Well at least its science related, at first I thought you were talking about the Vampire book and movie. --
"Go for a Metric America" Howard Ressel Project Design Engineer, Region 4 (585) 272-3372 >>> On 6/27/2010 at 1:12 AM, in message <1957828963.8256761277615576471.javamail.r...@sz0024a.emeryville.ca.mail.comcast net>, <[email protected]> wrote: > I don't often do this, but this group is so good about precise definitions, I > couldn't resist when I ran across this just now (and had no idea these > existed or where they are defined): > > > Civil Twilight Civil twilight is defined when the sun is 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. > > Nautical Twilight Nautical twilight is defined when the sun is 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 Astronomical twilight is defined when the sun is 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. > Ezra
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