Use Polarizing Film to Locate the Sun on Overcast Days

The sagas are ancient stories about the sea voyages that the Vikings took around the year 1000 CE. Most modern scholars believe the accounts are predominately fictional, but they contain enough factual details to give insight into the Viking way of life. One particularly interesting story concerns King Olav II, a famous warrior king who was so well thought of that he was canonized Saint Olav, the patron saint of Norway.

In the Saga of Saint Olav, Olav asks an Icelandic Viking named Sigurd to tell him the location of the sun during a foggy, cloudy day. Sigurd does so, and to check Sigurd’s answer, the saga says that Olav “grabbed a sunstone, looked at the sky and saw from where the light came, from which he knew the position of the invisible sun.”

That’s quite a trick. And, if it were possible to have a device that could find the exact position of the sun on a cloudy day, it would explain a lot about how Viking voyagers were able to sail to far-off places such as Greenland, Iceland, and even North America. Historians and archeologists have found no evidence that the Vikings had magnetic compasses, and the North Star was of little use to Viking navigators because of the perpetual daylight of the arctic summer when most voyaging took place.

So, the Vikings had to use the sun to navigate on their long-distance journeys, but in the world’s often-cloudy polar regions, days go by without the sun appearing. A solarsteinn (sunstone) that showed the sun’s position on overcast days would be a great advantage.

There is no agreement among historians that sunstones actually existed, but there are tantalizing possibilities. First, sunstones are mentioned several times in different sagas. Secondly, the Vikings did know of naturally occurring rock crystals such as calcite and cordierite, and these minerals have the ability to polarize light passing through them under the right conditions.

How a Sunstone Could Have Worked

As any honeybee or mouse-eared bat will tell you, the sky is polarized. Regular light vibrates in more than one plane and is referred to as unpolarized. Polarized light vibrates in a single plane. With the possible exception of the Vikings, people did not know this until 1812 when the great French scientist Francois Arago invented the first polarizing filter. Thanks to Arago and the scientists that followed him, we now know that the sky’s polarization is due to “scattering,” meaning that light waves are bounced around by particles in the atmosphere.

The interesting thing about the sky’s polarization is that it’s not a random phenomenon but one that always occurs in the same way. If you could see polarized light like a honeybee does, you would notice a particular bright spot in the sky, and that spot is always in the same place: a point on the celestial arc that is exactly 90° opposite the sun (Figure A). If you can see that “spot” you can figure out where the sun is — even when it is occluded by clouds.

Building a Sunstone

Only wealthy people like Olav could afford a sunstone made of rock crystal, but crafting your own sunstone with polarizing film is inexpensive and easy. The materials cost less than $15 and the project will take a couple of hours.

Project Steps

1.2.3.4.5.FIND THE HIDDEN SUN!

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To begin, use a saw to cut a 45° angle at one end of the 2×2 piece of pine (Figure B).

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Refer to Figure C to lay out and cut the 4″×4″ piece of basswood.

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3. Place the piece of cellophane at a 45° angle to the polarization axis of the film, which is probably the bottom of the square (Figure D). Hold it up to blue sky and rotate it. You should see the cellophane go from clear to opaque. If it does not, then: (1) you’re not using the right kind of tape, or (2) you need to change the angle of the cello tape to the axis of the polarizing film, or (3) the cello tape has been placed on the wrong side of the polarizing film. Once correct, tape the cello tape to the film.

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Align the polarizing film so that the cello tape faces out toward the sun. Rotate the film until the cello strip is perpendicular to the bottom edge of the basswood square (where the 1″ tab protrudes). Then glue the 3″×3″ polarizing film over the hole (Figure E). Trim film as necessary.

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Glue the basswood piece with the polarizing film onto the square end of the 2×2 block, and the craft mirror onto the 45° face of the block as shown in Figure F. The film and the mirror are now at a 135° angle. Let the glue dry completely.

FIND THE HIDDEN SUN!

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1. Hold the sunstone so that the cellophane strip runs vertically and the mirror is below the film holder. Make a guess as to where the sun is and then face the opposite direction. When you look at the sunstone you will note that the cellophane strip is darker (Figure G) or lighter (Figure H) than the surrounding area. Move the sunstone horizontally until the cellophane strip is nearly the same color as the polarized film on either side of it (Figure I).

2. Now, move the sunstone up or down until the cellophane strip completely disappears. You are now looking at a point exactly 90° on the celestial arc from where the sun is. Note: You need at least a small patch of blue sky to aim this sunstone.

3. Without moving your head or arms, look into the mirror. The spot in the center of the mirror is the location of the hidden sun. (Don’t stare at the sun if it is visible when you do this! Cover the mirror with paper when testing your sunstone on sunny days.) You may need to adjust the angle of the mirror to get the most accurate readings.