An Etymological Dictionary of Astronomy and Astrophysics
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The time representing the → Planck length divided by the → speed of light. It is expressed by: t_P = (Għ/c⁵)^1/2≅ 5.4 x 10^-44 s, where G is the → gravitational constant, ħ is the → reduced Planck's constant, and c is the → speed of light. At the Planck time, the mass density of the Universe is thought to approach the → Planck density.

→ Planck; → time.

A time interval over which an orbit precesses by 2π radians in its plane.

→ precession; → time.

In general relativity, the time as measured on a clock that travels with the observer in the same system. An accelerated clock will measure shorter time intervals between events than a non-accelerated clock between the same events. → twin paradox.

→ proper; → time.

The operation of a computer during the actual time that the related physical processes take place so that the results can be used to guide the physical processes.

→ real; → time; → operation.

Data processing that takes place instantaneously upon data entry or receipt of a command.

→ real; → time; → processing.

The time period necessary for a cloud of atomic hydrogen to be → ionized by the ultraviolet photons of a central → massive stars.

→ recombination; → time.

The characteristic length of time that is required for a system undergoing → relaxation to move to its equilibrium state. If the system follows an exponential law G = G₀ exp(-t / τ), the relaxation time is the time required for G to obtain the fraction 1/e of its initial value G₀.

→ relaxation; → time.

The time between an occurrence and a reaction produced by the occurrence.

→ response; → time.

A → general relativity effect whereby an → electromagnetic signal passing near a massive object takes, due to the curved → space-time, a slightly longer time to travel to a target than it would if the mass of the object were not present. The Shapiro time delay is one of the four classic solar system tests of general relativity. Radar reflections from → Mercury and → Mars are consistent with general relativity to an accuracy of about 5%. The Shapiro time delay is a significant contributor in → gravitational lens systems.

Irwin I. Shapiro, an American astrophysicist; → time; → delay.

The time based upon → Earth's rotation with respect to the stars, with the → sidereal day as the unit of measurement. At the moment when the → vernal equinox crosses the → meridian in → upper culmination, sidereal time is equal to zero hours for that observing position. The → hour angle of the vernal equinox is equal to sidereal time.

→ sidereal; → time.

The time based on the rotation of the Earth relative to the Sun. → mean solar time.

→ solar; → time.

A physical entity resulting from the union of space and time concepts. In its most simple version space-time is the four-dimensional continuum, having three spatial coordinates and one temporal coordinate, in which any → event or physical object is located. In → special relativity it is Minkowski's flat space-time. In → general relativity, it is described by a curved entity characterized by a → metric. Free-fall motion describes the → geodesic of this curved space-time. It may have additional dimensions in the context of speculative theories, such as → string theory.

→ space; → time.

→ curvature of space-time.

→ space; → time; → curvature.

A simple way of representing the → space-time continuum, usually including time and only one spatial dimension. The curve of a particle's equation of motion on a space-time diagram is called a → world line. Same as → Minkowski diagram.

→ space; → time; → diagram.

1) In Einstein's → theory of relativity, ds² = c²dt² - dx² - dy² - dz².
2) In → Newtonian mechanics, time interval: dt; space interval at fixed time: dl² = dx² + dy² + dz².

→ space-time; → interval.

→ space-time.

→ space; → time.

The time scale on which a stellar image changes significantly due to → atmospheric turbulence. It is proportional to the ratio r₀/Δv, where r₀ is the → Fried parameter and Δv the standard deviation of the distribution of wind velocities weighted by the turbulence structure coefficient. Typical lifetimes in the visible range from about 3 to 30 milliseconds.

→ speckle; → life; → time.

Omr "life-time;" from Ar. 'umr; pakâl, → speckle.

The time in any of the 24 internationally agreed time zones into which the Earth's surface is divided. The primary zone is centered on the Greenwich meridian (0° longitude).

→ standard; → time.

The time necessary for a star to form. It depends inversely on the stellar mass.

→ star formation; → time scale.

A → time series if it obeys the following criteria: 1) Constant → mean over time (t). 2) Constant → variance for all t, and 3) The → autocovariance function between X_t1 and X_t2 only depends on the interval t₁ and t₂.

→ stationary; → time; → series.