By the time that chronometers started to become cheaper and produced on a larger scale, there were two main methods to calculate longitude at sea. One, of course, was by the chronometer method. The idea behind this method is simple in nature. A chronometer, or many chronometers, would initially be set to a home port time, such as Greenwich, England. The ship would also have a ship clock alongside the chronometers.3 To determine longitude with the chronometers while out at sea, seamen would first wait for the sun to get directly overhead. When the sun was directly overhead, the ship clock would be wound and reset to noon.3 This could then be compared to the home port time on the chronometers. Once the time difference was calculated, one would then know their longitude. The chronometers themselves would have to also be wound up everyday (unless they were 8-day chronometers, such as the one in the exhibition), to give it continuous mechanical power to work. Mishaps certainly did happen. Sometimes keys, which were used to lock the chronometers in a box, were broken off, which then caused them to be forced open—risking instrument breakage.7 The rocking and rolling of the ship in rough seas could cause the chronometers to stop working. To counteract this, the chronometers were often given a dedicated room on the ship, where they would be mounted on gimbals in their own boxes. Sawdust was also sometimes packed into the boxes to lessen the vibrational effects coming from the ship in rough seas.3 Weather conditions could also directly effect the chronometers. On William Parry’s voyage to discover the Northwest Passage from 1819-1820, he remarks how several of his chronometers were affected by the cold.9 Parry reduced the effects of the cold by placing the chronometers in “cases filled with moderately-heated sand, which was renewed as occasion required.”9
The second major method that started to come into use during the early modern era was the lunar distance method. This method used the moons position relative to fixed stars in the sky to derive local time and the meridian.2 The observer would use a sextant to measure the angular distances of celestial objects from the moon, and then note the precise time of observation. Using the nautical almanacs of the year given for the locations of celestial objects, the observer could then compute his longitude.2 This method was brought to the fore when Nevil Maskelyne, Astronomer Royal, helped to create the first Nautical Almanac in the year 1767. The problem with this method is that the sky isn’t always clear enough to make measurements using celestial objects. Also, it often required hours of complex mathematical calculations by a dedicated astronomer.
This ultimately leads to the question: which method of determining longitude during the 18th and 19th centuries was more reliable? Although chronometers would take over in the mid-19th century, they were not yet dependable enough to be used on their own. They still tended to gain and lose time on voyages. Moreover, the lunar method was still very complex–requiring a dedicated astronomer and hours of complex calculations. The solution was simple: both of them were used in conjunction with each other. More specifically, lunar observations often acted as a “check” on the going rates of clocks. When opportunities presented themselves, these “checks” were often done in land-based astronomical observatories, known as tent observatories, when they presented themselves on voyages.3 On shore, the sailors would bring an astronomical clock, sextant, universal theodolite, chronometers, nautical almanacs, as well as many other instruments. The astronomical clock would contain sidereal time (the time it takes Earth to actually spin–23 h 56 min 4.0905 s), which helped astronomers locate celestial objects in the sky. Along with an almanac, one can easily point a telescope, or universal theodolite, to the proper coordinates in the sky. A sextant would then be used to measure angular distances with the moon from other objects. Again, using the nautical almanac, one could deduce local Greenwich time to use as a check against the chronometers, to see if they gained or lost time.3 However, being on land away from the brutal conditions of the sea did not hasten issues from arising. For instance, during one of Matthew Flinders stops on his circumnavigation of Australia, he realized that his universal theodolite was “virtually useless and the astronomical clock was not performing.”2 The issues were not limited to the instruments, either. During the year 1769, observers all over the world traveled to see the transit of Venus. An Astronomer named John Bradley was one of them. He set up a tent observatory on the Lizard Peninsula in Cornwall, where he met with “very dirty” weather.7 He eventually remarked that, “The Fatigue I have met with & Cold weather has almost laid me up with the Rhumatism.”7 These tent observatories add a very interesting element to voyages during the 18th century. Tent observatories were often used to gather data that could be difficult on a moving ship. But, as we have seen, the same afflictions that happen at sea can also happen in a far-away land.