An Etymological Dictionary of Astronomy and Astrophysics
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The → emission line of → neutral hydrogen in interstellar clouds. It lies in the radio spectrum at wavelength of about 21 cm, frequency 1420 MHz. Its existence was predicted by Hendrik van de Hulst in 1944 and discovered by Harold Ewen and Edward Purcell in 1951.

Twenty-one (21); → twenty + → one; → centimeter; → line.

A universal time supposed to be the same for all observers at any place in the Universe. Absolute time is one of the foundations of → Newtonian mechanics, but it fails to account for physical phenomena in → reference frames with relative motion. Its abandoning was one of the starting points of → special relativity. See also → absolute space; → space-time.

→ absolute; → time.

The time necessary for the → accretion of a definite amount of mass with a fixed → accretion rate.

→ accretion; → time.

An instrument which determines the altitude of an object with respect to a fixed level, such as sea level.

L. altus "high" + → -meter.

Farâzsanj, from farâz "above, over, aloft" + -sanj, → -meter; farâzyâb, from farâz + yâb "finder," from yâbidan "to find, discover, obtain."

The measurement of heights in the atmosphere (altitude) by an → altimeter.

→ altimeter; → -metry.

The → mean sidereal time corrected for the → nutation and shift in the obliquity of the ecliptic that occurs as a result of the Moon's gravitational effect. Apparent sidereal time differs from mean sidereal time in that the → true vernal equinox point is used.

→ apparent; → sidereal; → time.

The time based on the motion of the → apparent Sun and kept by dividing the day into 24 equal hours.

→ apparent; → solar; → time.

The precise time at which the gamma burst photons hit a detector. Measuring the time difference between the arrival time of the photons at different telescopes separated by known distances permits to determine the burst direction.

Arrival, n. from arrive + → -al. Arrive, from O.Fr. ariver "to come to land," from V.L. *arripare "to touch the shore," from L. ad ripam "to the shore," from → ad "to" + ripa "shore;" → time.

Zamân, → time; raseš n. from rasidan "to arrive," Mid.Pers. rasitan, O.Pers./Av. rasa- present stem of ar- "to move, go or come toward," cf. Skt. ar-, rcchati.

Time measured using atomic clocks.

→ atomic; → number.

A → coordinate time having its spatial origin at the solar system barycenter. It is intended to be used as the independent variable of time for all calculations pertaining to orbits of planets, asteroids, comets, and interplanetary spacecraft in the solar system. → Barycentric Dynamical Time (IDB).

→ barycenter; → coordinate; → time.

A time scale previously used in calculations of the orbits of solar system objects (planets, asteroids, comets, and interplanetary spacecrafts). It was based on the Terrestrial Dynamical Time, but took the relativistic effect of time dilation into account to move the origin to the solar system barycenter. It is now superseded by → Barycentric Coordinate Time (TCB).

→ barycenter; → dynamical; → time.

A unit of length in the → metric system, equal to one-hundredth of a meter, which is the current unit of length in the → International System of Units (SI).

→ centi-; → meter.

→ Mean solar time.

→ civil; → time.

The time over which a propagating → electromagnetic wave may be considered → coherent. The coherence time of an interferometer is the interval during which the fringe phase remains stable.

→ coherence; → time.

A → quantum walk taking place entirely in the position space. Continuous-time quantum walk was introduced by E. Farhi & S. Gutmann (1998, Phys. Rev. A 58, 915).

→ continuous; → time; → quantum; → walk.

1) The time in which a → white dwarf cools to half its temperature. It depends on the composition, the mass, and the actual luminosity at some point in time. Cooling time is given by the relation: t = 8.8 × 10⁶ (12/A) (M)^5/7 (μ/2)^-2/7 (L)^-5/7 in years, where M and L are mass and luminosity in solar units, A the mean → atomic mass, and μ the → mean molecular weight (Iben & Tutukov, 1984, ApJ 282, 615). See also → Mestel theory; → white dwarf crystallization.
2) The time needed by a → plasma to radiate its thermal energy. The cooling time is directly proportional to the square root of the temperature and inversely proportional to the density. It turns out that for the → intercluster medium in a → cluster of galaxies this time is longer than the → age of the Universe. At the centers of some clusters, however, the cooling time is smaller than the age of the Universe due to the presence of a → cooling flow.

→ cooling; → time.

In relativity, the proper time in the specified reference frame. Because of time dilation, this may differ from the time experienced by any participant in the events being considered. It is the time basis (or coordinate) to be used in the theory of motions referred to this system.

→ coordinate; → time.

An international high-precision time standard based on the Greenwich Mean Time and adjusted to compensate for divergence from atomic time. It is based on the non-uniform rotation of the Earth (UT1) and the perfectly uniform international atomic time (TAI). UTC differs from TAI by the total number of → leap seconds, so that UT1-UTC stays smaller than 0.9 sec in absolute value.

→ coordinate; → universal; → time.

The time as measured by a clock that is at rest relative to the expanding space, and that has been set to zero at the very beginning, the time of the hypothetical → Big Bang singularity. The cosmic time is interpreted as the → age of the Universe (Einstein-online).

→ cosmic; → time.

A concept used for checking the stability of a group of mass such as a → cluster of galaxies or a → star cluster. The crossing time is given by t_c = R/V, where R is the average projected radial distance of group members from the center of mass and V the Gaussian dispersion in internal velocity.

→ cross; → time.