a specific astronomical category. The book also includes references to the material, explanations, and examples. It used to be available up to one year in advance of its date, however the current 2024 edition became available only one month in advance; in December 2023.