Venturing out on the open ocean of the South Atlantic sailing at first with the currents and winds out to the Azores, Portuguese sailors soon reached the limits of traditional methods of navigation
The North Star
Without any
traditional star maps such as these on the left, sailors turned first
to the stars they did know and could rely on, the Pole Star and
constellations around it.
The North Star is not always in the same place. It moves (very
slowly) in a small circle around the pole.
Constellations
The major constellations of the zodiac were extremely important to time-keeping. Knowing the constellation visible in the sky as the sun rose was a secure way of keeping track of days and months when on board ship and the usual clues to the passage of seasons were invisible.
Precession of the Ecliptic Earth Facts