Two years ago, at dawn on a cold December morning, Kate and I piled rocks and stones into two cairns under an old maple tree in the yard. The tree stands at the crest of the hill that our property straddles. We were marking two points on a line to the southeast — a line that when extrapolated to the horizon hit the deep red bull’s-eye of the rising sun. It was the beginning of the shortest day of the year, the winter solstice. The sunrise was at the southern terminus of its annual journey up and down the eastern horizon.

People have been engaging in similar activities for thousands of years. Long before there was a month of December, more than 5,000 years ago, a remarkable tomb was constructed in a kidney-shaped mound atop a hill in what is today Newgrange, Ireland, about 30 miles northwest of Dublin. The cruciform chamber at the heart of the mound lies at the end of a narrow 62-foot passage. On the winter solstice, at four and a half minutes after sunrise, the rays of the sun slip through a slit above the door and pierce the passageway, illuminating carved designs in the chamber.