An excellent article on this is: Wagon, Stan. "Why December 21 Is the Longest Day of the Year" Mathematics Magazine, Vol. 63, No. 5, December, 1990, pp. 307-311.
Mathematics Magazine is a journal of the Mathematical Association of America (MAA) and is available in most all college libraries in the US. Despite the subtitle of the article, "How fundamental ideas from max-min theory explain the location of the extrema of the times of sunrise and sunset," there are no formal equations to wade through, but nice graphs and figures explaining the equation of time and effects such as offset sunrise and sunset extrema. Very readable. The title refers to the fact that since the equation of time has the greatest advance (slope) on December 22, this day is the longest in terms of time from solar noon to solar noon the next day. Also, John Shepherd is correct--the ecliptic angle is the predominant cause. "The sunrise/sunset phenomenon and the general shape of the graphs of solar noon and solar day length would be the same even if the earth's orbit were a circle centered at the sun!" If anyone who wants the article can't find it, send me an email with your address and I'll drop a photocopy off in the mail (post). Ron Doerfler [EMAIL PROTECTED]
