"Stan Gosnell" wrote in message
...
Jose wrote in news:Qbhbf.3995$Lv.1888
@newssvr23.news.prodigy.net:

Yes, there is nautical twilight and astronomical twilight

As Spock would say - "fascinating". What is the reason for all these
different twilights? (never mind, if the FAA can have two different
definitions of night, I might as well just go google myself.

Jose

Civil twilight is defined to begin in the morning, and to end in the
evening when the center of the Sun is geometrically 6 degrees below the
horizon. This is the limit at which twilight illumination is sufficient,
under good weather conditions, for terrestrial objects to be clearly
distinguished; at the beginning of morning civil twilight, or end of
evening civil twilight, the horizon is clearly defined and the brightest
stars are visible under good atmospheric conditions in the absence of
moonlight or other illumination. In the morning before the beginning of
civil twilight and in the evening after the end of civil twilight,
artificial illumination is normally required to carry on ordinary outdoor
activities. Complete darkness, however, ends sometime prior to the
beginning of morning civil twilight and begins sometime after the end of
evening civil twilight.

Nautical twilight is defined to begin in the morning, and to end in
the evening, when the center of the sun is geometrically 12 degrees below
the horizon. At the beginning or end of nautical twilight, under good
atmospheric conditions and in the absence of other illumination, general
outlines of ground objects may be distinguishable, but detailed outdoor
operations are not possible, and the horizon is indistinct.

Astronomical twilight is defined to begin in the morning, and to end
in the evening when the center of the Sun is geometrically 18 degrees
below the horizon. Before the beginning of astronomical twilight in the
morning and after the end of astronomical twilight in the evening the Sun
does not contribute to sky illumination; for a considerable interval
after the beginning of morning twilight and before the end of evening
twilight, sky illumination is so faint that it is practically
imperceptible.

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And stars of the 6th magnitude ( the faintest naked-eye stars) become
visible at the zenith.........