A Brief History of Navigation Instruments

A brief history of navigation instruments

==========

________________

A brief history of navigation instruments

Navigation is based largely on the spherical coordinates latitude -angular distance north or south of the equator - and longitude - angular distance east or west of a generally accepted reference location. Finding longitude requires comparing local time, measured by a heavenly body, with the local time at a reference location, kept by a clock. Measuring latitude, on the other hand, does not require an accurate time-piece. Refinement of instruments enabled sixteenth-century mariners to determine latitude with reasonable accuracy.

The celestial globe was a mounted sphere depicting the heavens instead of the earth. With the introduction in 1569, of practical, affordable sea charts, the costly and delicate celestial globe gradually fell out of use.

The astrolabe was used to determine latitude by measuring the angle between the horizon and the North Star (Polaris). The user held the astrolabe by a loop at the top, turned the alidade so that he could sight the star along its length, and read the altitude off the scale engraved on the ring - difficult tasks to perform on the deck of a heaving ship.

The quadrant, shaped like a quarter-circle, was another hand-held instrument of wood or brass. The user measured the altitude of Polaris by sighting through a peephole and taking a reading where a short plumb line intersected the scale on the outer edge of the arc.

The cross-staff was developed in the tenth-century. It consisted of a square staff 3.5-4 feet in length, bearing a scale, with four sliding cross-pieces or transversals of graduated lengths. The user held on end of the staff to his eye, then slid the transversal onto the far end and moved it back and forth until its upper and lower edges seemed to touch, respectively, the observed body and the horizon.

The magnetic compass, is an instrument developed, probably independently, by Chinese in the eleventh century and Europeans in the twelfth. A typical sixteenth-century compass consisted of a large magnetized needle fastened to the underside of a circular card on which the several directions were drawn. The needle was pivoted on a fine brass pin to enable it to swing freely. The compass card was suspended by gimbals (concentric mounting rings), which allowed the card to remain level regardless of the motion of the ship.

Charts not only gave the mariner an idea of where he was going, but also a means of plotting his past and present positions. Cartographers and mariners endured many of the same problems, such as inability to determine precise longitude. Consequently, most sixteenth-century charts were not very accurate by modern standards.

The