The art of navigating was a dangerous business. Sailors had to guess where they were and generally stay in sight of land before several inventions enabled them to go farther.

      Developed in China around 200 BC, the compass was first used as a fortune telling device. Iron Ore, or Magnetite, was used for compass needles. It naturally aligns itself in a North/South position. These needles were first floated in water on a reed, but later more elaborate compasses were used with markings for the constellations. First believed to be satanic when introduced in Europe, compasses were widely used for navigating in both Europe and China by the 11 th century.

A Chinese box compass.

     While mariners could find their latitude by using the North Star, they needed a way to find their longitude. Invented in 1757, the sextant could measure longitude as well as latitude. The navigator measures the angle of the horizon and the sun at 12 o'clock noon when the sun is straight over head. This angle is used to find the longitude of the boat and the navigator can calculate their position.

An early sextant, made around 1750.

 http://www.mat.uc.pt/~helios/Mestre/Novemb00/H61iflan.htm

        Before the sextant had been invented, sailors used latitude, not longitude to navigate. Ships could sail east and west along known latitude. To determine the latitude of a ship, the Mariner's Astrolabe was used in the 15 th and 16 th centuries. The altitude of the Sun at noon or the altitude of a known star when it was on the meridian could be found using the astrolabe. The star or Sun's declination was then looked up in an almanac and using the equation 90 degrees – altitude + declination = latitude. The astrolabe is not a super accurate device however it was still widely used.

      By using these navigational tools, sailors could navigate more accurately. This means that sailors could travel farther without dying. When sailors can sail further, they can conduct trade with other regions and further connect the world through sea.  An expedition is worthless if the explorer can't get back home.

A mariner's astrolabe.  This astrolabe is about 6 inches across.

http://astrolabes.org/mariner.htm