The two main factors to be determined by the navigator are the plotting of a course between two known points and the ship's position at any time.
As long as a ship sailed along the coast, determining its position was not too difficult. The pilot merely had to supplement his local knowledge with the use of a magnetic compass, a sandglass and a lead line to take soundings. However, out of sight of land the navigator had to determine the two coordinates of latitude and longitude. During the sixteenth and seventeenth centuries the navigator had but two options. To keep an account of the ships speed and changes or direction since leaving port or the last known sighting of land, this being know as "dead reckoning" or alternatively, to make use of known positions of the sun, moon and stars relative to the ship's position.