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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