Greetings to you both,
Another way I prefer to use, during spring and summer when the sun is north of
the equator, is to find the east-west line from the prime vertical (the great
circle passing through those points on the horizon and the zenith). If you
know the time of noon, then the sun will be on the prime vertical when Z = 90
or 270 degrees. Its hour angle T is easily calculated,
cos T = tan d / tan L, (d declination of sun, L latitude).
(The very slight error from interval change in declination can be reduced by a
second approximation using the found time. In practice, this is usually
negligible.)
The method is better because the sun is lower in the sky, lengthening its
shadow conveniently, and because the change in azimuth with time is less near
the horizon. The disadvantage is that the rest of the year the sun crosses the
PV below the horizon. Ah, well! It's often raining here in those months. You
might choose to try it, however.
Gino, it was very pleasant to have you both here in Portland for the conference
in '09. I hope to see you and Donald in Indianapolis.
Best wishes,
John
John F. Schilke, MD
N 45º 24' W 122º 39'
----- Original Message -----
From: GINO SCHIAVONE
Subject: RE: sundials in schools
One of the ways I like to find north is to determine the latitude and
longitude of a location with Goggle Earth (to Goggle Map). Then I enter that
information in the NOAA Sunrise/Sunset Calculator at
http://www.esrl.noaa.gov/gmd/grad/solcalc/sunrise.html which will tell me the
time of Solar Noon on any particular date.
Armed with this information I can use a plumb-bob over a point and mark a
second point on the shadow of the plumb-bob string at solar noon the moment the
sun is "southing". A line drawn between these two points is a north-south line.
A long way around the barn, but rather fun.
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