Yesterday, December 21st, you may have thought to be the winter solstice. But not this year. Although the solstice, almost by definition, comes around at exactly yearly intervals in an astronomical sense, our odd habit of inserting a leap day into the annual cycle now and then means it oscillates backwards and forwards on our calendar. Sometimes the solstice falls on December 21st; in other years, like this, if falls on the 22nd.
In ancient times the winter solstice was of very great significance. Because of the tilt of the Earth's axis relative to its orbital plane, the sun rises at different points on the horizon depending on the time of year.
At the spring and autumn equinoxes it rises in the east; at the summer solstice in late June it rises in the north-east quarter; and near the winter solstice at this time of year, the sun first peeps above the horizon at its most southerly point, very close to due south-east.
This reversal of the southerly trend of the rising sun was a sign of yet another waning winter, and a promise of the eventual return of longer, warmer days and fertility and growth.
It may be that the search for reassurance on this annual renaissance provides the answer to the megalithic passaged mound at Newgrange, Co Meath. Traditionally it was thought of as a burial mound, or something perhaps connected with religion, but in recent times archaeologists have focused on the view that the cairn may well be a precision instrument to monitor the sun's annual progress.
At the core of Newgrange lies a passage facing almost due southeast. At the entrance to this passage is what might be called a little "window" in the roof: for a few days around the winter solstice, the rising sun shines directly through the "roofbox", and at the time Newgrange was built, a well-defined illuminating beam of sunshine would have been projected on to the rear wall of a chamber deep within.
Gradual changes in Earth orbit in the intervening years have put the image somewhat out of focus, but at the winter solstice the streak of sunshine can still be seen at sunrise, illuminating the pitch-black floor of the structure's central chamber.
It is suggested that this arrangement allowed the people of the time to observe the annual turning of the sun, and to anticipate spring's imminent rebirth, the lengthening days, and the reawakening of the growing season.