Navigation

For thousands of years seafarers, who crossed the seas far from land, had only the sky to help them. They learned early on how to use the position of celestial bodies, the sun, moon and stars to guide their course.

The astrolobe is an ancient instrument for estimating the position of the sun and other planets with respect to the horizon and the meridian. It was perfected by the Arabs and used widely, especially by Portuguese and Spanish seamen, up until the 17th century. Other similar instruments in use from the 16th century were the cross-staff and Davis’ quadrant. An improved version of the octant is the modern sextant that measures angles up to 1200 and has an arc of 600 (1/6 of a circle) with subdivisions.

The position of any place on the earth’s surface is determined by its latitude and longitude. Throughout the centuries, it was relatively easy for seamen to guess the latitude of their position by the length of day time in relation to the season and by the angle between the direction of stars and the horizon (altitude) that they determined by eye or with instruments.

However, finding the longitude, that is at what distance west or east they were, was a great problem for many centuries until it was finally solved in the 18th century.

In July 1714, the English parliament passed an act (The Longitude Act) that offered a prize for the solution of the problem. It was won after many years by an English clockmaker, John Harrison, who made the first nautical clock (chronometer) that kept precise Greenwich mean time no matter what the conditions of the journey. Later, an English man, John Hadley, and an American, Thomas Godfrey, presented almost at the same time an instrument called an octant to determine the angular distance of the sun from the moon during the day and the moon from the stars at night. With the help of tables that gave this angular distance for each hour of the day and each day of the year, the longitude and latitude could be found.