For centuries Europeans and Arabs had sailed around its coasts using as reference the visible points of the littoral and their knowledge of the depths (probes) in different places. When, at the end of the 15th century, the Portuguese and Castilians began their more distant voyages of exploration and discovery, their navigation instruments were:
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Runner and ampule to determine the speed of the vessel (for estimated navigation).
- Probe to determine the depth and nature of the fund.
- Very rustic compasses to determine the North (magnetic).
- Ballestilla, astrolabe and quadrant to measure the height of a star on the horizon.
They knew the limitations and errors of navigating by esteem. They also knew that the magnetic declination was not constant but varied with place and time. They knew that the polar star was not located just above the celestial pole and knew how to correct the error introduced in the measurement of latitude by observing nearby stars (the "guards").
