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There have been some mails about NLC. I'll notify about some
points on geometrical reasons to NLC apparance. The borders
of visible NLC area are formed usually by horizon, Earth's
shadow and light sky in the north. Especially in the wide
displays to see a natural edge in NLC area is not so common.
The Earth's shadow gives often the southern edge of NLC. Sometimes
it is very defined and sharp, but often it is not so easy
to notice exactly. During the observation night, the motion
of the shadow can be observed quite easily. If the display is
very bright and the forms are strong, in the beginning of the
night, after it is dark enough, you can see the NLC area
extending even over zenith to southern sky. When the Sun goes
lower, the southern edge of the NLC is moving very fast
northwards. In the morning hours, the area is extending back
to south. By careful observing you can notice also the
shadow turning westwards, when the Sun goes to east.
Very interesting time is, when the Sun is near 6-8° below
the horizon. Then the motion of shadow is fastest (several
tens of degrees in south-north direction when Sun sets/rises
one degree). I have tried to calculate the location of
Earth shadow in 83 kms altitude and in which direction it is
visible. In principle it is quite an easy calculation. The
only problem is the know the screening height ie. the thickness
of the air layer which screens the sunlight.
-Veikko-
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