New discoveries at prehistoric passage graves and stone monuments in counties Meath and Tyrone show that our early ancestors really knew their astronomy.
Two large tombs in Meath are aligned with spring and autumn equinoxes and the northern monument-builders were particularly interested in the moon.
Details of the archaeoastronomy research on both sides of the Border will be discussed this afternoon at the UK National Astronomy Meeting under way at Dublin Castle.
The international meeting brings together leading astronomers and this is the first to be held outside the UK.
Dr Frank Prendergast of the Dublin Institute of Technology will talk about his discoveries at Loughcrew, Co Meath. The site, about 70 kilometres from Dublin, includes a number of passage graves and stone monuments that date from the Middle Neolithic Age, about 3,600-3,100 BC.
The Newgrange tombs are much better known but the Loughcrew monuments are unusual because of their alignments, Dr Prendergast has found. While Newgrange accurately marks the dawn of the winter solstice, the two largest Loughcrew tombs pick out the vernal and autumnal equinoxes, he will tell delegates.
On these days in March and September at dawn and for a period of some 20 minutes afterwards, the interiors of the tombs are illuminated by a shaft of sunlight, exposing the elaborate engravings on some of the stones inside.
Equinoctial orientations are uncommon and their interpretation is controversial, he says. Their alignments are also a challenge to pinpoint, as an observer must track the total annual range of the sun's rising direction and then divide it in half to provide the tomb's alignment.
Loughcrew is also unusual, he will tell delegates this afternoon, because of the pattern of orientation between many of the smaller satellite tombs. Some point towards each other and towards the two large focal tombs.
In contrast, the monument builders at Beaghmore, Co Tyrone, and similar sites in Fermanagh, Derry and Donegal seemed to have had a fascination with the moon, according to Prof Clive Ruggles of the University of Leicester. This cluster of Bronze Age monuments consists of interrelated stone circles, rows and cairns with apparent lunar alignments.