Why Did the Portuguese Invent the Science of Sailing?

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.

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