>>21602595Some authors point to the use of a set of sand clocks. It seems it was an Arab developed technique to sailing across the Mediterranean, where measuring longitude had some importance. It might have permeated to the kingdoms of Spain and Portugal. Magellan and then Elcano made extensive use of it with extraordinarily accurate results.
The technique was confirmed by measuring the difference in longitude between two Spanish ports, hence the actual size of the Earth was known before Columbus departed.
