What were the major difficulties in trying to navigate in the open seas before the modern era?

Before the Modern era, people used to navigate the seas using primitive charts and tools such as astrolabes and calibers to make observations of the Sun and stars and determine directions. The major difficulties in trying to navigate in the open seas were the unknown and unpredictable weather conditions in the open seas. For that reason, their navigation was limited to areas with relatively known winds and currents.