Sun Position calculations

I am currently implementing a sun/shadow study interface. This will allow a user to define the 'location' of the current drawing, in terms of latitude and longitude. Using calculations from http://www.srrb.noaa.gov/highlights/sunrise/azel.html the position of the sun from any location on Earth, will be possible. We can then calculate the position of a 'shadowlight' and display that in real-time using the shadowing engine in the system. This can also be video captured to produce a shadow study.