As explained in the excellent book and movie "Longitude" [why not go out and buy a copy today?],
one of the most challenging problems that faced mariners over the centuries was finding a way
to keep accurate time while at sea. Navigating by the position of the sun and stars [called "celestial
navigation"] was all well and good, but since the sun and stars changed position throughout the day
it was pretty useless if you didn't actually know what time it was when you checked the position.
Determing the height of the North Star would allow you to figure out your lattitude, 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 timepiece that wouldn't
be affected by the motion of the ship on the ocean.
Traditional clocks used weighted chains to provide the power and swinging pendulums to provide
accuracy -- neither of which worked too well aboard a ship. Pocket watches, on the other hand,
used springs for both power and regulation of that power, but weren't particularly accurate, at least
not accurate enough to keep constant time during a sea voyage that might last weeks or even months.
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 timpeiece that was simultaneously portable and accurate
enough for use aboard a ship. Abandoning his early attempts at creating a clock that could compensate
for the motion of the ship, he finally decided to focus instead on creating a spring driven and regulated
timepiece that was accurate enough for shipboard use. Although the specific design of his chronometer
proved too complicated for mass production, the basic idea of used a highly accurate watch became the
basis of all the chronometer designs that came after.
A standard ship's chronometer is, as stated, an extremely accurate -- if usually oversized -- pocket watch.
In order to reduce positional errors caused by the motion of the ship, they are typically placed within
special gimballed boxes that allow them to remain horizontal at all times [lesser quality watches that
are simply kept flat in a normal box aboard a ship are generally referred to as "deck watches"]. Because
these watches are designed to always remain in a single position, they are usually not "adjusted" to
compensate for positional errors the way that normal high grade watches are.
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 noticibly losing any time at all.
If anybody has some additional information about these watches that they would care to share with me, please send me an e-mail and let me know!
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This is a Hamilton Model #21 Marine Chronometer. These were used by the U.S. Navy as ships timepieces and were basically the most accurate timepieces ever designed. In good running condition, these can keep time to within a few seconds per week! The Model #21 has 14 jewels, an oversized fusee movement and an up-down wind indicator. The serial number on this one is 6019 and it dates to about 1945. |
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This is a Waltham 8-day watch that has been set up as a marine chronometer. It is stem wind and pendant set, has 15 jewels, and has wind indicator. The watch is housed in a gimballed brass tub which has been set into an inner and outer wooden box. Ca. 1920 |
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