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How a Chronometer is used in Navigation

Articles by Gary Sellick

If a ship were to navigate out of the sight of land before the late 1700’s,  its position had to be determined by dead reckoning.   This was an informed guess based on the relative measured speed and compass heading.   If the ship was in unfamiliar waters or the currants were unpredictable,  this was an uninformed guess.   Latitude,  the lines parallel with the equator,  could be determined with a sextant and a sighting of the star Polaris in the Northern Hemisphere and the sun in the southern.   The determination of longitude,  the lines running from pole to pole,  required the knowledge of the correct time at a known position on the face of the earth.   Due to the course of history and particularly the location of the invention of the chronometer,  Greenwich England is typically chosen for this arbitrary point.   With the correct charts and tables to tell the navigator the angle of a star or the sun above the horizon at Greenwich at any particular time,  the navigator would compare the angle above the horizon of the same star with his sextant from his position at the same exact time.   Knowing these two angles,  it was possible for the navigator to triangulate his distance,  east or west of Greenwich.   An accurate time keeper was a must for this method to work. For every second the calculations were off the calculated position was off by 1,500 feet. That is over a quarter of a mile.