Stone Age people in Ireland appear to have built tombs based on a detailed knowledge of how the Sun moves across the sky during the year.
By Helen Briggs
BBC News Online science reporter
Tombs at the archaeological site of Loughcrew in County Meath align with the rising Sun at the spring and autumn equinoxes.
Sunlight strikes the inside of the tomb at the autumn equinox (Image: knowth.com)
The inside of the chambers are spectacularly illuminated by a shaft of sunlight at dawn on these days, said Frank Prendergast of the Dublin Institute of Technology.
It suggests settlers in the area some 5 to 6,000 years ago knew the yearly cycle of the Sun and perhaps centred their lives around it.
Tombs found elsewhere in Ireland have been found to point towards the rising Sun at the summer and winter solstices.
At these times, the Sun reaches its most northerly and southerly points in the sky, which can be easily observed from any place on Earth.
The equinoxes - in late March and late September - are not so obvious and can only be pinpointed by tracking the passage of the Sun across the entire year.
Why tomb builders wished to do this remains a mystery but it suggests the Sun was at the heart of ritual and ceremonial practices of ancient people.
THE SUN'S PATH
Spring/autumn equinox - day and night are each 12 hours long and the Sun is at the midpoint of the sky
Summer solstice - the longest day of the year, when the Sun is at its most northern point in the sky
Winter solstice - the shortest day of the year, when the Sun is at its most southern point
"Archaeology now has a substantial body of evidence which would indicate a very sophisticated and advanced agrarian society," Frank Prendergast told BBC News Online.
"They would have attached a sense of sacredness to their landscape and the sky and they would have done that by building the monuments the way they did; decorating them with a kind of rock art; and associating some of these monuments with key astronomical events such as a significant rising and setting points of the Moon and Sun."
Window to the past
The findings are to be presented at the UK/Ireland National Astronomy Meeting in Dublin.
Details will also be revealed of how Bronze Age stone circles in Ulster relate to both the Sun and the Moon.
Archaeologists believe there could have been separate lunar and solar traditions, possibly at different times in history.
One of the two large cairns at Loughcrew (Image: knowth.com)
But Professor Clive Ruggles, of the University of Leicester, said great care was needed in interpreting them.
"Just because a monument is aligned in a direction that we would be tempted to interpret as astronomically significant, such as the direction of sunrise or sunset on one of the solstices, this might not have been intentional," he said.
He believes the study of astronomical alignments gives an insight into how people comprehended the world in the past.
"The builders were not 'astronomers' in the sense that we would mean it today, but celestial objects and cycles were important to them in keeping their own lives in harmony with their world," he explained.