While reading Carry on Mr. Bowditch, we researched the History of a Sextant and found this information compiled by Peter Ifland.
So what do navigators need to find their position on the earth’s surface by observing the stars?
1. They need an Almanac prepared by the astronomers to forecast precisely where the heavenly bodies, the sun, moon planets and selected navigational stars, are going to be, hour by hour, years into the future, relative to the observatory that prepared the almanac, Greenwich, England in modern times.
2. They need a chronometer or some other means of telling the time back at the observatory that was the reference point for the data in the almanac,
3. It is the cartographer’s job to provide accurate charts so that navigators can establish their position in latitude and longitude or in reference to landmasses or the hazards of rocks and shoals.
4. The navigators need a quick and easy mathematical method for reducing the data from their celestial observations to a position on the chart
5. Finally, navigators need an angle-measuring instrument, a sextant, to measure the angle of the celestial body above a horizontal line of reference.
How do navigators use the stars, including our sun, the moon, and planets to find their way? Well, for at least two millennia, navigators have known how to determine their latitude — their position north or south of the equator. At the North Pole, which is 90 degrees latitude, Polaris (the North Star) is directly overhead at an altitude of 90 degrees. At the equator, which is zero degrees latitude, Polaris is on the horizon with zero degrees altitude. Between the equator and the North Pole, the angle of Polaris above the horizon is a direct measure of terrestrial latitude. If we were to go outside tonight and look in the northern sky, we would find Polaris at about 40 degrees 13 minutes altitude – the latitude of Coimbra.
In ancient times, the navigator who was planning to sail out of sight of land would simply measure the altitude of Polaris as he left homeport, in today’s terms measuring the latitude of home port. To return after a long voyage, he needed only to sail north or south, as appropriate, to bring Polaris to the altitude of home port, then turn left or right as as appropriate and “sail down the latitude,” keeping Polaris at a constant angle.
http://www.mat.uc.pt/~helios/Mestre/Novemb00/H61iflan.htm
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