It functions as an observatory on a few certain days of year and can plot and trace the path of the Venus etc (depending on time of year). Light actually reflects of shapes/crystal in rocks..in areas thought that sun would not reach thru the 'observatory slot', but it does at certain very specific times...And there's more...As i say look at the book "Uriel's Machine"
http://www.randomhouse.co.uk/features/uriel/main.html