OK, the moon is on an 18½ year cycle, and its extreme points are the major and minor standstills. Callanish was built as a lunar observatory and at the major standstill, if you stand at stone 8 (ie down at the bottom of the avenue (near the tearoom, oppostite end to the visitor's centre) the moon will skim along the horizon to your left, disappear (behind the outcrop the visitor's centre is built behind) and then reappear between 2 of the stones in the centre of the monument (you can have someone positioned to be the man in the moon) and disappear again ... as far as I understand it! I'm a beginner as far as astroarchaeology goes - suggest "Stones of Wonder" at http://www.placedirectory.com/stones/stones1.htm ... or Ron and Margaret Curtis!
Glad to hear you had a good time and didn't get stuck in any bogs.
Sals