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Topband: Sunrise times

To: topband@contesting.com
Subject: Topband: Sunrise times
From: Charles Bibb <zedkay@bellsouth.net>
Date: Sat, 21 Apr 2007 15:37:07 -0500
List-post: <mailto:topband@contesting.com>
At 10:49 AM 4/21/2007, W7lr@aol.com wrote:
>I wonder how the weather man defines or determines sunrise time?


I use the U.S. Naval Observatory's web site to obtain precise sunrise 
and sunset times. Their definition of sunrise is the time at which 
the upper edge of the sun's disc it exactly even with the local horizon.

One thing you have to remember about published sunrise times it that 
these figures denote when the sun's rays will just begin to strike 
the Earth's surface at a specific location. As far as radio 
propagation is concerned, we should be more concerned with when the 
sun's rays begin to strike the D-region of the ionosphere some 60-odd 
miles directly above our heads. At that altitude, sunrise comes 
significantly earlier than on the surface.

I'm not sure that the exact geometry of the situation matters too 
much, though, because I think the speed with which the rising D-layer 
ionization reaches full absorption of topband signals is more a 
function of the intensity of the solar radiation striking it than 
with the time differential between ground-level sunrise and D-layer 
sunrise, and so varies with solar activity.  However, it does mean 
that any observed peak in received signal strength will happen before 
your local published sunrise time.

The reverse is true for sunset times.

73,

Charles - K5ZK


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