John, >From "Astronomical Algorithms," First Edition, by Jean Meeus (Willmann-Bell):
length of a degree of longitude: d = (3.14159/180) R where R is the radius of the parallel circle at latitude p, R = a * (cos p)/(square root(1 - e^2 * sin^2 p)) where "^2" denotes squared, e = earth's eccentricity = 0.08181922, a = earth's equatorial radius = 6378.14 km, p is latitude. This assumes that the earth's shape is an ellipsoid (International Astronomical Union 1976 ellipsoidal parameters used) rather than a sphere. For a sphere: e = 0, and thus R = a * cos p. Thus, the length of a degree is approximately proportional to the cosine of latitude rather than the latitude directly. A second edition of the book is now available. This should suffice for most sundial applications. In practice, I would think that one would use a topographic map or a GPS system to determine the latitude and longitude of the sundial. Gordon Gordon Uber [EMAIL PROTECTED] Reynen & Uber Web Design http://www.ubr.com/rey&ubr/ Webmaster: Clocks and Time http://www.ubr.com/clocks/
