The Moder Dy: Steering by the Waves in Shetland’s Seas

Shetland fishermen of the 18th and 19th century are reported to have used a phenomenon known as the moder dy to find their way back to land in poor visibility. Drawing on oceanographic principles, possible origins for the moder dy are discussed and it is suggested that it could have been swell waves arriving in Shetland waters from distant sources. Swell waves can travel very long distances on the ocean and have distinctive characteristics (in particular long wavelengths) that distinguish them from locally generated waves. As the direction of these waves would have remained fairly constant over the duration of fishing trips they could have provided fishermen with a relatively stable reference framework. This could have allowed fishermen to construct a mental map of the seascape that they traversed during their fishing trips which could have been used for navigation when necessary.