Following the Sun

Imagine that you are lost at sea attempting to across the Atlantic Ocean. You are surrounded by hairy and hungry shipmates, and have no compass or friendly satellite navigation system with a celebrity voice. If you're lucky, you have a very basic chart.

What do you do? It may sound unnerving but this was the situation for most seafarers before the magnetic compass or concept of longitude came along. The Vikings in particular are well known for being successful sailors, perhaps even discovering the Americas before Columbus, thanks in part to their understanding that salt would preserve food for long journeys and to a simple but ingenious method of navigation.

Observing swell, wave size and direction, and most importantly the position of the Sun and stars, can ensure a ship's successful progress across the sea. However, navigating by the sky during either day or night would have been impossible when fog, cloud or the long polar twilights of the northern hemisphere obscured visibility. When this happened on a Viking voyage, a sunstone was held high to locate the sun and continue the journey.

This 'sunstone' has been identified as calcite crystal Researchers from Royal Society A have carried out experiments with calcite to demonstrate how the Vikings could have 'performed a precise navigation under different conditions… using absorbing dichroic crystals as polarisers to detect a hidden sun direction using the polarised skylight.'

The light emitted by the Sun is not polarised. The electromagnetic waves (including visible light) vibrate in all directions perpendicular to the direction in which they are travelling. However, when it reaches Earth's atmosphere light scatters and becomes polarised in a particular direction. A calcite crystal absorbs light at different rates, known as birefringence, becoming brighter and turning yellow when in the path of the Sun. After noting the position of the Sun when the sky was clear, if fog descended then holding up the calcite and observing when it brightened would show which direction was East and as such the way home. Spars of calcite have also been found on 16th century voyaging vessels, nearly 400 years after the Vikings, giving further weight to the theory of their use as navigational tools.

However, not everyone is convinced by this theory, partly because the 'voyaging season' was summer, when neither inclement weather nor polar twilight would have been a significant navigational issue. Additionally, when particularly overcast the sunstone and the polarisation method would likely not have worked.

Glossary

Birefringence

Also known as double refraction, describes the splitting of a light ray in two when passed through certain substances.

Dichroic

A property attributed to some substances in reference to light travelling through them. They experience a varying absorption rate depending on the polarisation of the light resulting in colour change.