Astrology, as practiced by the Egyptians, Greeks, and Hindus, which taught people about geography, direction, and navigation. No, it’s not about your sign. Pictured is Ptolemy.
The lateen sail, which improved over square sails used by Roman and other dominant navies. The lateen sail allowed us to manipulate sails according to wind direction, rather than having to wait for a wind going the way we wanted to and having to sit around if that fickle wind changed on us. It allowed for translatlantic crossing, int particular, because with a triangle sail you could slide from Europe down towards the equator, pick up the trade winds; return by sliding up the east coast of the new world (now peopled by a strange tribe of individuals in suits with magic devices they click all the time) and catch the western trade winds.
The compass, which is apparent enough.