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

According to → general relativity, → space-time is curved by the presence of → matter. The curvature is described in terms of → Riemann's geometry. In → cosmological models three types of curvature are considered: positive (spherical, → closed Universe), zero (Euclidean, → flat Universe), and negative (hyperbolic, → open Universe). See also → curvature constant.

→ curvature; → space-time.

A system of adjusting the official local time in some countries in order to provide a better match between the hours of daylight and the active hours of work and school. The "saved" daylight is spent on evening activities which get more daylight, rather than being "wasted" while people sleep past dawn. Although known also as summer time, it includes the spring season and nearly half of autumn.

→ day; → light; saving, from save, from O.Fr. sauver, from L.L. salvare "to secure," from L. salvus "safe," PIE *solwos, from base *sol- "whole" (cf. O.Pers. haruva-, Av. haurva- "whole, intact," Mod.Pers. har "every, all; any," Skt. sarva- "whole, entire," Gk. holos "whole"); → time.

Vaxt, written vaqtوقت but pronounced vaxtوخت, is a Pers. word meaning "portion (of time)". Its variants and related words in Mod./Mid.Pers. are: baxt "what is alloted, 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 alloted (luck, fortune)," baxədra- "part, portion," baγa- "master, god," O.Pers. bāji- "tribute, tax," cf. Skt. bhaj- "to share, divide, distribute, apportion," bhájati "divides," bhakta- "alloted; 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."
nur→ light.
anduzi, verbal noun of anduxtan "to save; acquire, gather," from Mid.Pers. handôxtan, handôz- "to gain, acquire, amass," from *ham-tuj-, from ham- "together," → com- + *tuj- "to save, gather, (re)pay," cf. Skt. tuj- "to promote, be strong, move quickly."

The time interval when the Sun is above the horizon for a given position.

→ day; → time.

Ruzhangâm, from ruz→ day + hangâm "time, hour, season," Mid.Pers. hangâm "time, epoch, season," Av. ham-gam- "to meet together," from ham- "together," → com- + gam- "to come; to go," jamaiti "goes," O.Pers. gam- "to come; to go," Mod.Pers. âmadan "to come," 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 root *gwem- "to go, come."

The time required for the amplitude of a vibrating system to decrease to 1/e of its initial value.
Of an orbiting object, its lifetime in a non stable orbit.

→ decay; → time.

The time-scale of geologic processes which is millions or billions of years in contrast to the few thousand years claimed by supporters of the → creationism. The concept of "deep time" was first described in 1788 by the Scottish geologist James Hutton (1726-1797) in the Transactions of the Royal Society of Edinburgh. The term was coined by the American author John McPhee (1931-).

→ deep; → time.

Same as → delay.

→ delay; → time.

The time it takes for a given → chemical species to be reduced below a significant level of → abundance in a → compound or → object.

→ depletion; → time.

A → quantum walk involving a probabilistic → operator that changes the direction while leaving the position fixed, and a shift operator that changes the position. Discrete-time quantum walk was introduced by J. Watrous (2001, Journal of Computer and System Sciences 62, 376)

→ discrete; → time; → quantum; → walk.

The independent variable in the theories which describe the motions of bodies in the solar system. The most widely used form of it, known as Terrestrial Time (TT) or Terrestrial Dynamical Time (TDT) uses a fundamental 86,400 Systeme Internationale seconds (one day) as its fundamental unit. → Terrestrial Time; → Terrestrial Dynamical Time; → Barycentric Dynamical Time.

→ dynamical; → time.

1) The characteristic time it takes a protostellar cloud to collapse if the pressure supporting it against gravity were suddenly removed; also known as the → free-fall time.
2) → crossing time for a stellar system like a galaxy.

→ dynamical; → time-scale.

The time within which the amplitude of an oscillation increases or decreases by a factor e (= 2.71828...).

From e the base of the natural, or Napierian, system of logarithms; folding, from -fold suffix meaning "of so many parts," or denoting multiplication by the number indicated by the stem or word to which the suffix is attached (as in twofold; manifold), from O.E. -feald, related to Ger. -falt; Gk. altos, -plos, -plus; → time.

