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abstraction > mathematical concept > number > quantity > time

time
subject			fact

time	has unit time unit
	has definition A dimension distinguishing past, present, and future. In relativity, time is portrayed as a geometrical dimension, analogous to the dimensions of space.
	is a kind of quantity
number	has value

Kinds of time :

• crossing time (2 facts) - The time it takes a particle to travel from one point in its orbit to another point 180° away.
• delay time (2 facts) - Time lapse between the time a signal (e.g., a radar beam) is propagated out to a distant object and the time it is received after the object bounces it back.
• epoch (7 facts) (T) - An orbital element representing the time of perihelion passage., A point of time selected as a fixed reference., An arbitrary fixed instant of time or date used as a chronological reference datum for calendars (see calendar), celestial reference systems, star catalogs, or orbital motions (see orbit)., A date and time that specifies the reference system to which celestial coordinates are referred. Prior to 1984 coordinates of star catalogs were commonly referred to the mean equator and equinox of the beginning of a Besselian year. Beginning with 1984 the Julian year has been used, as denoted by the prefix J, e.g., J2000.0.
• half-life (4 facts) (τ_1/2) - Time it takes for the number of particles to halve. For a radioactive substance, the length of time required for half the atoms to disintegrate.
• hold-time (2 facts) - The time taken to use up all the liquid cryogens, like LN₂, in a cooled CCD cryostat.
• Hubble time (6 facts) (H₀^-1) - The Hubble time is one divided by the Hubble constant, which gives a number from 10 to 20 billion years. For a flat universe with no cosmological constant, the age of the universe is two-thirds of the Hubble time., The inverse of the Hubble constant and a crude measure of the universe's age. For a Hubble constant of 50, one can calculate that the Hubble time is 19.6 billion years; for a Hubble constant of 80, the Hubble time is 12.2 billion years. If there is no cosmological constant, the universe is younger than the Hubble time. In particular, if the mass density of the universe (designated Ω) is 0.1, the universe's age is 90 percent of the Hubble time; if Ω is 1.0, the universe's age is 67 percent of the Hubble time.
• integration time (2 facts) - The interval of time used to collect photons of light on a detector and build-up a strong signal.
• Kelvin timescale (1 kind, 7 facts) - The time it takes a star to contract gravitationally from infinite radius down to its present radius by radiating its thermal energy.
• lifetime (6 facts) (τ, mean life) - The time it takes for a sample of identical particles to decay to 1/e of its initial population.
• lookback time (3 facts) - Phenomenon that, owing to the finite velocity of light, the more distant an object being observed, the older is the information received from it. A galaxy one billion light-years away, for instance, is seen as it looked one billion years ago., The time in the past at which the light we now receive from a distant object was emitted. Galaxies of a certain type (redshift and luminosity) can be seen only at a certain distance.
• nuclear time scale (1 kind, 6 facts) - Time required for a star to evolve a significant distance off the main sequence; the time it takes a star to convert all its available hydrogen into helium.
• period (4 facts) (P) - An orbital element representing the time required to complete an orbit. This parameter is required when determining the orbit of a binary star system in which the mass is not known.
• Planck time (6 facts) (t_P)
• rise time (2 facts) - The time required for the vehicle to achieve its optimum height (in rocket or balloon astronomy).