Midwinter calculations

EVERY winter on December 21 a moment of magic occurs at Newgrange in Co. Meath, as it has done for the last 5,000 years.

At dawn, a pencil of sunlight on the shortest day of the year penetrates the main Neolithic burial chamber in Newgrange, aka Brú na Bóinne, and the place is flooded with winter light.

The huge megalithic structure. built around 3,300 BC is perfectly aligned so that the first rays of the solstice sun pierce through a small opening in the chamber's roof. The light then travels some nineteen yards along a narrow stone passage to focus on the entrance to the grave. For seventeen minutes the chamber is illuminated, and then it returns to darkness for the next year as the light departs down the passage.

Brú na Bóinne, or ‘the Palace on the Boyne’, the earliest building in the world with an astronomically based alignment, was clearly built by people with an understanding of the heavens and the seasons. They may have known that it is the axial tilt of the earth which causes the turn of the seasons. the tilt of the Earth’s axis is least aligned with the sun, providing us with the least daylight of the year. After December 21, the nights will begin to get shorter as our planet tilts back towards the sun.

That the world isn't flat, and that a phenomenon such as the solstice is caused by the earth's rotation in relation to the sun, only became widely known in the last few centuries. Thus the migration of birds remained a mystery until relatively recently – indeed, the birds knew the earth was round long before we did. It was assumed they hibernated underwater during the winter months.

When it was eventually discovered that the world is round and spins on its axis, the blameless birds once again got it in the neck. It was observed that birds take off at sunrise, shortly after the dawn chorus. On the opposite side of the world, they are landing at sunset. This was obviously what caused the earth to spin. An imaginative idea, but — for those of you absent from school that day — totally erroneous.

Whether the birds know about the axial tilt of the earth, we can only speculate — but when you look at some of their navigational feats, you can’t help wondering.

What we can say with utmost certainty is that this year the earth’s axial tilt is furthest away from the sun at 16.28 on December 21 (some years it’s on December 22). This split second is technically speaking the Winter Solstice, although colloquially the term is used to mean Midwinter’s Day.

Blink and you’ll miss it.