Madam, - Eileen Battersby (December 22nd) claims that the winter solstice lasts five days. Astronomically speaking, the winter solstice has no duration: it is just the instant of time which separates autumn from winter and this year it occurred at 7.04 a.m. on December 22nd.
This raises the question: how well could the builders of Newgrange have defined the seasons? Clearly, the Newgrange light-box is a crude device for this purpose but the answer lies in the much smaller Cairn T at Loughcrew, Co Meath. Cairn T defines the time of the equinoxes to within a day and our ancestors could have been aware that the equinoxes divide the year into unequal intervals differing by nearly eight days.
Tim O'Brien, in his book Light Years Ago, has shown that they could even have detected the four-year cycle of the Sun. It is only by considering these megalithic monuments in combination that we can appreciate their true calendrical significance. - Yours etc.,
IAN ELLIOTT,
Pinecroft,
Kilternan,
Dublin 18.