[oclug] (OT - maybe) time question

[Francis J. A. Pinteric]

On Sat, 06 Nov 2004 09:57:58 -0500
Normand Fisher <fishern at ncf.ca> wrote:
>
> In the world of aviation, official night is when the sun disk is 6
> degree below the horizon, therefore 30 minutes after sunset.
> 

Indeed, there are various definitions of when the sun actually sets
depending on the end use. The six degree rule you mentioned is based on
to the theoretical size of the solar disc which differs from the
apparent (visual) size of the disc and depends on a number of factors,
not to mention the actual position of the earth in its orbit and the
position of the observer on the earth (longitude, latitude and
altitude).

The Main Branch of the public library has a complete set of Astronomical
Ephemerides as well as the Nautical Almanac, both of which give brief
explanations for the different kinds of sunrise, sunset and twilight. At
my instigation, the library also has a copy of the"Explanatory
Supplement to the Astronomical Ephemeris" that have detailed discussions
on the theories and mathematical and geometical methods used to create
the ephemeris tables.

>>>--fja->

-- 
Bloggers: useless people with useless lives proving it 
daily for all the world to see. (2004)
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