Knowth, Dowth, and Newgrange are Neolithic Passage tombs located in the Boyne Valley of eastern Ireland. They make up what archaeologist refer to as a necropolis—basically a city of the dead. Together, these three tombs are some of the oldest examples of monumental Neolithic architecture in existence.
Newgrange was begun first, starting around 3200 BC. It consists of a large circular mound with an inner stone passageway and chambers. Human bones and possible grave good or votive offerings have been found in these chambers. The mound has a retaining wall at the front, made mostly of white quartz cobblestones, and it is ringed by engraved kerbstones.
The passage into Newgrange is aligned for the sunrise on the Winter Solstice, which appears to have been one of the most important events in Neolithic religion. The Winter Solstice, for those for whom the terminology is new, is the shortest day of the year, usually celebrated on or around December 21st. If more modern pagan rituals are anything to go by—which is debatable—the significance of celebrating this Solstice is because the days afterwards will get longer and, if the people worshipped some kind of a sun god, the Winter Solstice marks the death and ritual rebirth of the sun.
While people were buried in the tomb, there is debate among scholars as to whether Newgrange and related monuments were primarily burials or if they were also temples, in the same way that in medieval Christian churches important people had tombs within the nave.
Like churches, the Bruinne a Boyne monuments were decorated, in that the large stones surrounding and inside the tombs are decorated with Megalithic Art such as spirals, concentric circles, triangles, zigzags and images that have been interpreted as the sun, the moon, and the human face.
Knowth, which is bigger in size than Newgrange, was started around the same time, and consists of a large main mound and seventeen satellite tombs. It contains two passages leading to graves, and like Newgrange was decorated, containing more than two hundred examples of megalithic art. Unlike Newgrange, archaeologists believe the passages were oriented towards the equinoxes rather than the solstice.
Dowth, the third passage tomb at Bruinne a Boyne, appears to have been built later than the others. It has two passages like Knowth but shares an astronomical alignment with Newgrange. In particular, from November to February the rays of the evening sun reach into the passage and then the chamber of Dowth South. During the winter solstice itself, the light of the low sun moves along the left side of the passage, then into the circular chamber, where three stones are lit up by the sun. The convex central stone reflects the sunlight in to a dark recess, lighting up the decorated stones there. The rays then recede slowly along the right side of the passage and, after about two hours, the sun withdraws.