Readers who saw last Tuesday's Irish Times report on plans for tomorrow morning's live RTE broadcast from Newgrange may have been confused with suggestions that this event marked the winter solstice.
Duchas, the Heritage Service, and RTE joined forces to enable television viewers to watch this dramatic event live as it happens - weather permitting, of course. Unfortunately the weather remains "finely balanced", according to a Met Eireann forecaster, who put the odds for a clear sky at about "60 to 40" in favour. A weather front is due to cross the country tonight, followed by clearer conditions spreading from the west. There was a "reasonable chance" that the clear weather would reach Newgrange in time, he said, but this was not certain.
The Newgrange solstice this year was also billed as a millennium event, given that it takes place as the solar year ends at the apparent end of this millennium.
Purists know, however, that the millennium does not turn until December 31st, 2000, the end of the thousandth year since the last millennium ended, so the Newgrange event is a year ahead of itself.
Astronomers have also pointed out that the television broadcast will be screened 22 hours and 46 minutes before the actual astronomical event that Newgrange celebrates, the moment of the winter solstice.
The winter solstice marks the moment when the sun lies directly overhead the imaginary line called the Tropic of Capricorn, the southern-most limit of the sun's reach, on December 22nd at 7.44 a.m. Irish time this year.
From this time on the sun in the sky tracks gradually northward, passing over the Equator on the day we note the Vernal Equinox. Eventually it reaches its northernmost reach, on midsummer's day and the 2000 summer solstice occurs on June 21st at 1.48 a.m.
Of course we are the ones doing the moving as we orbit the sun. The apparent change in the sun's position is dictated by the planet's "tilt" relative to the plane of the solar system. The builders of Newgrange may not have know why the sun changed position in the sky but they knew how to find when the sun reached its lowest point on the horizon at the solstice.
"Literally it [solstice] means the sun standing still," explained Dr Ian Elliott, an astronomer at Dunsink Observatory which is part of the Dublin Institute for Advanced Studies (DIAS). People 5,000 years ago may have feared that the sun would just keep on going and "wouldn't come back again", he said. Newgrange provided a useful way to demonstrate that the sun had hit its low and was once again on the way back.
The real magic of Newgrange is associated with its fine alignment with the winter solstice sun. Dawn sunlight passing through a small opening above the passage grave's entrance reaches back into the 19 metre long underground tunnel, almost to a large chamber at the rear of the grave.
The achievement of the builders of Newgrange was for a time obscured by the fact that the light didn't quite make it all the way to the back. But Prof Tom Ray of DIAS was able to show that when Newgrange was built 5,000 years ago, the light did reach all the way to the rear of the tunnel.
He showed that the variation in the cast of the light was caused by the known changes to the earth's tilt or obliquity. The tilt is 23.5 degrees off "north" today but was 24 degrees in 3,200 BC.
In fact the earth's tilt follows a long-term variation with a 41,000 year period, explained Dr Elliott. Obliquity ranges from 21.5 degrees to 24.5 degrees. He also pointed out that the presence of the moon helps to make life liveable on the earth.
The tilt is what gives us the seasons from summer to winter. "The presence of the moon prevents the tilt of the earth's axis from varying wildly," he said. Without its moderating influence this axis "wobble" would be erratic and unpredictable.
And what of the decision at Newgrange to celebrate the solstice almost a full day ahead of time? The manager of visitor services there, Ms Clare Tuffy, has a simple answer. "Some years the winter solstice happens on the 21st and other times on the 22nd. We tend to celebrate it on the 21st by tradition," she said.
"There is very little difference with regard to the sunlight seen inside the chamber." In fact, the "solstice" sunlight was observed on each of the days from December 19th through 23 as it tracked further into the chamber and then receded, she added.
There was also an apparent contradiction, again easily explained, Ms Tuffy said, between the time of sunrise on the 21st, which she put at 8.54 a.m. and "the first pencil of sunlight which enters the chamber at 8.58 a.m.".
There is a low hill on the horizon outside the chamber which delays the sun's arrival for those few minutes, as it did 5,000 years ago when our Neolithic ancestors first experienced the marvel that is Newgrange.
RTE broadcasts live from Newgrange tomorrow for an hour from 8.30 a.m. The coverage as the sunlight enters the chamber will also be broadcast over the internet via its website: http:// www.rte.ie
