Knowth Calendar Stone - oblique view

This is kerb stone 52 at Knowth, the famous 'Calendar Stone'. Author Martin Brennan was the first to propose that this stone demonstrates that the builders of Knowth were competent astronomers and were observing and calculating the 19-year cycle of the moon known as the Metonic Cycle - almost three millennia before it was said to have been first described by a fifth century Greek astronomer named Meton. The crescent and full moon symbols are said to represent the various phases of a 29-day synodic lunar month.