Kelly et all,
Unfortunately the answer is not simple.
The official definition of sunrise and sunset is when the Sun is 0.8 deg. below
the horizon.
Civil twilight is when the center of the Sun is 6 deg. below the horizon.
Nautical twilight is when the center of the Sun is 12 deg. below the horizon.
Astronomical twilight is when the center of the sun is 18 deg. below the
horizon.
At sunrise the F layer begins receiving solar radiation when the sun is 20 deg.
below the horizon and is fully illuminated when the Sun is 14 deg. below the
horizon. The D layer starts becoming illuminated when the Sun is 9 deg. below
the horizon. The opposite holds for sunset.
For the low bands (160-30 meters) I would use the 9 deg. below horizon and the
high bands (20-10 meters) when the Sun is 20 deg. below horizon.
Bottom line in my professional opinion is that it doesn't really matter which
one you use. Actual day to day variation of the height of and level of
ionization in the D and F layers varies more from other factors besides
illumination by the Sun.
I personally use the free and excellent W6EL propagation software and I think
it defines sunrise and sunset as when the center of the Sun is on the horizon.
73,
Thomas F. Giella, KN4LF
Retired Space & Atmospheric Weather Forecaster
Plant City, FL, USA
Grid Square EL87WX
Lat & Long 27 58 33.6397 N 82 09 52.4052 W
kn4lf@arrl.net
Propagation eGroup: http://lists.contesting.com/mailman/listinfo/propagation
PropNET Beacon Program: http://www.propnet.org
KN4LF Daily Solar Space Weather & Geomagnetic Data Archive:
http://www.kn4lf.com/kn4lf5.htm
KN4LF HF/MF Frequency Radio Propagation Theory Notes:
http://www.kn4lf.com/kn4lf8.htm
KN4LF Amateur & SWL Radio History: http://www.kn4lf.com/index.htm
----- Original Message -----
From: Kelly Taylor
To: propagation@contesting.com
Sent: Thursday, November 25, 2004 3:24 PM
Subject: [Propagation] Twilight?
Im configuring Geoclock and some propagation software for CQWW contest and
wanted to know which type of Twilight do hams use when predicting Grayline
openings? Civil, Nautical, Astronomical? Or does it matter? It seems like there
is a big difference.
Thanks
Kelly
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