Julian Cope presents Head Heritage

The usual idea, you'll have to correct me if I'm wrong, is observation of an annual 'alignment' to winter/summer sol. etc., then construction of the monument on the correct axis? Could this be different; observation of one spectacular event and one monument to mark the same? Or how soon would something similar, with all those factors, be likely to happen? Though if, as you suggested, you're waiting 18.6 years for something you're not going to be able to see anyway...