If you know where you are then you start there and given speed and heading you can know where you are going. The sextant should be used. There will always be errors due to current and wind speeds either adding to or subtracting from your speed so you may be pushed off course a tad.
When I was a helmsman we used two tools to navigate the helm. We had a gyrocompass which gave us true N,S,E,W and we had a compass. Depending on where you are on the earth your heading will be adjusted due to flux in the magnetic field.
A course might be give to steer 345 by 352. One would be your gyroscope heading and the other your magnetic compass heading. If the two did not line up then something was amiss in navigation. There is an offset in heading depending on where in the world you are. I remember our Lead navigator was a 1st class petty officer using the sextant at night to check our position. Over him was an LTJG and the XO a LT.
The XO ran us aground. The 1st class petty officer got us where we were going safely.
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Self-education is, I firmly believe, the only kind of education there is.
 ~Isaac Asimov~
Mercfulfate
将補
日本帝國海軍
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