The correct answer to this open question is the following.
Two important navigational instruments used at Prince Henry's school were the compass and the astrolabe.
The compass was very helpful in locating the direction and navigate to that place. To calculate the latitude distance from the equator, south, and north, with the North Star reference, the astrolabe was the correct instrument to use. Other two instruments were used in Prince Henry's navigation school: the cross-staff and the black-staff. The first one was used to identify the altitude above the horizon. The second one, to find the latitude from the horizon.
