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

A uniform atomic time scale for apparent geocentric ephemerides defined by a 1979 IAU resolution, which replaced Ephemeris Time. TDT is independent of the variable rotation of the Earth, and the length of the tropical year is defined in days of 86,400 seconds of international atomic time. In 1991 it was replaced by Terrestrial Time.

→ terrestrial; → dynamical; → time.

The modern astronomical standard for the passage of time on the surface of the Earth. It is the → coordinate time scale consistent with the theory of general relativity for an observer on the surface of the Earth. TT was renamed from Terrestrial Dynamical Time (TDT) in 1991. The fundamental unit of TT is the day of 86,400 SI seconds. It is related to the International Atomic Time by the relation: TDT = TAI + 32.184 sec.

→ terrestrial; → time.

That part of the working time of a telescope devoted to tests of coupled instruments or the telescope itself.

→ test; → time.

1) A non-spatial sequential relation in which events occur in apparently irreversible succession from the past through the present to the future. → time's arrow.
2) A limited period or interval, as between two successive events.

M.E.; O.E. tima "limited space of time," from P.Gmc. *timon "time" (cf. O.N. timi "time," Swed. timme "an hour"), akin to L. tempus (genitive temporis) "time" (Fr. temps, Sp. tiempo, It. tempo); maybe related to Pers. Tabari tum, tomon, temen "time;" Aftari ton "time."

Zamân "time," from Mid.Pers. zamân, jamân "time," zamânak "period, epoch;" loaned into Aramaic and Ar., loaned into Arm. žam, žamanak "time;" prefixed Sogdian nγm "time, moment, hour;" Proto-Iranian *gām- "to go, to come;" cf. Av. gam- "to come; to go," jamaiti "goes;" O.Pers. gam- "to come; to go;" Mod./Mid.Pers. gâm "step, pace," âmadan "to come;" cf. Skt. gamati "goes;" Gk. bainein "to go, walk, step;" L. venire "to come;" Tocharian A käm- "to come;" O.H.G. queman "to come;" E. come; PIE base *g^wem- "to go, come."
Gâh "time; place;" Mid.Pers. gâh, gâs "time;" O.Pers. gāθu-; Av. gātav-, gātu- "place, throne, spot;" cf. Skt. gâtu- "going, motion; free space for moving; place of abode;" PIE *gwem- "to go, come."
Vaqt, pronounced vaxt (وخت), but written vaqt (وقت), is a Pers. word meaning "portion (of time)". Its variants and related words in Mod./Mid.Pers. are: baxt "what is allotted, fate, fortune," baxš "portion, part, division," baxšidan, baxtan "to divide, distribute, grant," Av. base bag- "to attribute, allot, distribute," baxš- "to apportion, divide, give to," baxta- "what is allotted (luck, fortune)," baxədra- "part, portion," baγa- "master, god," O.Pers. bāji- "tribute, tax," cf. Skt. bhaj- "to share, divide, distribute, apportion," bhájati "divides," bhakta- "allotted; occupied with; a share; food or a meal, time of eating?," Gk. phagein "to eat (to have a share of food)"; PIE base *bhag- "to share out, apportion."
Tâmen "time," from Tabari temen, tumun, tum "time," pərtəmən "long time;" Lori temen "age, length of life;" Aftari ton; Lâri, Garâši taim "time span" (related to L. tempus?).

The assignment of telescope time by an expert panel to proposals after evaluating the merits of the observation projects.

→ time; → allocation.

Th speed of response of a detector, usually measured as 1/(2πν), where ν is the chopping frequency at which the responsivity fails to 1/√2 of its maximum value.

→ time; → constant.

1) The amount of time required for a → signal to travel from one point to another in an → electric circuit.
2) → gravitational lensing time delay.

→ time; → delay.

A distance-like quantity derived from → gravitational lensing time delay. It is given by a combination of three angular diameter distances in a strong lens system: D_Δt = (1 + z_L)[D_A(EL)D_A(ES) / D_A(LS)], where z_L is the → redshift of the → gravitational lens, while D_A(EL), D_A(ES), and D_A(LS) are the angular diameter distances from the Earth to the lens, from the Earth to the source, and from the lens to the source, respectively. As each of the distance is proportional to the inverse of H₀, D_Δt is proportional to 1/H₀.

→ time; → delay; → distance.

A phenomenon related to special and general relativity.
1) In → special relativity, the apparent shortening of time that occurs at speeds approaching that of light. A clock moving relative to a stationary observer will appear to slow down by a factor √(1- v²/c²), where v is the velocity and c the speed of light. → twins paradox.
2) In → general relativity, a clock in a stronger gravitational field runs more slowly. The dilation factor is given by: √(1- 2GM/rc²), where G is the gravitational constant, M the mass of the object creating the gravitational field, r a radial coordinate of the observer, which is analogous to the classical distance from the center of the object, and c the speed of light.

→ time; dilation, verbal noun of dilate, from M.E. dilaten, from O.Fr. dilater, from L. dilatare "make wider, enlarge," from → dis- "apart" + latus "wide."

Farâxeš, → dilation; zamân, → time.

One of the → orbital elements, the time when the → secondary body reaches → periapsis.

→ time; → periapsis; → passage.

Same → temporal resolution.

→ time; → resolution.

A transformation operating on time in the equations of motion of a dynamical system in which t is replaced by -t.

→ time; → reversal.

A measure of duration of a specific process, such as → crossing time, → dynamical time scale, → evolutionary time scale, → Kelvin-Helmholtz time scale, → nuclear time scale, → photon escape time, → relaxation time, → star formation time scale.

→ time; → scale.

A → sequence of values of a → variable in successive time order, usually at fixed intervals of time.

→ time; → series.

Any of the 24 zones on the Earth surface delimited by → meridians at approximately 15° intervals. In each time zone a common standard time is used, and the time is one hour earlier than the zone immediately to the east.

→ time; → zone.

The sequence of all natural processes in which the → entropy increases. In other words, the fact that these processes all move in one direction in time and are → irreversible. The past is distinctly different from the future; things always grow older, never younger.

→ time; arrow, M.E. arewe, arwe, from O.E. arwan, earh "arrow," from P.Gmc. *arkhwo (cf. Goth. arhwanza), from PIE base *arku- "bow and/or arrow," source of Latin arcus, → arc.

Peykân "arrow," → Sagitta; zamân, → time.

Of, pertaining to, or describing an → event belonging to the interior of the → light cone.

→ time; → like.

The → space-time interval between two → events if it is real, i.e. ds² > 0.

→ timelike; → interval.

Any mechanical, electric, or electronic device, such as a clock or watch, designed to measure and display the passage of time.

→ time; → piece.

Zamân-šomâr, literally "time counter," from zamân, → time, + šomâr "counter," from šomârdan "to count," from Mid.Pers. ôšmârtan, ôšmurtan "to reckon, calculate, enumerate, account for," from Av. base (š)mar- "to have in mind, remember, recall," pati-šmar- "to recall; to long for," hišmar-, cf. Skt. smar- "to remember, become aware," smarati "he remembers," L. memor, memoria, Gk. mermera "care," merimna "anxious thought, sorrow," martyr "witness."

The time interval between the release of an electron at the photocathode and the arrival of an electron at the anode. Transit time is not a single-valued quantity, but has a bell-shaped distribution.

→ transit; → time.