dipper@utastro.UUCP (Debbie Byrd) (12/20/84)
The invention of the quadrant helped sailors to navigate better. More on how a quadrant works -- after this. December 20 Ancient Mariners A listener in West Virginia wrote to ask how ancient mariners charted their courses by the stars. Well, until the eighteenth century, mariners could only approximate their location at sea, using the stars. They could -- and did -- estimate their LATITUDE, for example by observing how high the pole star was above the northern horizon. But LONGITUDE was only a guess based on dead reckoning -- and thousands of sailors lost their lives as their ships crashed on rocks and shores, due to faulty positions. In the year 1731 an instrument was invented that greatly advanced the art of navigation. This instrument was the quadrant -- forerunner of the modern sextant. In such an instrument, light from a star reaches the observer's eye after being reflected TWICE by mirrors. One mirror is attached to a MOVING arm marked off in degrees and minutes of angle. The other mirror is FIXED in relation to a small hole used to see the horizon directly. The observer slides the arm holding the moving mirror, until the star is seen to be reflected into the SAME field of view as the horizon, with the star just appearing to touch the horizon. A marker can then be read against the graduated scale to record the ANGLE that the star lies above the horizon. In turn that angle depends on exactly WHERE the observer is on the Earth at that instant. By getting such a measure on two different stars, even a hundred years ago a careful navigator could determine BOTH latitude and longitude with an accuracy of a mile or two. Script by Diana Hadley and Harlan Smith. (c) Copyright 1983, 1984 McDonald Observatory, University of Texas at Austin