The Compass

The compass, the principal instrument of navigation, is a device that indicates direction on the Earth's surface. Before the advent of the compass, seamen estimated directions largely with reference to the wind. Two basic types of compasses exist: a magnetic compass and a gyroscopic compass.

The magnetic compass utilizes the force known as magnetism. The earth itself has a magnetized core, two magnetic poles, and lines of force that form a magnetic field. The ancient Chinese were aware that the earth's magnetism, through the medium of the lodestone, could be used to indicate horizontal directions. Mediterranean seamen of the 12th century were probably the first to use a magnetic compass at sea.

There are two types of magnetic compasses: the dry card and the liquid. The dry card compass consists of a magnetized needle mounted on a pivot at the center of a fixed graduated card so as to permit the needle to swing freely in the horizontal plane.

In the liquid compass, the most stable mariner's type, the card is mounted in a sealed bowl filled with a liquid of low freezing point. The buoyancy of the card is adjusted so that it floats, thus ensuring the minimum possible friction between the cap and the pivot. Frictional force between the cap and the pivot reduces the sensitivity of the compass. The liquid compass is used more often than the dry compass.

The needle oscillation encountered in ship and aircraft compasses led to the development of the gyrocompass in the early twentieth century. Since it is not north-seeking, it is unaffected by magnetism. Because of this ability to maintain its orientation, it is useful as a direction indicator. The gyrocompass consists of a rapidly spinning wheel set in a framework that permits it to tilt freely in any direction. It is valuable in airplanes for holding the course in rough air.

Credit for inventing the first practical gyrocompass in 1907 belongs to the German inventor Hermann Anschutz-Kaempfe. The first American gyrocompass, designed by Elmer A. Sperry, was produced in 1911.

What is a Sextant?

A sextant is a hand-held navigational instrument used to determine the altitudes of celestial bodies. It does this by measuring the angle of a celestial body from the horizon. The altitude of the sun at high noon can then be used to determine latitude.

The name comes from the Latin sextus, or "one-sixth." The sextant's arc spans 60 degrees, or one-sixth of a circle.

The sextant was invented by the English Captain John Campbell in 1757. It met the needs of the more scientific navigators, including the great explorer Captain Cook. Before its invention, sailors used the octant, or reflecting quadrant, which could measure up to 90 degrees. The sextant will read angles up to 120 degrees, because of the double reflection of the arc of the instrument.

In the 1770s, Jesse Ramsden of London introduced new sextants that revolutionized the graduation of both circular arcs and linear scales. So accurate were the scales produced by Ramsden's dividing engines that it became possible to make sextants much smaller without losing accuracy.