Weather vane This decorative 11th century weather vane, found in a church is Norway, probably adorned the prow of a Viking ship. Today, weather vanes on churches and roofs tell you which way the wind is blowing. But Viking versions had a different purpose. The regularly spaced marks along the edge of the instrument suggest that it was used to measure the height of the sun based on the shadow it would cast.



(Image: Universitetets Oldsaksamling, Oslo/Werner Forman Archive)

Sundial from Canterbury This is a copy of a 10th-century Saxon pocket sundial, based on an item found in 1939 at Canterbury cathedral in Kent, UK. The gnomon consisted of a gold pin topped with an animal head with jewelled eyes.



(Image: Science Museum / Science & Society Picture Library) Advertisement

Alderney sunstone Viking weather vanes and medieval sundials are fine for when the sun is shining. But what happens when there's no sun? In 2002, archaeologists found a white crystal block in the remains of a 16th-century Tudor ship off the island of Alderney in the English Channel. Discovered close to a set of navigation instruments, the crystal proved to be calcite. The Vikings perhaps used the optical properties of calcite as a "sunstone" to reveal the direction of the sun, even on cloudy days. Historians believe this Tudor example could have been used to correct magnetic compass errors caused by the presence of iron cannons. There is no evidence that it was used to guide the ship directly.



(Image: Guy Ropers/PA)

Viking sunstone compass Physicist Guy Ropars and a team at the University of Rennes in France have designed a modern compass based on a Viking sunstone. It also uses a crystal mounted inside a cylindrical wooden box to locate the sun in the sky. Ropars is now working with a US company to commercialise the compass.



(Image: Guy Ropars)