How did sailors find their way in the sea?

In early times, sailors used to orient themselves to the stars. In order to calculate directions, they learnt to study the positions of the fixed stars, which, unlike other celestial bodies, appeared to be always present at the same point in the night sky. With the help of a simple geometric device called astrolabe, they would measure the angle between the horizon and the fixed stars at night, and the angle between the horizon and the sun during the day. With the help of their findings, they could calculate the distance to the north or the south of the equator. They also used a special navigator clock to determine how far they had come to the east or west of their home harbour. The astrolabes were replaced later by the modern sextants, and ships today navigate with the help of GPS devices that receive information regarding their position on the sea from satellites.