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

The → energy at all → wavelengths that is incident on unit area of surface in unit time. It is measured in Watts per square meter.

Irradiance, from ir- variant of → in- (by assimilation) before r + radi(ant), → radiation, + -ance a suffix used to form nouns either from adjectives in -ant or from verbs.

Tâbešdâri, from tâbeš, → radiation, + dâri, verbal noun from dâštan "to have, hold," → property.

A unit of angular measure; one radian is that angle with an intercepted arc on a circle equal in length to the radius of the circle.

From radi(us) + -an an originally adj. suffix.

1) Generally, the → radiant energy per unit → solid angle per unit of → projected area of the → source. It is usually expressed in → watt per → steradian per → square  → meter (W m^-2 sr^-1). Same as steradiancy.
2) Of any particular → wavelength within the interval covered by a → spectral line, the → energy per unit → surface per steradian, per wavelength; denoted I_λ. The term radiance is often loosely replaced by "→ intensity." The radiance of the whole line is given by I = ∫ I_λ dλ. The radiance of an → emission line depends, among other things, upon the → number of → atoms per unit area in the → line of sight (the → column density) in the → upper level of the line.

From radia(nt), → radiant, + → -ance.

Tâbešmandi, noun from tâbešmand "possessing radiation," from tâbeš, → radiation, + -mand a suffix denoting possession; Mid.Pers. -ômand suffix forming adjectives of quality.

1) Sending out rays of light; bright; shining.
See also: → radiant energy, → radiant flux, → radiant intensity.
2) The point in the sky from which → meteors in a → meteor shower appear to radiate or come. See also: → radiant drift.

M.E., from M.Fr. radiant, from L. radiantem (nominative radians) "shining," pr.p. of radiare "to shine, radiate," → radiation.

1) Tâbandé, tâbeši adj. from tâbidan, → radiate.
2) Tâbsar, from tâb "light, radiation," → radiation, + sar "head, top, summit, point," → head.

The apparent slow motion of the → radiant of a → meteor shower from night to night against the background stars due to the Earth moving in its orbit around the Sun.

→ radiant; → drift.

The energy that is transmitted in the form of → radiation, in particular as → electromagnetic radiation.

→ radiant; → energy.

Rate of flow of energy as → radiation.

→ radiant; → flux.

A measure of the amount of radiation emitted from a point expressed as the radiant flux per unit solid angle leaving this source.

→ radiant; → intensity.

The radiative power per unit area in all wavelengths from the Sun received by the Earth at its average distance from the Sun. Its mean value is called the → solar constant. The solar irradiance changes over a year by about 6.6% due to the variation in the Earth/Sun distance. Moreover, solar activity variations cause irradiance changes of up to 1%.

→ solar; → irradiance.

The solid angle subtended at the center of a sphere by an area on its surface numerically equal to the square of the radius. → square degree.

From ste(reo)-, → stereo- + → radian.