This was my second visit in two days, there was much to see, and the line was short — there were maybe 10 of us waiting. My turn came, and I climbed the outer stairs, then went down into the entryway. The stone in front of the entrance is a large oval that has been carefully incised with carved connecting spirals.

Perhaps more unique is the “roofbox” — something like an open window set onto the heavy lintel stone that defines the doorway. The two people who went before me came out, and I entered the passage. Like many ancient passages, it wasn’t built for tall women like me. Heavy stone slabs lean inward; it is a tight corridor; I felt as if I was walking slightly down hill. There was a little light, and I avoided bumping my head, but I could hardly see the interior engravings. At the bottom of the path, it was difficult to gauge how far I’d come.

There was nothing grand about the passage until I got to the bottom, and turned around. The guide told us what to look at. And then, the demonstration — how, for 16 minutes on the morning of the winter solstice, and only then, the sun shines through the roofbox, down the passage right to the bottom, right to what might be some sort of altar with a basin sitting on it. Someone outside shone an electric light through the roofbox (which was discovered in 1963), demonstrating the effect. I imagined the miracle of such a thing, total darkness pierced so precisely by sunlight. The tools used to build the monument were made entirely of stone, bone and wood.

Later, I drove to the mouth of the River Boyne, on the Irish Sea, 31 miles north of the River Liffey, which flows through Dublin. The landscape is flat, and, this day, a little damp and gloomy. The shingle beach, edged by wild green plants of all types, stretches toward a broad estuary. Even to me, the river seemed to promise an easy and productive trip inland. It is evident why countless men in ships accepted the invitation — in 2013 a boat made of logs, possibly 5,500 years old with “a pair of oval shaped blisters on the upper edge” indicating oars, according to reports, was found near Drogheda, not far from where a dredging operation in 2006 revealed a Viking ship.