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In an old Norse saga, a king holds a clear crystal up to a gray, snowing sky and works out where the sun is hiding. Scientists have spent more than ten years testing whether a crystal could really do that, and the answer so far is: it could.
The Deep Dive: The Crystal That Sees Double
Around the year 1000, Viking crews were sailing from Norway toward Iceland and Greenland, days at a time with no land in sight. Imagine crossing an ocean with no GPS and no compass: the first European writing about the magnetic compass didn’t appear until about 1190. On clear days they steered by the sun and stars. But the North Atlantic loves fog, snow, and clouds, and when those rolled in, the sun vanished.
Enter the sunstone. A saga written down in the 1200s describes a king on a snowy day with no blue sky anywhere. To check a guess about where the sun is hiding, the king holds up a sunstone and looks at the sky. The saga never says what the stone was. The best guess is Iceland spar, a clear calcite crystal found in Iceland and Scandinavia. Look through it and one dot becomes two, like wearing glasses that see double.
Guess what? Even when clouds hide the sun, the whole sky is covered in a hidden pattern. Sunlight bounces around in the air, and its light waves line up in certain directions. That’s called polarization, and your eyes can’t normally see it. Honeybees and desert ants can, and they use it to find their way. A crystal can read it too: turn it, and the two dots brighten and dim in a pattern that points back toward the sun.
Did Vikings really do this? Nobody knows yet. No one has dug a sunstone out of a Viking ship, and the closest find is a calcite crystal from an English warship that sank in 1592, found beside its navigation tools. Tests show Iceland spar can locate the sun within a few degrees, even after sunset. A totally gray sky makes it harder, but the pattern doesn’t disappear.
Polarized sunglasses block light that’s lined up in one direction, which is why glare off a lake or a car hood fades when you put them on. Most laptop and TV screens give off polarized light too, and that’s exactly what the activity below uses.
Hands-on Activity
Find the Hidden Direction in Light
What you need: a laptop, monitor, or TV, polarized sunglasses, clear packing tape, an index card, and scissors.
Cut a window in the index card, about the size of a matchbox.
Open a blank white page on your screen and turn the brightness up.
Hold one sunglasses lens in front of one eye and look at the white screen. Slowly turn the lens (or tilt your head) through one full turn. The screen dims and brightens twice. That’s light with a direction, the same thing a sunstone reads.
Stick strips of tape across the card’s window, overlapping them so some spots have one layer, some two, some three or four.
Lay the card flat on the screen and look through the sunglasses. Colors appear, and each number of layers gives a different color.
Turn the card, then turn the sunglasses, and watch the colors shift. Criss-cross a few strips to make a stained-glass picture.
Your screen sends out light that wiggles in one direction, the tape twists it by different amounts, and the sunglasses sort it into colors. A sunstone does a similar sorting job on light from the sky.