Navigation Facts

  • Early sailors found their way by staying near land, looking for ‘landmarks’ on shore. Away from land they steered by stars, so had only a vague idea of direction in the day.
  • After C 1100 European sailors used a magnetic compass needle to find north.
  • A compass only gives you a direction to steer in; it does not tell you where you are.
  • The astrolabe was used for navigation from about 1350. This measured the angle of a star above the horizon, or the Sun at noon, and so gave a rough idea of a sailor’s latitude.
  • From the 1500s the cross-staff gave a more accurate measure of latitude at night from the angle between the Pole Star and the horizon.
  • From the mid-1700s until the 1950s, sailors measured latitude with a mirror sextant. This had two mirrors. It gave the angle of a star (or the Sun) when one mirror was adjusted until the star was at horizon height in the other.
  • For centuries the only way to find longitude – how far east or west – was by dead reckoning. This meant trailing a knotted rope in the water to keep track of speed, and so estimate how far you had come.
  • You can find longitude by comparing the Sun’s height with its height at the same time at a longitude you know. But early pendulum clocks did not work well enough aboard ship to give the correct time.
  • The longitude problem was solved in the 1700s when John Harrison made a very accurate spring-driven clock or chronometer.
  • Ships can now find their position with pinpoint accuracy using the Global Positioning System or GPS. This works by electronically comparing signals from a ring of satellites, north or south of the Equator they were. GPS satellite AO. In the Global Positioning System, sometimes known as ‘satellite navigation’, 24 satellites in orbit around the Earth send out radio signals to the surface. At any place on the planet, signals from at least three satellites can be detected and compared by a hand-held receiver, to fix a location within 10 to 50 meters.
  • A navigational instrument for measuring latitude from the angle of certain stars.