Hi Roger,
A very nice derivation can be found in "A Choice of Sundials", by
Winthrop W. Dolan, pg. 20-21. For those seeking a more intuitive
approach, the graphic, inside the book's dust jacket, of a horizontal
plane "embedded" within the celestial sphere gives a marvelous
description of why the hour lines are drawn at the angles they are.
Simply put, the hour lines of the dial must reach out to intersect the
projected meridian lines. An example of the graphic (made for lat=36.5d)
can be viewed at the URL below.
ftp://ftp.gcstudio.com/pub/misc/gcdial.jpg
Best,
Luke
Roger Bailey wrote:
>
> My challenge to all for today is to simply explain
> the basic formula for horizontal dials: Tan HA = Sin L x Tan t.
>
