A stone you hold up against an overcast sky that tells you where the sun is: it is one of the finest stories told about seafaring in the North. It appears in physics papers, in museums and in every second documentary. On one point it is remarkably thin. No grave, no shipwreck and no settlement of the Viking Age has ever yielded a sunstone. What we have is one word, two crystal finds from later periods, and a long series of experiments.
In Old Norse the thing is called sólarsteinn, sunstone. The word is attested – but the texts it appears in are younger than the Viking Age, and they are not about navigation.
The only passage that describes anyone actually using one is in Rauðúlfs þáttr, an edifying tale of the late twelfth or thirteenth century, preserved among other places in Flateyjarbók. King Ólafr Haraldsson is a guest of a man named Rauðúlfr. Under an overcast, snowy sky the king asks his host's son Sigurðr where the sun stands. Sigurðr answers. The king then has the sunstone fetched, holds it up, and sees where the beam from the stone points – exactly where Sigurðr had said.
The scene takes place on land, not on board. It belongs to a story chiefly concerned with visions, number symbolism and royal foreknowledge, and it serves to confirm the young man's gift. Reading it as a report on seamanship is a twentieth-century move.
Besides that, Icelandic church and monastic inventories of the fourteenth and fifteenth centuries list a sólarsteinn among their holdings. What the object was, and what it was for, is never stated. A crystal? A burning glass? An ornament? The lists say nothing. Hrafns saga Sveinbjarnarsonar, from the thirteenth century, names the sunstone too, without letting any use be read off it.
One misunderstanding to clear up at once: the modern gemstone sold as "sunstone", a shimmering feldspar, has nothing to do with any of this.
In 1967 the Danish archaeologist Thorkild Ramskou put forward the proposal that has dominated the field ever since: the sólarsteinn was a birefringent crystal, calcite or cordierite, with which one can determine the direction of polarisation of skylight.
The physics is not in dispute. Sunlight scattered by the atmosphere is polarised, in rings around the sun. That pattern persists even when the sun itself is behind cloud or just below the horizon – provided some patch of blue or at least bright sky is visible. A calcite crystal splits a beam into two images whose brightness shifts as the crystal is turned. Turn it until both images are equally bright, and the crystal stands in a fixed relation to the direction of polarisation – and so to the sun.
The finest calcite in the world comes, of all places, from Iceland: silfurberg from the Helgustaðir quarry near Eskifjörður in the east of the island. It was on this material that Rasmus Bartholin first described double refraction in 1669. The snag is that quarrying there begins in the seventeenth century. That Viking Age Icelanders knew and used the deposit cannot be shown.
In 2002 a cloudy, heavily degraded lump of calcite about the size of a bar of soap was recovered from a wreck off Alderney in the Channel. The ship was an Elizabethan vessel that sank in 1592. The crystal lay close to navigational equipment, including a pair of dividers.
In 2013 a group around Albert Le Floch and Guy Ropars published a study of it in the Proceedings of the Royal Society A. Their proposal: the crystal may have served as a reference optical compass – a backup for the magnetic compass, which the iron guns on board would have disturbed.
The find is real, its context is interesting, and the conclusion is an offer, not a fact. Above all: 1592 is roughly five hundred years after the Viking Age. As evidence for tenth-century navigation the Alderney stone is worthless. It shows only that somebody in the early modern period had calcite on board.
Since the 2000s the question has mostly been worked on by physicists, and worked on thoroughly.
Gábor Horváth and colleagues spent years measuring the polarisation patterns of the sky, including under fog and cloud; the foundational paper by Hegedűs, Åkesson, Wehner and Horváth appeared in 2007. The result: under thin cloud and fog the pattern often remains usable, under a thick overcast it does not.
Guy Ropars and his team published laboratory work in 2011 and 2012 in which calcite yielded the direction of the sun to within a few degrees. In 2018 Dénes Száz and Gábor Horváth ran a thousand simulated voyages from Norway to Greenland in Royal Society Open Science. Their finding: a navigator who checked the sky at least every three hours nearly always arrived in the simulation. One who checked less often frequently did not.
All of this shows that the method can work. It does not show that it was used. Nor is it as simple as documentaries make it look. The same Budapest group examined in 2016, in Royal Society Open Science, how accurately people can align the crystal at all – that first step alone produces sizeable errors depending on material and practice, and those errors carry into every step that follows.
