If you don't know of it, you might find this very useful -
http://weba.viawest.net/users/fcs/ho/

The thought that wherever the sun rises/sets, so might the moon opens up a big field of speculation I reckon. E.g. the setting sun reappears at Maeshowe and various other sites in the area, so what about the moon? Is Callanish not alone?

http://www.iol.ie/~geniet/maeshowe/eng/flashing.htm
http://www.maeshowe.co.uk/latest/latest1998/11jan1999a.html