Blue Water

One of my most important jobs, when I served as a Third Class Quartermaster on a U.S. Navy oiler in the early 1970s, was to attend to the ship’s three chronometers. The three exquisite timepieces were binnacle mounted in wooden boxes and kept in a special felt-lined drawer in the chartroom. Every morning, I would ask the radio shack to tune the chartroom speaker to a particular shortwave frequency, where I could listen to the “time tick” broadcast from the U.S. Naval Observatory. The announcer would say, “At the tone the time will be exactly fifteen hours and three minutes Greenwich Mean Time.” At the tone, I would start a stopwatch, then proceed to compare each of the three chronometers. I would log the difference and calculate the daily decay rate for each of them. Another of my jobs, during abandon ship drills, was to gently remove one of the boxed chronometers from its drawer and carry it, along with a sextant, almanac and sight reduction tables, to the lifeboat station, to ensure the survivors could navigate to the nearest friendly shore if the ship were to sink.
 

Why all this fastidiousness about time?
 

Accurate timekeeping is critical when taking observations of celestial bodies. It’s also important in dead reckoning. Remember that prior to the advent of GPS the primary means of navigating at sea was through celestial sights, and it was not uncommon for a ship to navigate by dead reckoning for days at a time, estimating the ship’s position based on course and speed steered since the last fix.
 

Prior to the eighteenth century, there was no practical way for sailors to measure longitude at sea. It was possible to measure latitude without a time reference, by taking sights on the sun at local apparent noon (LAN) and on Polaris, the North Star, at morning or evening dusk. But longitude calculation required an accurate time reference, and human science had not yet come up with an accurate timepiece that could function for months at a time on a rolling and pitching ship at sea. So for many centuries, mariners “sailed the squares” when making open ocean transits – sailing up or down the coast to the desired line of latitude and sailing back up or down the coast after making landfall on the other side. The invention of the marine chronometer by an English clockmaker named John Harrison in the 1730s revolutionized the art of marine navigation by making longitude calculation possible. Thus, when Captain Cook made his famous voyages of discovery, he was able to create reasonably accurate charts. His original surveys are still the basis for many nautical charts today.
 

As an aside, you might be interested in the bells traditionally used to chime the hours on ships at sea. Watchkeeping was (and still is) generally based on a four hour rotation. Prior to the invention of the chronometer, an hour glass was used to mark the time, albeit with marginal accuracy. At the first half hour of the watch (0830), the officer of the deck would turn the hour glass and ring a single bell. At the next half hour, he would turn the glass again and sound two bells. Then three, four, five and so on. Finally, at the last hour of the watch, eight bells were sounded, signaling time for changing the watch on deck. The oncoming watch would start over with one bell. And so it would continue throughout the day and night.