Martin Byrne, a researcher and artist in County Sligo, Ireland, studied for many years the Neolithic tombs at Carrowkeel in the Bricklieve mountains. He thinks that at least one of these tombs, the Cairn G, is positioned so that the light from the Moon could peep into the inner chamber at midwinter.
      The tomb's "lightbox" is only the third ever discovered (the other two being the Newgrange Neolithic complex in Ireland and the recently discovered Crantit tomb in Orkney, Scotland) and is by far the most complex. Given the number of Neolithic tombs in the Carrowkeel area this was one of the most sacred regions of ancient Ireland. Over a dozen mountain-top cairns can be seen looking across the hills of Sligo.
      Carefully set into the entrance of Cairn G is a hole that is positioned to let the Sun's rays into the inner chamber for a month either side of midsummer. But according to Martin Byrne, it would also let in the light of the setting full moon on either side of the winter solstice.
      Capturing the Moon may have been the main purpose of the tomb, because it is pointing at a hill called Knocknarea, which means "Hill of the Moon". Knocknarea is said to be the burial place of the wild Queen Maeve, one of the major figures in the Irish saga, the Tain Bo Cualnge.
      The tomb points to the most northerly point the setting Moon reaches on the horizon, an event that only happens every 18.6 years.
      Positioning the Carrowkeel tomb would have required a sophisticated understanding of the cycles of the Moon, as well as patient and careful observations over many years. Further evidence that the Moon was important is that Knocknarea is on a peninsula called Cuil Irra, "the Remote Angle of the Moon".

Source: BBC News