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

A physical quantity that describes the rate of change of temperature with displacement in a given direction from a given reference point. Same as → thermal gradient.

→ temperature; → gradient.

Meteo.: A reversal in the normal temperature decrease, the temperature rising with increased elevation in the atmosphere instead of falling. A layer in which temperature increases with altitude.

→ temperature; → inversion.

Prefix denoting one million million (10¹²).

From Gk. teras "monster."

A temperature scale, measured in → kelvin (K), that is related to the energy possessed by matter; it was formerly known as → absolute temperature. The zero point on the scale (0 K) is absolute zero. Thermodynamic temperature can be converted to temperature on the → Celsius scale by subtracting 273.15.

→ thermodynamic; → temperature.

In celestial mechanics, a combination of orbital elements commonly used to distinguish between comets and asteroids. Objects whose Tisserand's parameter value is smaller than 3 are considered to be dynamically cometary, and those with a value larger than 3 asteroidal. Also called Tisserand's invariant.

Named after François Félix Tisserand (1845-1896), French astronomer, Director of the Paris Observatory (1892).

The maximum permissible error or variation in a dimension of an object.

M.E., from O.Fr. tolerance, from L. tolerantia "endurance," from tolerans, pr.p. of tolerare "to bear, endure, tolerate."

Ravâdâri, noun from ravâdâr "consenter; judging right; lawful," from ravâ "admissible; allowable; tolerated" (from raftan "to go, walk; to flow;" Mid.Pers. raftan, raw-, Proto-Iranian *rab/f- "to go; to attack" + -dâr "having, possessor" (from dâštan "to have, to possess," Mid.Pers. dâštan, O.Pers./Av. root dar- "to hold, keep back, maitain, keep in mind," Skt. dhr-, dharma- "law," Gk. thronos "elevated seat, throne," L. firmus "firm, stable," Lith. daryti "to make," PIE *dher- "to hold, support").

A geometrical method in land surveying for the determination of the relative position of points. In contrast to → triangulation, trilateration involves measuring the lengths of the three sides of touching or overlapping triangles and not their angles.

→ tri-; → lateral; → side; → -tion.

The abundance of a chemical element being lower than a reference value, in particular compared to that of the Sun.

→ under-; → abundance.

A linear operator whose inverse is its → adjoint. In addition to → Hermitian operators, unitary operators constitute a fundamentally important class of quantum-mechanical operators.

→ unitary; → operator.

1) An act of uttering; a spoken word, statement, or vocal sound.
2) Something uttered; a word or words uttered; a cry, animal's call, or the like.
3) Linguistics: Any speech sequence consisting of one or more words and preceded and followed by silence: it may be coextensive with a sentence (Dictionary.com).

→ utter; → -ance.

A new kind of optical telescope with a 8.4-m diameter → primary mirror currently under construction in Chile and scheduled to begin operations in October 2023. Initially named Large Synoptic Survey Telescope (LSST), it will use a 3200 megapixel camera and an automated data processing system. It will have a large → field of view almost 10 square degrees of sky, or 40 times the size of the full moon. The LSST will move quickly between images to rapidly → survey the sky. From its mountain top site in the Andes (Cerro Pachon, a 2,682-m high mountain in Coquimbo Region), the LSST will take more than 800 panoramic images each night with its 3.2 billion-pixel camera, recording the entire visible sky twice each week. Each patch of sky it images will be visited 1000 times during the survey, each of its 30-second observations will be able to detect objects 10 million times fainter than visible with the human eye. The LSST's combination of telescope, mirror, camera, → data processing, and survey will capture changes in billions of faint objects. Hence, the data it provides will be used to create an animated, three-dimensional cosmic map with unprecedented depth and detail. This map will serve many purposes, from locating the → dark matter and characterizing the properties of the → dark energy, to tracking transient objects, to studying our own Milky Way Galaxy in depth. It will even be used to detect and track → potentially hazardous asteroids that might impact the Earth.

Named after Vera C. Rubin (1928-2016) whose work on galaxy rotation rates supported the existence of dark matter in galactic halos.

The mean temperature at which a gravitationally → bound system would satisfy the → virial theorem. For a system of mass M and radius R with constant density, the gravitational energy per unit mass is W = GM/R. The kinetic energy per unit mass is E = (3/2)kT_vir/μ, where k is → Boltzmann's constant and μ the mean molecular weight. According to the virial theorem, E = W/2, which leads to the virial temperature T_vir = (1/3)(GM/kR).

→ virial; → temperature.

One of the fundamental forces of nature that accounts for some particle interaction, such as → beta decay (→ radioactivity), the decay of free → neutrons, → neutrino interactions, and so forth. It is short-ranged, dominating at distances of 10^-16 cm and occurs at a rate slower than that of the → strong interaction by a factor of about 10^-13, hence its name. Although the weak interaction also includes interactions in which no neutrinos are emitted, neutrino emission accompanies all weak interactions of interest to astrophysics. Weak interaction plays an important role in the evolution of the stars from birth to death. For example, the → proton-proton reaction is a weak interaction. Also called → weak force or → weak nuclear force.

→ weak; → interaction.