An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

A precise pendulum clock with separate dials for seconds, minutes, and hours. It was originally used by astronomers to calculate astronomical time.

→ astronomical; → clock.

A modern clock, in which the characteristic frequencies of certain atoms (most commonly chosen cesium 133) are utilized for precision time measurement. → atomic fountain clock.

→ atomic; → clock.

An → atomic clock based on the principle of the → atomic fountain. A ball of atoms, usually → cesium (¹³³Cs), created by the → laser cooling technique, is trapped in the intersection region of six laser beams. The ball is thrown upward by a laser beam and passes twice through a cavity where the atoms interact with the → microwave radiation generated by an → oscillator. The ball reaches the summit of its trajectory (about 1 m above the cooling zone) and then due to gravity falls through the same microwave cavity. The microwave radiation causes the electrons of the cesium atoms to move between two specific → energy states as they pass through the cavity. The clock is based on a → hyperfine transition (9.192631770 GHz) between two energy states in the electronic → ground state of the atom. The upper hyperfine state can in principle radiate to the lower state by → spontaneous emission, but the process takes a very long time -- thousands of years. Selection and detection of the hyperfine state is performed via → optical pumping and laser induced resonance fluorescence. In a carefully controlled setup, a relative uncertainty of 10 ^-16 can be reached for the cesium clock. This means an accuracy of 1 sec every 300 million years. This fluorescence is measured by a detector. The entire process is repeated until the maximum fluorescence of the cesium atoms is determined. This determination is used to lock the oscillator to the atomic frequency of cesium, which is used to define the SI → second. The first atomic fountain for metrological use was developed at the Paris Observatory (A. Clairon et al. 1996, Proc. 5th Symp. Frequency Standards and Metrology, p. 45).

→ atomic fountain; → clock.

→ atomic fountain clock.

→ cesium; → clock.

A device (not carried or worn) for measuring and showing the time. See also: → hour, → gnomon, → clepsydra.

M.E. clokke "clock with bells," from O.Fr. cloque "bell" (Fr. cloche, Du. klok, Ger. Glocke), M.L. clocca "bell," of Celtic origin.

Sâat from Ar.

Successive risings and lowerings of voltage on the electrodes of a CCD in order to move the electrons from one pixel to the next.

In the same direction as the rotating hands of a clock when viewed from in front.

From → clock + wise "way, manner," O.E. wise (adj.), from wis, from P.Gmc. *wisaz (cf. Du. wijs, Ger. weise "wise"), PIE base *weid-/*wid- "to see, to know;" cf. Av vaeda "I know," Skt. veda "I know," Gk. oida "I know".

Sâ'atsu, from sâ'at, → clock, + su "direction," Mid.Pers. sôg, sôk "side, direction".

In a direction opposite to the rotating hands of a clock.

From counter- "contrary; opposite; opposing," + → clockwise.

Pâdsâ'atsu, from pâd-, → counter-, + sâ'atsu, → clockwise.

In star formation models involving magnetized accretion, a process whereby the stellar rotational → angular velocity becomes equal to the → Keplerian angular velocity of the → accretion disk. This happens at the → corotation radius. Disk locking is believed to be responsible for efficient loss of stellar → angular momentum during the → pre-main sequence contraction of → T Tauri stars. These stars are expected to spin up by a factor of about 3 due to contraction after being magnetically disconnected from the → circumstellar disk. However, observations show that a large fraction of pre-main sequence stars evolve at nearly constant angular velocity through the first 4 Myr. This process results from → magnetic braking. The idea of magnetic disk locking originated with the theory developed by Ghosh & Lamb (1979, ApJ 232, 259) for → neutron stars.

M.E., from O.E. lucan "to lock, to close," from loc "bolt, fastening, enclosure;" cf. M.L.G. lok, O.H.G. loh, O.N. lok "a cover, lid," Goth. -luk in usluk "opening," Ger. Loch "opening, hole," Du. luck "shutter."

Girkard, from gir "hold, block," from gereftan "to seize, hold, take," → eclipse, + kard noun from kardan "to do," → work.

Reduction of the radiative flux in a model atmosphere due to absorption by a large number of lines. Line blocking affects the radiative transfer, ionisation and temperature structures, and the atmosphere's hydrodynamics.

→ line; block, from M.E. blok "log, stump," from O.Fr. bloc "log, block," via M.Du. bloc "trunk of a tree."

Bandâri "blocking," from band âvardan "to block," from band "dam, tie, chain," from bastan "to bind, shut, close up" (Mid.Pers. bastan, band; Av./O.Pers. band- "to bind, fetter," banda- "band, tie;" cf. Skt. bandh- "to bind, tie, fasten," bandhah "a tying, bandage;" Goth bandi "that which binds;" O.Fr. bande, bende; O.E. bend; M.E. bende; E. band; PIE base *bendh- "to bind") + âvari verbal noun of âvardan "to cause, produce; to bring" (Mid.Pers. âwurtan, âvaritan; Av. ābar- "to bring; to possess," from prefix ā- + Av./O.Pers. bar- "to bear, carry," bareθre "to bear (infinitive)," bareθri "a female that bears (children), a mother;" Mod.Pers. bordan "to carry;" Skt. bharati "he carries;" Gk. pherein; L. fero "to carry").

1) A device fitted to a gate, door, drawer, lid, etc, to keep it firmly closed and often to prevent access by unauthorized persons.
2) To fasten (a door, gate, etc) or (of a door, etc) to become fastened with a lock, bolt, etc, so as to prevent entry or exit (Dictionary.com).

M.E., from O.E. loc "fastening, bar;" cognate with M.L.G. lok, O.H.G. loh "dungeon," Ger. Loch "opening, hole," O.N. lok "a cover, lid;" akin to O.E. lucan "to shut."

Qofl, loan from Ar.

A region in the sky lying roughly between the → pointer stars of the → Big Dipper that is almost free from → neutral hydrogen gas in the → Galaxy. It is centered at R.A. 10h 45m, Dec. +57° 20', has an area of 15 square degrees, and a → column density of N_{H I} ≤ 5 x 10¹⁸ cm^-2. The Lockman hole is one of the favorite directions for obtaining a clear and unobstructed view of objects in deep space, far beyond our own Galaxy.

Named after Felix J. Lockman et al., 1986, ApJ 302, 432; → hole.

In electronics, a technique of adjusting the phase of an oscillator signal so that it will follow the phase of a reference signal.

→ phase; lock, from O.E. loc "bolt, fastening, enclosure;" cf. O.N. lok "fastening, lock," Goth. usluks "opening," O.H.G. loh "dungeon," Ger. Loch "opening, hole," Du. luck "shutter, trapdoor."

Fâz, → phase; bast "fastening, lock," from bastan, from Mid.Pers. bastan/vastan "to bind, shut," Av./O.Pers. band- "to bind, fetter," banda- "band, tie," Skt. bandh- "to bind, tie, fasten," PIE *bhendh- "to bind," cf. Ger. binden, E. bind, → band.

The process whereby the → rotation period of a → primary body becomes identical to the → orbital period of a → secondary body. Tidal locking results from → tidal braking and leads to → synchronous rotation.

→ tidal; → lock; → -ing.

The description of a → system of two bodies undergoing → tidal locking.

→ tidal; → -ly; → lock.

An ancient form of clock, used by several civilizations, consisting of a water container with a small hole from which the water slowly dripped. Time was reckoned by the level of the water remaining in the container.

→ water; → clock

Pang "a copper bason with a small hole in the bottom, for water in which it is placed to flow through, used for measuring time" used in Iran.