Chris, Mat, Jim
Many thanks - each of your responses added that extra bit to my knowledge. I think I have all I need for the next step of my journey. I am trying to build a virtual sundial correct for any Latitude and Longitude - which may seem a bit of a strange task. But Stetchup has rather a good model of the Earth's movement around the Sun. It correctly distinguishes between solar time - as seen on a sundial - from civil time - as read from your watch. So, with its ability to cast shadows correctly, a sundial simulator should not be too difficult.
I'll keep you posted !
