The user measured the altitude of Polaris by sighting through a peephole and taking a reading where a short plumb line intersected the scale on the outer edge of the arc. Only one transversal was used at a time, its selection being based upon the height of the heavenly body in the sky - the higher the body, the longer the transversal. The user held on end of the staff to his eye, then slid the transversal onto the far end and moved it back and forth until its upper and lower edges seemed to touch, respectively, the observed body and the horizon. The location of the transversal on the scale was converted by a table into degrees of latitude. Polaris is often obscured by clouds, fog, or daylight, and it is below the horizon for anyone in the Southern Hemisphere.
Darkness often makes the horizon hard to find.
So navigators learned to use the astrolabe, quadrant, and cross-staff with the sun. A piece of smoked glass was frequently used to keep the user from blinding himself. Under lock and key, for use by the captain and pilot only, were highly prized declination tables or astronomical charts showing calculated heights of the sun above the equator at noon for every day of the year. The foregoing instruments provided invaluable information, but their use depended on the visibility of heavenly bodies. As a result, mariners relied on the magnetic compass , an instrument developed, probably independently, by Chinese in the eleventh century and Europeans in the twelfth.