Bob Frick wrote:
We dont change clocks here so
> after 22 Dec our sunrises will start to get earlier and earlier.
Bob--
Maybe I am missing something here, but I do not believe your sunrises
will turn around on December 22, no matter what clocks you are using.
Your northern-hemisphere sunrise should continue to occur later each
morning for about a month after the winter solstice---even though the
days are growing longer.
I was startled to observe this a few seasons ago--before becoming a
lowband DXer, I had not noticed. I pursued this with several
astronomers, most recently Derek, AA5BT, who sent along a detailed
explanation. I published it in my newsletter last month.
Without going into the nuts and bolts too deeply, it is primarily due to
two factors:
* the Earth's elliptical orbit, which causes it to go faster when closer
to the sun, thus changing the length of the solar day--the time to
rotate around to the same angle to the sun---over the year.
* the earth's inclination within its orbit, which causes the sun to move
north/south in the sky, but which also adds a seasonal east/west
component to those changes due to the elliptical orbit.
These lead to some interesting phenomena.
The absolute time of "mid-day" is not fixed, but varies sinusoidally
over the year. Near the solstices--the extrema of the sun's
excursion--the amount of daylight stays approximately constant for
several weeks. Through most of January, "midday" is moving later faster
than sunrise is moving earlier---hence sunrise continues to be later. By
the end of January, "midday's" movement has slowed enough to allow the
lengthening daylight hours to catch up and overcome it---and sunrise
begins to come earlier. The opposite occurs in July.
Unfortunately, none of this is likely to put me or any other west coast
station in your 160 meter log.
(With thanks to AA5BT),
Garry Shapiro, NI6T
Editor, The DXer
newsletter of the Northern California DX Club
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