Civil twilight: Difference between revisions

Latest revision as of 14:51, 20 February 2012

civil twilight

The period between astronomical sunrise or sunset and the time when the sun's unrefracted center is at elevation h₀ = -6°.

Local topography above the astronomical horizon will make local sunset occur before astronomical sunset. During a clear evening's civil twilight, horizontal illuminance decreases from ∼585–410 lux to ∼3.5–2 lux. As is true of nautical and astronomical twilight, civil twilight's length varies greatly with latitude and time of year. At h₀ = -6°, the ambient illuminance under clear skies historically was deemed just adequate for doing outdoor work without artificial illumination. However, perceptually demanding tasks, such as driving an automobile, require higher illuminances. Also at h₀ = -6°, luminances near the clear sky's zenith are low enough to make the brightest stars visible.
Compare astronomical twilight.

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-<div class="definition"><div class="short_definition">The period between astronomical [[sunrise]] or [[sunset]] and the time when the sun's  unrefracted center is at [[elevation]] ''h''<sub>0</sub> = &minus;6&deg;.</div><br/> <div class="paragraph">Local [[topography]] above the [[astronomical horizon]] will make local sunset occur before astronomical  sunset. During a [[clear]] evening's civil twilight, horizontal [[illuminance]] decreases from  &sim;585&ndash;410 [[lux]] to &sim;3.5&ndash;2 lux. As is true of nautical and [[astronomical twilight]], civil twilight's  length varies greatly with latitude and time of year. At ''h''<sub>0</sub> = &minus;6&deg;, the ambient illuminance under  clear skies historically was deemed just adequate for doing outdoor work without artificial [[illumination]].  However, perceptually demanding tasks, such as driving an automobile, require higher  illuminances. Also at ''h''<sub>0</sub> = &minus;6&deg;, luminances near the [[clear sky]]'s [[zenith]] are low enough to make  the brightest stars visible. <br/>''Compare'' [[astronomical twilight]].</div><br/> </div>
+<div class="definition"><div class="short_definition">The period between astronomical [[sunrise]] or [[sunset]] and the time when the sun's  unrefracted center is at [[elevation]] ''h''<sub>0</sub> = -6&#x000b0;.</div><br/> <div class="paragraph">Local [[topography]] above the [[astronomical horizon]] will make local sunset occur before astronomical  sunset. During a [[clear]] evening's civil twilight, horizontal [[illuminance]] decreases from  &sim;585&ndash;410 [[lux]] to &sim;3.5&ndash;2 lux. As is true of nautical and [[astronomical twilight]], civil twilight's  length varies greatly with latitude and time of year. At ''h''<sub>0</sub> = -6&#x000b0;, the ambient illuminance under  clear skies historically was deemed just adequate for doing outdoor work without artificial [[illumination]].  However, perceptually demanding tasks, such as driving an automobile, require higher  illuminances. Also at ''h''<sub>0</sub> = -6&#x000b0;, luminances near the [[clear sky]]'s [[zenith]] are low enough to make  the brightest stars visible. <br/>''Compare'' [[astronomical twilight]].</div><br/> </div>
 </div>