Navigation schools were set up to teach navigators how astronomy could be used in expeditions.
For the Portuguese, the fundamental principal of astronomy was the movement of the sun. The navigators were able to identify the latitude they were on (the position in relation to the north-south axis) by comparing the position of the Sun in relation to the horizon.
Astronomers prepared tables with the position of the Sun, the date and its position in the sky. The navigators used instruments such as the astrolabe, to check the position of the Sun and they compared it to the tables.
This was a fundamental breakthrough. Until then the latitude had been defined by the position of the polar star, which cannot be seen from below the equator.
But this method of navigation was not always reliable. On cloudy days, when the sun was hidden, there was no way of working out the latitude.
The ability to identify the longitude (east-west) of the boat was developed much later. It was already possible, in the 16th century, to have an approximate idea of the longitude by estimating the distance covered, but there was a margin of error of up to 350 km. It was only at the end of the 18th century that a more exact method was discovered.