Ships, Clocks and Stars: the Quest for Longitude tells the amazing story of how the problem of determining longitude at sea was solved. The exhibition explains the rival methods and shows the incredible craftsmanship and ingenuity of clockmaker John Harrison, whose timepieces finally gave sailors a practical means to calculate their longitude in a simple manner.
Why was it so hard to sort out a means of finding longitude, when it seems finding latitude had been a relatively simple process?
The short answer is that latitude had reference points easily available and they were also easy enough to measure and use for guidance, even without fancy instruments. However, the references for longitude were much less obvious and needed very accurate tools and tables of information for their measurement to be usable.
Reference points are a key to either being lost or knowing where you are. If you are lost, there is nothing familiar around you as a reference to guide you on your journey or locate where you might be and get you back to where you came from.
In contemporary terms, latitude is your location north or south of the equator and longitude is your location east or west of the prime meridian, 0 degrees. Both are imaginary lines circling the earth and on a modern chart they establish a grid of lines at right angles to each other that enable you to locate your position. These terms, and their associated method of mapping with a geometric grid, are relatively recent within human history.
However, latitude, or the same thing by another name, has been widely used by humans since they took to exploring the oceans and maybe even earlier, perhaps as people moved to new places on land, leaving a locality with known landmarks. This is because latitude has an obvious reference in the sky – a map or chart featuring the sun during the day and the stars, moon and planets at night. As you move north and south, the map in the sky changes.
You could ‘measure’ where you were in relation to the changes in the location of celestial bodies. Their relative position as they rose and set was simple enough to observe, but also their height above the horizon changed as you moved north and south, and this could be estimated or even measured with quite reasonable accuracy by different methods.
This whole reference in the sky formed a chart of its own, and although it changed through the night and through the seasons, and even presented a different view of the bodies depending on your location, the changes were all gradual and formed a pattern which many communities and civilisations were able to observe and record, often in stunning detail and insight. This accumulated knowledge and understanding was passed on and people were able to determine anything from a broad indication through to a reasonably precise location of where they were in a north-to-south direction, either relative to another known point or, in later times, relative to the equator.
The quest for determining longitude developed a number of possible methods, three of which were potentially quite accurate. Observations of Jupiter’s moons could be used and worked well on land, and observations of the moon through the lunar distance method presented a very precise answer, too. Also, the concept of using the difference in time between a known location and your location as a means of calculating your longitude was also widely known. However, all of these needed very precise observations of the different celestial bodies, and in the case of the first two, very detailed recordings of their patterns of movement, and tedious calculations to arrive at an answer. The time of day was also needed, and needed very accurately. The instruments required – telescopes, sextants, timepieces and so forth – were gradually improving in accuracy, but did not begin to meet the requirements needed until the 1700s and into the 1800s.
The difference in time or timekeeping method that eventually gave practical access to longitude at sea used a specific reference as a datum for time that was within the east-to-west moving chart in the sky. Lying within this east-to-west movement was a north-to-south oscillation. All of the bodies were rising in the sky to a high point above the horizon and then setting again in the west. This change in height above the horizon could be seen on earth as a north/south vector, or drift, within the east/west motion. The highest point was due north or south in the sky and always occurred at the midpoint between when a body rose and set, and this midpoint was a constant reference as it occurred at the same time each day.
It was one of these midpoints, along with Harrison solving the technical problem of creating a clock that kept time accurately for days on end, which helped provide a practical solution to the longitude problem. The point is called local noon, the point at which the sun was highest in the sky. Whereas sunrise and sunset change time each day, local noon was always the same time and thus a precise reference point for checking time each day wherever you were. If you could then just check your time against the time at another known location, you could calculate your longitude relative to the known location.
In fact the high point of other bodies could do this as well, but the sun was an obvious one. Local noon at the prime meridian of 0 degrees, which passes through the Royal Observatory at Greenwich in London, eventually became the accepted datum for longitude and time, leading to the well-known term Greenwich Mean Time. GMT is actually an average or ‘mean’ for local noon at the Greenwich Meridian because there are some minor fluctuations in local noon that need to be allowed for. You can read more about that, along with some other intriguing issues that had to be resolved, and explore some wide-ranging background to time, distance and speed in the digital story Longitude – A story comes full circle.
Ships, Clocks & Stars: The Quest for Longitude has been produced by the National Maritime Museum, part of Royal Museums Greenwich, London.
Proudly supported by United Technologies Corporation.