Patti M. Walsh

Sprawled along the River Boyne, in County Meath, Ireland, is a complex of the largest passage tombs (burial mounds) in Western Europe.

Comprising an eight-square mile UNESCO World Heritage site, and carbon-dated to circa 3200 BC, Newgrange, Knowth, and Dowth are hundreds of years older than Stonehenge and the Giza pyramids.

Some cultural anthropologists suggest that visiting a passage tomb allows one to make a symbolic journey into the Land of the Dead, and to return, reborn, to the Land of the Living.

High on my life’s bucket list was that journey.

Visiting a passage tomb allows one to make a symbolic journey into the Land of the Dead … High on my life’s bucket list was that journey.

Newgrange, arguably the most famous of the area’s mounds, has been dubbed the world’s oldest solar observatory. Every December during the winter solstice, the Sun illuminates its passage and chamber, based on calculations that hold true today. It is the only passage tomb in the Brú na Bóinne that visitors can enter..

Mildly disappointed that my tour began with Knowth and not Newgrange, I nonetheless joined 20 other seekers and climbed aboard a bus on September 1, 2023. Excitement built we rolled through a bucolic landscape, dotted with cattle and sheep under vast expanses of cumulus-fluffed blue sky. I imagined the lush panorama looked much as it did 5,000 years ago.

Passage tombs, as the name implies, consist of a passage leading to a chamber where the remains of the dead were placed. The passage and chamber were covered by stones and earth, retained at the base by large kerbstones.

Surrounded by at least 18 smaller satellite monuments, the central mound at Knowth is nearly a football-field in diameter (93 by 104 yards) and covers nearly two acres. The satellites, which are as large as 22 yards in diameter, predate the colossal central cairn.

“Knowth is not only the largest of the area’s passage tombs,” the docent told us, “But it also has the highest concentration of megalithic art in all of Europe.”

Originally encircled by 127 kerbstones, 124 are in still place. According to the Fr. Michael O’Flanagan History & Heritage Centre, Knowth contains 50% of the total engraved stones in Ireland and more than a third of the total examples of megalithic art in all of Western Europe.

“Megalithic art,” the docent explained, “was created by pecking the stone using stone, flint, bone, and ivory.”

This was the Stone Age.  Metal had not yet been discovered.

Most of the motifs are spirals, lozenges, and serpentiforms. Knowth, however, also features the oldest known illustrations of the moon and markers for the equinoxes.

My disappointment was soon redefined as awe.

Megalithic Astronomers

In Irish mythology, Knowth is Cnoc Bui, home to the goddess Buí and her consort, Lugh of the Long Arm. The pair of standing stones positioned outside Knowth’s west portal may represent them.

To archaeo-astronomers, however, the stones suggest that the neolithic people revered the equinox.

The tall stone is known as a gnomon; lying beyond it is a kerbstone that bears one vertical line. Not only does the shadow of the gnomon align with that carved vertical line on the vernal and autumnal equinoxes, but sunlight penetrates the passage at equinox and illuminates the orthostats at the rear of the tunnel.

Kerbstone 52 is commonly known as the Calendar Stone. It may not look it to the untrained eye, but this stone shows that the artisans were aware that the solar year, which is 365 days long, does not contain an equal number of synodic periods of the moon (full moon to full moon or new moon to new moon).

The neolithic astronomers studied the movements of the moon over long periods of time. They depicted the 29 days of the synodic lunar month with 22 crescent shapes and 7 circles or double circles. The crescent shapes represent the early and late phases of the moon, and the circles, that week in the middle of the month when the moon is full or almost full.

“The stone men knew calculus while today’s high school students don’t know arithmetic.”

Although they knew the phenomenon by a different name, they had etched in stone the Metonic Cycle, which was “discovered” by the Athenian astronomer Meton thousands of years later, in the fifth century BC.

The Metonic Cycle is a period of 19 years, in which there are 235 lunations, or synodic months, after which the Moon’s phases recur on the same days of the solar year, or year of the seasons.

A synodic period is exactly 29.531 days long. Therefore, 12 synodic lunar months is exactly 354.372 days long. But this is 11 days shorter than a solar year. The Neolithic mound-builders knew this, and used the Calendar Stone to record their calculations of the numbers of synodic lunar months in solar years.

Call it what you will, but after learning this, a friend quipped, “So, the stone men knew calculus while today’s high school students don’t know arithmetic.”