LongitudeThe history and development of horology is intimately tied up with the problem of discovering a ship's longitude when at sea.
Accurate navigation is, of course, dependant on knowing one's position. Maps of the world indicate position using a two figure grid reference: degrees of latitude and longitude. Latitude indicates position in a North/South direction, longitude gives the East/West coordinate. Both are needed for cartographers and navigators to locate a position on a map.
Measuring latitude is relatively straightforward since it has a genuine astronomical basis. It is physically "real": zero degrees latitude is the equator, the place where the sun and the moon pass directly overhead at noon. Sailors can judge their latitude with relative ease by observing the sun or fixed stars.
Longitude on the other hand has no such physical basis. The location of zero degrees longitude is an arbitrary one dependant on the whims of politics and fashion. Today, zero degrees longitude passes through Greenwich in London and has done so since the International Meridian Conference of 1884. But prior to that zero degrees longitude represented points on the planet as diverse as Rome, Jerusalem and Washington.
Inability to assess longitude accurately made navigation difficult and led to the wreck of many ships. Eventually the British Parliament passed the Longitude Act of 1714 and set up the Board of Longitude to award a huge prize to the first person to accurately measure longitude at sea. Many complex astronomical solutions were devised but in the end horology won.
One thing that does relate to longitude physically is time. Solar noon - the point when the sun is at its highest - occurs at a different time at different longitudes. This difference can be calculated reliably. If you know the amount of time by which local noon differs from noon in a fixed place then you can determine your longitude.
This provides a theoretically simple method for determining longitude: Before leaving port, set a clock accurately for your home town. Then at sea compare local noon with the time showing on this clock.
Unfortunately this theoretical solution was far from simple in practice. Clocks of the era were not sufficiently accurate for the task. Even those that came close could not cope with conditions at sea. The motion of the waves, changes in temperature, even local distortions in the Earth's magnetic field all led to clocks becoming wildly inaccurate.
Many clock and watch makers tried their hand at producing an adequate timekeeper. In the end the longitude prize was awarded to John Harrison who invented the first true shipboard chronometer. This was the famous H-4 watch which is today on display in the National Maritime Museum at Greenwich.
For much, much more of the fascinating story of longitude I thoroughly recommend the book Longitude by Dava Sobel