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Subject: Re: Sunset, sunrise, civil & nautical twilight
daveweilacher@XXX.XXX
Date: Wed Jan 30 2002 - 16:53:44 EST
Thanks & thanks. Betwixt the two of you, I've got it.
Original Message:
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From: R.H. van Gent r.h.vangent@XXX.XXX
Date: Wed, 30 Jan 2002 22:39:32 +0100
To: NAVIGATION-L@XXX.XXX
Subject: Re: [NAV-L] Sunset, sunrise, civil & nautical twilight
"daveweilacher@XXX.XXX" wrote:
> A definition I've read for these is:
>
> Sunset occurs right when the top of the sun disappears over the horizon. Civil twilight occurs when the sun is 6 degrees below the horizon, and nautical twilight occurs at 12 degrees.
Hi,
These are the standard astronomical (and nautical, I guess) definitions
for the various dusk and dawn intervals tabulated in the (local)
astronomical almanacs.
For sunset/sunset, the centre of the Sun is assumed to be 90 degrees 50
arc minutes distant from the zenith (the 50 arc minutes accounts for
both refraction and the Sun's semi-diameter, in an average sense). Note
that this is in fact only correct at sea-level, for non-zero altitudes a
small correction can be made if this is deemed to be necessary.
Civil twilight is when the Sun's centre is 96 degrees distant from the
zenith and nautical and astronomical twilight when the Sun's centre is
102 and 108 degrees distant from the zenith.
> Sunset might make sense but the 6 and 12 degree notion doesn't seem right to me.
>
> The closer to a pole you are, the longer twilight lasts (yes or no?) in which case the degrees don't work.
That is true (twilight lasts longer) and at certain latitudes some of
the darker twilight periods do not even occur during certain periods of
the year.
For instance, for latitudes higher than 48.5 degrees astronomical
twilight does not occur around the summer solstice. This is of some
concern to Muslim communities in Northern Europe and elsewhere as the
lawful period for one their five compulsory prayers can never occur.
For latitudes higher than 54.5 degrees you will likewise have a period
with no nautical twilight around the summer solstice and higher than
60.5 degrees no civil twilight either.
The same of course also occurs on the southern hemisphere near to the
(northern hemisphere) winter solstice.
Regards,
=======================================================
* Robert H. van Gent *
* E-mail: r.h.vangent@XXX.XXX *
* Homepage: http://www.phys.uu.nl/~vgent/homepage.htm *
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