The megalithic burial site, which dates back more than 5,000 years, is unique, in that it aligns with both the rising and the setting sun on the Winter Solstice.
It has separate tombs, one facing east and one with a westerly aspect. Conditions permitting, the east facing chamber lights up at sunrise and the west facing chamber is illuminated at sunset, c 3.40pm. Newgrange is only illuminated at sunrise.
Picture, by Chris Corlett of the National Monuments Service, Department of Culture, Heritage and the Gaeltacht, was taken at sunset on Tuesday December 19. It shows the alignment as it would occur today, if the conditions allow.
Knockroe, in the foothills of Slievenamon, is one of a series of megalithic tombs in the area.
Dr Muiris O’Suillivan, Professor Emeritus Archeaology, University College Dublin, who led the excavation of the site over a 25 year period, will be among those gathering at Knockroe today for the solstice. He calls the site the “Newgrange of the south".
According Dr O’Sullivan, the Knockroe tomb, which is richly illustrated with artwork, was designed from the beginning to be an impressive monument.
He has described the design as very similar to that of Knowth, not far from Newgrange, in the Boyne Valley, Co Meath.
Earlier this year, An Bord Pleanála shot down plans for eight 126-metre high turbines in the area.
Locals and conservationists campaigning against the plans submitted by a Dutch company had warned that the development would have impacted hugely on the Knockroe Passage Tomb.