The Obelisks of Zahara de los Atunes – Andalusia’s mysterious structures and their scientific history

Halfway between Cadiz and Gibraltar there is a puzzling structure. It looks like the site of some ancient cult, but in fact it was used for scientific purposes. Read how it was used in Andalusia to determine the nautical mile and measure the speed of ships.

Zahara de los Atunes is the most popular resort along the coast in the western Andalusian province of Cadiz. It is particularly famous for its beautiful, wide sandy beaches. The most enigmatic landmarks in this resort are the four stone obelisks rising above the dunes.

These obelisks mark the nautical mile – a unit equivalent to exactly 1,852 metres. One obelisk, six and nine metres high, stands on each side.

This arrangement was intended to provide good visibility of the structure. The obelisks were primarily used to calculate the speed of the ships at different engine speeds. In order to minimise the influence of wind and currents on the measurements, the ships sailed a designated section in two directions as part of the test.

The entire structure was erected in 1947, when the local Hydrographic Institute base was established. The base closed in 1986 and the structure now only serves a historic function. Next to the obelisks are plaques describing their history and explaining what a nautical mile is. It is a measure of distance of one minute of latitude – it is used in nautical navigation and aviation.