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on 5/19/04 4:37 PM, Jim Thompson at jim2@JIMTHOMPSON.NET wrote:

> What about this method for emergency calcuation of declination?
>
> 1. Label a compass rose June 22 at 000o, Sept 23 at 090o, Dec 22 at 180o and
> March 21 at 270o.
> 2. The radius on the vertical axis is the declination of the sun. A
> horizontal line from any date around the circle intersects that vertical
> radius.
> 3. Measure the length of the vertical axis from the center to the
> intersection of the horizontal line, divide that length by the full radius,
> and multiply that ratio by 22.5o.
> 4. Error is +/- 0.5o.
>
> Jim Thompson
> jim2@jimthompson.net
> www.jimthompson.net
> Outgoing mail scanned by Norton Antivirus
> -----------------------------------------
>
I am not sure I understand what Jim is saying, but here is what I have been
preaching for many years.  I would very much appreciate someone telling me
if I am wrong, and if so, how much wrong!
I tell people to take a piece of paper and draw horizontal lines, each
separated by an equal amount.  Label them +30, +20,+10,0,-10,-20,-30.  Draw
a circle centered on the 0 line so that the top of the circle is on 23.5,
and the bottom is on -23.5. Label the cardinal points June 21, Sept 23, Dec
22, and March 21.  Then I tell them to fill in the dates around the circle
(easier said than done), and read the declination directly.

I am guessing that if the Earth were in a circular orbit around the sun
instead of elliptical, then my analogue would be OK. Does the ellipticity of
the orbit make this wrong?

Ken Gebhart

   

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