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Calendrical Interpretation of Spirals in Irish Megalithic Art

The tumuli of Newgrange and Knowth in Ireland are among the most monumental heritages of the Neolithic era. The megalithic constructions date back to around 3'200 BC, centuries before the completion of Stonehenge and the Egyptian pyramids. Passageways inside the mounds have been shown to be aligned such as the rising sun illuminates the interior chambers on the winter solstice at Newgrange and around the equinoxes at Knowth. Many of the kerb and interior stones are covered with petroglyphs, in particular with spiral motifs. Despite several attempts to classify and interpret them, they remain enigmatic. Here we show that some of the most elaborated spirals are likely calendrical representations. We use a new, 'dynamic' approach adding a temporal dimension to the rock art. We identify in the detailed spiral motifs up to five different manifestations of the solar and lunar cycles, which could easily be observed in prehistoric Ireland. Corroborating evidence comes from adjacent motifs or a special location of the stone. Although the work is speculative by nature, many clues give confidence in the interpretation. If correct, it would imply that one or a few individuals developed skills for celestial observations, the design of complex motifs and their preservation on the rock. This resembles a scientific process, but is more likely the expression of a ritual engagement with the sky.