Chronometer, Reproduction to Look like a 1785 Chronometer made by John Arnold and Sons, London, England, The Mariners’ Museum(1933.0162).
Time keeping on a ship has some special problems: the great variation of temperature on a ship at sea, the motion, and the humidity can all affect the accuracy of the time keeper.
At first, sailors could track time just by estimating the position of the sun. From sunrise to noon was about six hours, and another six hours from noon to sunset. There were actually several times during the year when the day was 12 hours long and this system worked fine. But most days are either longer or shorter. Of course, for several thousands of years, no one cared that much about the exact time and this estimating was acceptable. The sand-glass or hour-glass was developed about 800-900 AD and allowed for a reasonably accurate measure of the passage of time.
When modern sailors tried to "keep" time they found the sand-glass met their needs quite well. They used at least two or three variations. To keep track of their speed they used a device called a "Chip Log" which required a sand-glass of 30 seconds duration. To keep track of how long they worked, they used a sand-glass of 30 minutes duration. Now the duty time aboard ship, called a watch, is traditionally four hours, and this can be traced back to Egyptian times thousands of years ago, so you might expect a sand-glass lasting four hours. But this would be too heavy and the sand in it more likely to clog. Instead, sailors used a sand-glass of 30 minutes length. By the way, the content of the "sand" glass usually was not sand, but a mixture of ground-up sea shells, stone, egg shells, marble or other materials which would be less like to stick together than sand. A "glass" of an hour's duration might also be found aboard ship.
To make sure the ship's boy was paying attention, for it was usually his job to turn the sand-glass every 30 minutes, and to let everyone know the time, each time the glass was turned, the ship's boy would ring the ship's bell. It would be rung once for each half hour of a four hour watch. So after the second hour, four bells would be rung; after the third hour, six bells. To enable people's ears to distinguish how many bells there were, they were rung in pairs, two rings at a time. When eight bells were rung, four hours had passed and the next watch hearing the eight bells would come up to relieve the crew currently working. The count would begin again with one bell being rung after half an hour into the new watch, and so on till eight bells would be rung again. With 24 hours in the day, there were six watch.
Noon was a significant event to the sailor right up until modern times and electronic navigation. Before electronic navigation, the sailor was able to find his position on the voyage by measuring the altitude of the sun above the horizon at noon. This was so important, that, at sea, the day began at noon, not at midnight. In the morning it might be Thursday, but at noon, it became Friday. This was true right up through 1924.
Accurate time keeping was highly desirable, especially for navigating across the ocean, but it was not possible until the invention of a very accurate watch in 1759 by John Harrison of England. While it was accurate, it was also expensive and could cost as much as an entire year's salary for the captain of a ship. As more people produced these accurate clocks, called "Chronometers", the price came down. Today, a $10 electric watch is as accurate as the older chronometer of the late 1700's. costing over 1,000 times as much.