Re: How did sailors determine the local time on board their ship

Keeping accurate time on a ship was such an important part of navigation 
that the English government offered a 20,000 pound prize to someone who 
could invent a device to do it.  The prize was won in 1762 by John 
Harrison.

A ship's watch, or chronometer, must endure changes in temperature, 
humidity, and movement of the ship.  The early clocks that used pendulum's 
did not work very well since the direction of gravity would roll with the 
ship.   Harrison's clock used a spring movement. His prize winning clock 
was accurate to 0.1 sec in 156 days at sea.

Navigators would keep the clock set to Greenwich time (0 deg longitude) and 
would calculate the ship's East-West position by comparing the time at 
Greenwich with the local noon (when the sun was at it's peak in the sky) 
where the ship was.  Each hour difference was equal to 360 degrees/24 
(since the earth rotates a full 360 degree revolution in 24 hours) or 15 
degrees of longitude from Greenwich.  Knowing the size of the earth's 
diameter gives you the distance in miles. (distance=angle in degrees/360 x 
diameter of earth in miles x pi)

The diameter of the earth was fairly accurately known from celestial 
observations and other mathematical means.

To know your north-south position was easier, since the height of the sun 
at noon was directly related to how far you were above or below the 
equator and the time of the year.  A sextant was used to measure the height 
of the sun in degrees.

I found this info about John Harrison and chronometer's at the Grolier 
Encyclopedia web site, which refererences a book called "Longitude" by 
Sobel.