Barycentric Dynamical Time (TDB)
zamân-e tavânik-e gerânigâhi
Fr.: temps dynamique barycentrique (TDB)
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 It is now superseded by → Barycentric Coordinate Time (TCB).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.
complex dynamical variable
vartande-ye tavânik-e hamtâft
Fr.: variable dynamique complexe
A → dynamical variable which has an → imaginary number part.
Of or pertaining to force or power; of or pertaining to force related to motion.
Adj. from → dynamics.
Fr.: âge dynamique
Age based on dynamical properties of a system. For example, the time derived for a system to evolve from an initial state to its present state, based on velocity and dimension (size) measurements.
Fr.: rupture dynamique
The process whereby a → bound system, such as a → binary system or a → globular cluster, is broken apart.
→ dynamical; → disruption.
Fr.: équilibre dynamique
Of a physical system, a condition in which the parts of the system are in continuous motion, but they move in opposing directions at equal rates so that the system as a whole remains in equilibrium.
→ dynamical; → equilibrium.
Fr.: frottement dynamique
The gravitational interaction between a relatively massive body and a field of much less massive bodies through which the massive body travels. As a result, the moving body loses → momentum and → kinetic energy. An example of dynamical friction is the sinking of massive stars to the center of a → star cluster, a process called → mass segregation. Dynamical friction plays an important role in → stellar dynamics. It was first quantified by Chandrasekhar (1943).
Fr.: loi dynamique
A law that describes the motion of individual particles in a system, in contrast to → statistical laws.
Fr.: masse dynamique
The mass of an object derived indirectly from theoretical formulae based on the laws governing the behavior of a → dynamical system.
Fr.: parallaxe dynamique
A method for deriving the distance to a binary star. The angular diameter of the orbit of the stars around each other and their apparent brightness are observed. By applying Kepler's laws and the mass-luminosity relation, the distance of the binary star can be calculated.
Fr.: relaxation dynamique
The evolution over time of a gravitationally → bound system consisting of N components because of encounters between the components, as studied in → stellar dynamics. Due to this process, in a → star cluster, → low-mass stars may acquire larger random velocities, and consequently occupy a larger volume than → high-mass stars. As a result, massive stars sink to the cluster centre on a time-scale that is inversely proportional to their mass. See also → mass segregation.
→ dynamical; → relaxation.
Fr.: courant dynamique
A group of stars pervading the Solar neighbourhood and travelling in the → Galaxy with a similar spatial velocity, such as the → Ursa Major star cluster, The term dynamical stream is more appropriate than the traditional term supercluster since it involves stars of di fferent ages, not born at the same place nor at the same time. A possible explanation for the presence of young groups in the same area as those streams is that they have been put there by the → spiral wave associated with their formation place, while kinematics of the older stars of the sample have also been disturbed by the same wave. The seemingly peculiar chemical composition of the Hyades-Pleiades stream suggests that this stream originates from a specific galactocentric distance and that it was perturbed by a spiral wave at a certain moment and radially pushed by the wave in the solar neighbourhood. This would explain why this stream is composed of stars sharing a common metallicity but not a common age (Famaey et al. 2005, A&A 430, 165).
Fr.: système dynamique
A system composed of one or more entities in which one state develops into another state over the course of time.
Fr.: temps dynamique
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 scale
marpel-e zamâni-ye tavânik
Fr.: échelle de temps dynamique
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.
→ dynamical; → time-scale.
Fr.: variable dynamique
Mechanics: One of the variables used to describe a system in classical mechanics, such as coordinates (of a particle), components of velocity, momentum, angular momentum, and functions of these quantities.
real dynamical variable
vartande-ye tavânik-e hasyâ
Fr.: variable dynamique réel
A → dynamical variable which does not have an → imaginary number part.
Terrestrial Dynamical Time
zamân-e tavânik-e zamini
Fr.: temps dynamique terrestre
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.