Two time zones, a 24-hour time indicator, date, current time and Astrolabe!
The Astrolabe was developed at Greek school in Alexandria about 160 B.C. by Hipparchus. Great scientific strides forward at that time were the result of combining the Greek sciences with Babylonian mathematics. This was made possible by the conquests of Alexander the Great who established a vast empire throughout the Mediterranean. From that point in history, the Astrolabe was known to scholars and was used as a slide rule of the Heavens.
Direction, time, angles, and the position of the celestial bodies could all be calculated. When Prince Henry the Navigator established his seafaring fleet, he began using the Astrolabe to navigate the ships. For many years this gave the Portuguese the exclusive ability to navigate open waters, which the other countries could not do. When Sir Francis Drake raided ports along the South American coast he was forced to flee from the Spanish ships. Drake attached a Portuguese ship and took its navigator hostage to guide him on his round the world voyage, thus avoiding the Spanish Fleet.
All the great voyagers in the age of exploration navigated with the Astrolabe, including Columbus, Magellan, and Drake. About 1391 Chaucer wrote his Treatise on the Astrolabe for his son.
The primary use of the mariner's Astrolabe was to find the latitude of a ship at sea. In use, the Astrolabe a mariner would use is suspended (above eye level) and the altitude of the sun or a star above the horizon is measured using a rotating disc. The measurement is taken when the sun or star is at its maximum altitude. The star's declination of the sun's declination for the day can be found from an almanac. The latitude is then found by solving the equation latitude=(90 degrees - measured altitude)+declination. The term (90 degrees - altitude) is called the zenith distance.
Astrolabe is not a very accurate navigational instrument and errors of several degrees in latitude were common.