The Uunartoq disc

Why this is extraordinary

Under the ruins of a Norse convent on the Uunartoq fjord, in the south of Greenland, the 1948 dig found half of a small oak disc in layers older than the building. It is about 7 centimetres across. Triangular notches march around the curved rim, so the broken edge looks a little like a cog that was never finished. Lines, some straight and some curved, are cut on the flat face. Archaeologists placed the piece around the year 1200. The fjord is near 60 to 61 degrees north, in the Eastern Settlement. A place with the same name far up the west coast is not where this wood was found.

The reading that made the disc famous treats it as a sun compass. A short pin, a gnomon, would stand at the centre. Through the day the tip of its shadow follows a curve. That curve is different on the equinox, when the sun rises due east and the day is split in half, and on a day near the summer solstice, when at this latitude the sun climbs high and the noon shadow is short. If those two curves are the ones cut in the oak, you could turn the disc until the shadow tip sat on the right curve, and the notches would then tell you a direction relative to the sun. A 2013 study goes further and reads some of the straight scratches as the construction lines a maker would draw in order to place the curves, and it sees the disc as a noon board as well as a compass, a way to know when it is local noon and how long the shadow ought to be. That is a sky instrument, if the lines are those lines. You observe it by watching a shadow, not by sighting a star at night.

The catch is the size of the claim and the size of the wood. A 2020 comparison of this disc with two others, from the Baltic, says the sun-compass idea has been repeated without a footing in the historical sources, and that the three objects do not share a function. None of them, in that argument, was for navigation. Other suggestions are a toy, or a confession token, a notched reminder of religious duties. The tokens that get cited are 17th- and 18th-century and Lutheran, and medieval Greenland was Catholic, so that parallel was offered tentatively. A crystal called a sunstone, which some experiments show could find the sun through cloud by the way the light is polarised, is a different hypothesis. It is not this object. No crystal was in the layer with the oak. Two later tests are not tests of this oak. A 2014 paper calls the fragment far from optimal as a sun compass, and it reports an error of about plus or minus 4 degrees only when an artificially hidden sun stood between 10 degrees above the horizon and 8 degrees below it. A 2018 simulation of a thousand passages reaches about 92 to 100 percent success if north is reset at least every three hours, and falls under 4 percent at the summer solstice if the gap is five hours. Drift and current are left out of that model. The ships stop at night. Neither paper puts a crystal in the 1948 layer.

The picture at the top is a drawing. It makes the notches and the lines easy to see, and it can also make a reading look more certain than a worn groove is. The second picture is a modern replica, a full disc on a handle with a pin. It is useful for imagining the compass in the hand. It is not the find, which is half, handle-less, and pin-less. What this changes: if the curves really are the equinox and the near-solstice shadows at 61 degrees north, the Eastern Settlement had a wooden dial for the sun. If they are scratches that happen to be close, the ocean crossing goes back to stars, swell, and birds, and this oak is still unexplained. The last written page of the settlement, the wedding at Hvalsey, is a later century and a different argument.