  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.