Zamân, → time; e, as above; tâyi noun of tâ multiplicative suffix, also "fold, plait, wrinkle; like, resembling."

The time required for the redistribution of → angular momentum due to → meridional circulation. The Eddington-Sweet time for a uniformly → rotating star is expressed as: τ_ES = τ_KH . GM / (Ω² R³), where τ_KH is the → Kelvin-Helmholtz time scale, R, M, and L designate the radius, mass, and luminosity respectively, Ω the → angular velocity, and G the → gravitational constant. The Eddington-Sweet time scale can be approximated by τ_ES≅ τ_KH / χ, where χ is the ratio of the → centrifugal force to → gravity. For the Sun, χ ≅ 10^-5 resulting in an Eddington-Sweet time scale which is too long (10¹² years), i.e. unimportant. In contrast, for a rotating → massive star  χ is not so much less than 1. Hence the Eddington-Sweet circulation is very important in massive stars.

Named after the prominent British astrophysicist Arthur S. Eddington (1882-1944), who was the first to suggest these currents (in The Internal Constitution of the Stars, Dover Pub. Inc., New York, 1926) and P. A. Sweet who later quantified them (1950, MNRAS 110, 548); → time scale.

The time during which a → microlensing event occurs. It is given by the equation t_E = R_E/v, where R_E is the → Einstein radius, v is the magnitude of the relative transverse velocity between source and lens projected onto the lens plane. The characteristic time-scale of → microlensing events is about 25 days.

→ Einstein; → time-scale.

The time required for → light to cross the classical radius of the electron (→ electron radius): t_e = r_e/c ≅ 10^-23 s.

→ elementary particle; → time.

The uniform time-scale used as the independent variable to calculate the orbits in the solar system prior to 1984. Ephemeris Time was adopted in 1960 to deal with irregularities in the → Earth's rotation that had been found to affect the course of mean solar time. The definition of Ephemeris Time is based on Newcomb's analytical theory of the Earth's motion around the Sun (Newcomb 1898), according to which the geometric mean longitude of the Sun with respect to the Earth-Moon barycenter is expressed by:
L = 279° 41' 48".04 + 129 602 768".13 T + 1''.089 T²,
where L refers to the → mean equinox of date while T measures time from noon 1900 January 0 GMT in Julian centuries of 36525 days. Ephemeris Time is therefore defined as the instant near the beginning of the calendar year A.D. 1900 when the mean longitude of the Sun was 279° 41' 48''.04, at which instant the measure of ET was 1900 January 0, 12h precisely. In this system the fundamental unit was the → ephemeris second, which was defined so that the → tropical year at the epoch 1900.0 should be exactly 31 556 925,9747 seconds of ephemerides. Ephemeris Time was inconvenient in many ways and was supeseded with the → Terrestrial Dynamical Time (TDT), whose fundamental unit is the SI second.

→ ephemeris; → time.

The difference, due to Earth's elliptical orbit and variable orbital velocity, between apparent solar time and mean solar time. It varies throughout the year, and slightly from year to year. At present, it reaches extremes of about -14 minutes in February, and about +16 minutes in November. The equation of time is visually illustrated by an → analemma.

→ equation; → time.

The characteristic time it takes an evolving astronomical object to pass from a step to another.

→ evolutionary; → time scale.

The length of time during which the receiver is irradiated.

→ exposure; → time.

The characteristic time it would take a body to collapse under its own → gravitational attraction, if no other forces existed to oppose the collapse. It is given by: t_ff = (3π/32 ρ₀ G)^1/2, where ρ₀ denotes the initial density and G the → gravitational constant. Free-fall time is independent of the starting radius. Also known as → dynamical time scale.

→ free fall; → time.

The proper time experienced by a clock at rest in a coordinate frame co-moving with the center of the Earth, i.e. a clock that performs exactly the same movements as the Earth but is outside the Earth's gravity well. TCG was defined in 1991 by the International Astronomical Union as one of the replacements for Barycentric Dynamical Time (TDB).

→ geocentric; → coordinate; → time.