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

Date: Wed Aug 23 23:26:35 2000
Posted By: Jim Stana, Mechanical Design/Analysis Manager, Lockheed Martin Orlando
Area of science: Earth Sciences
ID: 965407998.Es
Message:
```
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.

```

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