On the eve of the 2020 winter solstice, creative production studio Algorithm.ie brought new life to a stone carving believed to be the world's oldest sundial. As part of Ireland's "Shine Your Light" program, organized by Creative Ireland and Raidió Teilifís Éireann, the company used projections and lighting design to illuminate the ancient Kerbstone at the Knowth Stone Age Passage Tomb at the historic Brú na Bóinne neolithic monument.
A bit more about the stones, from Mythical Ireland:
Kerb stone 15: One of the most striking of all the kerb stones at Knowth, and one of the best known, this has been called the Sundial Stone. Martin Brennan in his 1980 book The Boyne Valley Vision (Dolmen) suggested that the large fanned symbol at the centre of the stone was a sundial, and that, in fact, the people who built the great monuments of Brú na Bóinne were the 'Master Diallers of the New Stone Age'. Australian engineer and author Neil L. Thomas, in his 1988 book Irish Symbols of 3500BC (Mercier Press) suggested that K15 represents a complex solar calendar, an "exact 365-day, sixteen month, four week month, five day week solar calendar". The proposition that the stone acted as a sundial appears incongruous mainly due to the position of the stone – the sundial is on a vertical face, and the plane of the surface of the stone faces towards the east. It cannot function as a practical sundial in its current position, and there is no evidence to suggest it was ever positioned with its surface horizontal. However, it does look like a sundial. In my book about Dronehenge, I made a suggestion that the main fan-like emblem on K15 looks a little bit like half a henge. Perhaps this was a guide as to how the henges should be laid out? It's unlikely, I know, and the theory is extremely tenuous to say the least, but you sometimes have to "say what you see" with megalithic art and that's as close to a convincing interpretation as you'll be likely to get.