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From: Veikko M{kel{ (PVTMAKELA_at_hidden_email_address.net)
Date: 07/10/1995


   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-