Any amateur (or professional, for that matter) photographers out there will know the potential benefit of using a polarising filter for enhancing your photos. A piece of polaroid in front of the camera lens will darken the sky if correctly oriented, by cutting out light scattered (and then itself polarised) off particles in the atmosphere. A polarising filter is most effective when pointing your camera in directions that are at a compass bearing 90˚ away from the sun. So, if it is mid-day you will find that the polarising filter will show the sky darkest when you face east or west. However, if you’re facing the sun, or have the sun directly behind you, a polarising filter won’t have any beneficial effect for your photos.
It is believed that the Vikings used the same phenomenon as a navigational aid. It seems that the Vikings sailed around the North Atlantic navigating by the sun. Noting the rising, mid-day, and setting sun they could identify south. But how do you manage that when the sun is obscured by cloud? It happens from time to time on the eastern and northern fringes of the North Atlantic, believe me! Well, this is where the polarisation of sunlight that is scattered by water droplets in the atmosphere comes in handy. If you have a polarising filter on your camera, we know that you can identify where the sun is by how effectively it cuts out glare, even when cloudy. One Icelandic saga appears to refer to this effect. It is reported there that King Olaf asked his vassal, Sigurd, the direction of the sun on a snowy day. He then checked Sigurd’s answer by “fetching the sunstone, he held it up and looked at the sky through the stone to confirm Sigurd’s answer”.
So, what stone did the Vikings use for polaroid? Well, it turns out that certain minerals can discriminate between light polarised in different directions. For example, some are “pleochroic” … their colour appears different depending on the polarisation direction of light shining through them (rather similar to polaroid, but usually not as dramatically). Most also show differences in their refractive index depending on that polarisation direction too. While such refractive differences are small for the majority of minerals, there are some that show really strong effects of this type, most notably calcite (CaCO3), also known as Iceland Spar. Calcite crystals can be found as large well-formed rhombs in Iceland (and elsewhere). When light shines through them, it travels faster or slower depending on its polarisation direction, so a single beam of light entering the crystal is refracted into two beams on exit (stay with me here …). This can be seen by placing a single mark on one side of the crystal and looking through it from the other side. The single mark shows up double. The two marks correspond to the two polarisation directions of light shining through the crystal from the sky beyond. Even on a cloudy day, the two marks will have equal intensity if you are looking in the direction of the sun, but one will appear dark and one light if you are looking at 90˚ to the sun. Hey presto! We have an optical compass. Actually, “sky-compass” devices like this have been the subject of patents in the last century or so, and were even used by trans-arctic aircrew in recent history.
So, coming back to the Alderney wreck. The calcite rhomb found in the wreck has been partially dolomitised (Mg replacing Ca in the calcite lattice) which acted to preserve it. No such Iceland spar crystals have been found in Viking sites, however. It is suggested that the reason for the lack of direct evidence of Viking spar crystals is that, as revered objects, they were consigned to the funeral pyres of their owners. Calcite (CaCO3) breaks down to CaO plus CO2 on heating (the process for making lime from limestone, in a lime kiln), so any Viking sun compass treated this way would inevitably have been destroyed. The more recent Alderney sun compass is the first physical proof of the use of calcite for this purpose in ancient times.
Vikings – mineral physics technicians?
Image: calcite sunstone and polaroid filter compared, a montage from www.making-it-up-as-i-go.c