An ancient Irish tomb may have been built with a light chamber aligned not only to the Sun, but to the Moon as well.

Building it would have required many years of observations of the motions of the Moon by the tomb's architects. The tomb could also explain the Moon-inspired names of local landmarks.

..and strikes the far walls

The tomb's "lightbox" is only the third ever discovered and is by far the most complex. It reveals the astonishingly-detailed astronomical knowledge of the ancient people.

BBC News Online recently reported that a team of archaeologists from Glasgow University had discovered a lightbox in the roof of a prehistoric tomb in Orkney, Scotland.

It allowed the rays of the sun to reach the innermost part of the tomb at the start and end of the winter. At that time, only one other lightbox was known, at the Newgrange Neolithic complex in Ireland.

Martin Byrne believes capturing the Moonlight could have been the main aim

The latest, and most remarkable yet, was revealed by Martin Byrne, a researcher and artist in County Sligo, Ireland. His work on the Neolithic tombs at Carrowkeel suggests they were positioned so that the light from the Moon could peep into the inner chamber at midwinter.

Carrowkeel is in the Bricklieve mountains. Given the number of Neolithic tombs in the area this was one of the most sacred regions of ancient Ireland. Over a dozen mountain-top cairns can be seen looking across the misty 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.

The entrance to the lightbox

Indeed, capturing the Moon may have been the main purpose of the tomb - 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.

Sunset over the "Hill of the Moon"

Further evidence that the Moon was important is that Knocknarea is on a peninsula called Cuil Irra, "the Remote Angle of the Moon".

This part of Ireland also hosts the largest prehistoric stone village in the country and many ring forts, earthworks and standing stones.