WORLD

For centuries, Viking seafarers ruled the North Atlantic, braving open seas to travel thousands of kilometers to their colonies in Iceland and Greenland—all without magnetic compasses. How they performed such a feat has long puzzled scientists. Now, one group of researchers has an answer, based on computer simulations—and legendary crystals. (Science)

What were Viking sunstones?

Teachers, scroll down for a quick list of key resources in our Teachers Toolkit.

Discussion Ideas

• Vikings successfully navigated the treacherous North Atlantic for 300 years, regularly using their spectacular longships to sail between Scandinavia, Iceland, and Greenland. What were their chief navigational tools?
  • waypoints. Landmarks, such as islands in the North Sea or the coast of Iceland, would be the most familiar and obvious way to navigate.
  • sun. The North Atlantic has up to 24 hours of continuous sunlight in the summer, the most likely time for Viking voyages. Viking navigators must have relied heavily on the sun to find their geographic orientation (position in relation to true north) on the open sea.
  • stars. Vikings undoubtedly used celestial navigation to orient themselves when the sun went down. Polaris, the North Star, was was probably the most important star they consulted.
  • sun compass. Sun compasses use the position of the sun’s shadow to help navigators orient themselves. No Viking sun compasses have been positively identified.
  • sunstones. Viking sunstones describe various minerals that, when held to an overcast or twilight sky, allow navigators to determine the position of the sun.
    • Sunstones are described in medieval texts such as the “Saga of St. Olaf”:
      • “The king made people look out and they could nowhere see a clear sky. Then he asked Sigurður to tell where the sun was at that time. He gave a clear assertion. Then the king made them fetch the solar stone and held it up and saw where light radiated from the stone and thus directly verified Sigurður’s prediction.”
    • No Viking sunstones have been positively identified.

• How do sunstones work?
  • sky-polarimetric Viking navigation. (Of course.) “Several types of minerals … can split a beam of sunlight to form two images, with polarized light taking a slightly different path than the main beam. By looking at the sky through such a crystal and then rotating it so the two images are equally bright, it’s possible to spot the rings of polarized light that surround the sun, even under cloudy skies. Identifying the sun’s location would give mariners a sure point of reference during long sea journeys.”
    • Polarized light describes light in which all waves vibrate in a single plane. Polarized light exhibits different properties than unpolarized light. Learn more about polarized light here.

• How did researchers test the efficiency of sunstones?
  • simulation and modeling. Researchers used computers to simulate more than 3,000 voyages between Bergen, Norway, and Hvarf, Greenland. A voyage was determined to be successful if it was completed in less than three weeks and came near enough to the coastline to see Greenlandic mountains. Researchers included the following variables:
    • timing. Voyages were limited to the time between the spring equinox and the summer solstice. This time of year has calmer seas and longer daylight hours, both of which are advantages to long voyages.
    • weather. The voyages encountered differing amounts of cloud cover, testing how well the sunstones worked in both relatively clear and low-visibility conditions.
    • minerals. All three crystals most associated with Viking sunstones (calcite, cordierite, and a variety of tourmaline known as “Icelandic spar”) were tested.
    • frequency of readings. Researchers varied how often their Vikings checked their sunstones, between every six hours and every hour.

• According to the simulations, how accurate were the sunstones in helping navigate the North Atlantic? Which variable made the biggest difference in success?
  • Sunstones were pretty accurate! The frequency of readings made the biggest difference. When checked at least once every three hours, ships made landfall a whopping 92% to 100% of the time. (!)
    • Vikings had diminishing returns the less frequently they checked their orientation—up to about 59% for checking at least once every four hours, and less than 1% for checking every five or six.

• What happened to the real-life Vikings that didn’t make landfall in Greenland? Take another look at that map for an intriguing possibility.
  • Honestly, most of them probably either turned around and went back to Iceland or Europe, or met a watery grave. (Bigger and more unsinkable ships have met similar ends in the North Atlantic.)
  • Look at that map—a lot of those failed Greenlandic journeys end up in Newfoundland and Labrador, Canada. “Some researchers have suggested that Viking explorers who ended up passing south of Greenland discovered America long before Columbus did.”

• The authors of the paper say “To date, this is the most detailed and precise rating of sky-polarimetric navigation that is achievable without testing this method directly on the high seas.”
  • So, are there any explorers out there willing to test their navigational abilities in the North Atlantic, equipped with a beautiful longboat and a nice rock?

TEACHERS TOOLKIT

Science: Viking seafarers may have navigated with legendary crystals

Nat Geo: Viking ‘Sunstone’ May Have Existed

(extra credit!) Royal Society Open Science: Success of sky-polarimetric Viking navigation: revealing the chance Viking sailors could reach Greenland from Norway