Ship's Chronometer

A Little Information on the Ship's Chronometer

One of the most challenging problems that faced mariners over the centuries was finding a way to keep accurate time while at sea. Determining the height of the North Star would allow you to figure out your latitude, but in order to determine your longitude you needed to know the difference between ship's time and the time at your home port, and in order to do that you needed an accurate clock that wouldn't be affected by the motion of the ship on the ocean.

Traditional clocks used on land used weighted chains which powered the clock and swinging pendulums to provide accuracy. Aboard a ship, both of these devices worked poorly. Pocket watches, which were popular in the early years of time keeping, used springs for both power and regulation of that power, but were not accurate enough to keep constant time during a sea voyage that might last weeks or even months.

John Harrison to the rescue! Who is John Harrison? A man of simple birth and high intelligence with no formal education or apprenticeship to any watchmaker, constructed a series of virtually friction-free clocks that required no lubrication and no cleaning. These watches were made from materials impervious to rust and kept their moving parts perfectly balanced in relation to one another even while onboard a ship, pitching and tossing in the sea. He did away with the pendulum and combined different metals inside his works in such a way that when one component expanded or contracted with changes in temperature, the other counteracted the change and kept the clock's rate constant.

In the year 1761, John Harrison's famous "Chronometer #4" was successfully used during a long sea voyage and proved that it was possible to create a time piece that was simultaneously portable and accurate enough for use aboard a ship. This famous time piece became the basis of all the chronometer designs that came after.

In order to reduce positional errors caused by the motion of the ship, chronometers are typically placed within special gimbaled boxes that allow them to remain horizontal at all times.

A good railroad grade watch was supposed to be accurate to within 30 seconds a week. A good marine chronometer, on the other hand, could go months without noticeably losing any time at all.

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