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

Acoustics: Effect produced when sound is reflected or thrown back on meeting a solid obstacle.
Radio.: A wave returned to the transmitter with sufficient magnitude and delay to be distinguished from the directly transmitted wave. In radar, the portion of the energy of the transmitted pulse reflected back to the receiver.

From L. echo, from Gk. echo, personified as a mountain nymph, from ekhe "sound."

Pažvâk, literally "return sound," from paž "back, against, opposite," varaint pâd- (Mid.Pers. pât-, from O.Pers. paity "agaist, back, opposite to, toward, face to face, in front of;" Av. paiti, akin to Skt. práti "toward, against, again, back, in return, opposite;" Pali pati-; Gk. proti, pros "face to face with, toward, in addition to, near;" PIE *proti) + vâk "sound," Mid./Mod.Pers. vâng/bâng "sound, clamour;" Av. vacah- "word," from vac- "to speak, say;" cf. Mod.Pers. vâžé "word," âvâz "voice, sound, song" (Skt. vakti "speaks, says," vacas- "word;" Gk. epos "word;" L. vox "voice;" PIE base *wek- "to speak").

Reflection of light from a stellar outburst by successively more distant clouds of dust surrounding the star. For example, the light echoes from two shells of dust near supernova 1987A, or those of star V838 Mon.

→ light; → echo.

The reflection of → radio waves from transmitters located on the ground by a → meteor or by the corresponding trail left behind. When a meteor enters the Earth's upper atmosphere it excites the air molecules, producing a streak of light and leaving a trail of ionization behind it tens of kilometers long. This ionized trail occurs typically at a height of about 85 to 105 km, and may persist for less than 1 second up to several minutes.

→ meteor; → echo.