The second building block of the story is a piece of wood. In 1948 the Danish archaeologist Christen Vebæk, digging at the Norse site Ø149 on the Uunartoq fjord in southern Greenland – the grounds of a Benedictine nunnery – found a round piece of wood about seven centimetres across and one centimetre thick, with a hole of not quite two centimetres in the middle. Roughly half of it survives. Notches are cut around the rim; scratched lines run across the face.
The Danish sea captain and naval historian Carl V. Sølver declared it the fragment of a bearing dial: a sun compass in which a pin at the centre casts a shadow whose tip traces a curve across the day. When the shadow meets the scratched curve, north is fixed. Søren Thirslund developed the reading further, museums adopted it, and many books now print it as fact.
It is not. Christian Keller and Arne Emil Christensen have pointed out that the object is ambiguous, and offered another reading: a skriftaskífa, a confession disc used by clergy to record who had come to confession. That is not secure either – the known comparative pieces are later. But it is a reminder that the disc was lying in a convent, not in a ship.
In 2014 the group around Balázs Bernáth and Gábor Horváth proposed that the disc served as a "twilight compass", used together with a calcite crystal, and could have given north for nearly an hour after sunset. That is a construction built on a reading that is itself unproven. A similar piece of wood from Wolin in Poland is also discussed – and also disputed.
It is worth turning the question round: what do you need to get from Norway to Iceland without a crystal? Less than you would think. The full picture is in our article How the Vikings navigated; here only the points against which the sunstone has to be measured.
Latitude sailing. You run up or down the coast until the sun at midday reaches the familiar height above the horizon, then hold east or west and keep that height. Directions of this kind survive in the Hauksbók version of Landnámabók: a route from Hernar in Norway to Hvarf at the southern tip of Greenland, passing north of Shetland and south of the Faroes, with notes on how the islands should stand on the horizon as you pass. That is navigation by landmark and solar altitude, with no crystal anywhere in sight.
Landmarks and signs. Cloud banks over glaciers are visible from far out. Iceland announces itself that way long before any land appears. Add the colour of the water, driftwood, the run of the swell, whales.
Birds. Landnámabók tells of Hrafna-Flóki, who took three ravens with him. The first flew back to the Faroes, the second returned to the ship, the third flew on ahead – and Iceland lay that way. Whether it ever happened we do not know; Landnámabók is centuries younger than the event. The technique itself is old and known from several cultures.
And failure. Old Norse has its own word for losing your bearings at sea: hafvilla. It turns up repeatedly in the sagas. Ships made landfall where they had not intended, or never made landfall at all. That soberness belongs to the picture – people with a reliable miracle crystal would not have needed the word. How risky the North Atlantic routes were is clear from the voyages to Iceland, Greenland and Vinland, and how thin the western link became in the end from the story of the Greenlanders.
No Viking Age sunstone has ever been found. No crystal in a boat burial, none in a wreck, none in a shipyard or harbour settlement, none in any of the great hoards. Nothing.
That is not proof that none existed – calcite is soft, soluble in water and poorly preserved in the ground, and an unremarkable lump of stone is not necessarily recognised as an instrument on a dig. But it is where we stand. And as long as that is so, the sunstone belongs in the category "possible, not proven" – along with a good deal else that gets said about the ships of the North.
Linguistic evidence is the word sólarsteinn in Icelandic texts and inventories of the thirteenth to fifteenth centuries. What it denoted is never stated.
Later tradition is the single scene of use, Rauðúlfs þáttr – on land, in an edifying tale, written down at least two hundred years after the Viking Age.
Archaeological evidence is one calcite crystal from a wreck of 1592 and one wooden fragment from a Greenlandic convent whose function is disputed. Both are post-Viking or ambiguous. A Viking Age sunstone is not among them.
Modern interpretation covers Ramskou's calcite hypothesis of 1967, the polarisation experiments since 2007, the simulations of 2018 and the reading of the Uunartoq disc as a twilight compass. All of it is serious work. None of it is a find.
We prefer the documented story to the prettier one. People who crossed the North Atlantic without a compass, by solar altitude, swell, cloud and birds, do not need a miracle object to be impressive. For courage. For memory. For legends that live on.
Viking navigation · The longships · The Vikings in the North Atlantic – Iceland, Greenland & Vinland · The Greenlanders · The Icelanders · The Faroese.
