I must go down to the seas again, to the lonely sea and the sky, And all I ask is a tall ship and a star to steer her by,

Click here to read the whole poem, one of my favourites - Sea Fever - John Masefield (1878-1967) (English Poet Laureate, 1930-1967).  Carrying on from my last blog dealing with our need to know the time, what is the connection between tall ships, stars and accurate time keeping?  I found this out when when I visited the National Maritime Museum Greenwich.
Increasing voyages of exploration and commerce meant that precise navigation became essential. The method used by Christopher Columbus  - Dead Reckoning- just wasn't good enough nor was Celestial Navigation and Sight Reduction Navigation using stars, the moon and sun to work out your position in latitude. In order to navigate accurately you needed to know where you were in both latitude (North to South) and longitude( East to West). Latitude was relatively easy to determine, if you knew the coordinates of a celestial object, the position of the object in the sky, the local sidereal time, apply some spherical trigonometry then you can determine your latitude.

A Sextant
 Used to measure the angle between any two visible objects. Its primary use is to determine the angle between a celestial object and the horizon which is known as the object's altitude.

Longitude was a more difficult problem. Initially longitude position was determined by the positions of the Moon but the lines of longitude differed between countries and charts.  In 1675 the Royal Observatory was established at Greenwich, England and here accurate data on the Moon was slowly accumulated. In 1767 the publication of English Nautical Almanac eventually led to the universal adoption of the meridian of Greenwich as the prime meridian, i.e. a line of longitude, at which longitude is defined to be 0°. The next step was to have a reliable way to compare the time where you were, to the time at the prime meridian, an hour is equal to 15° of longitude, you couldn't rely on the Ship's Boy turning the sand glass. Even the most reliable pendulum clocks of the time were unable to cope with the movement of the ships, changes in temperature and humidity. In 1714 this led to the launching of a competition by Act of Parliament, with a  £20,000 prize, to be the first to construct a marine chronometer precise and accurate enough to  provide longitude to within half-a-degree (2 minutes of time).
Stepping on the the podium in first place was Lincolnshire carpenter and self-taught clockmaker John Harrison (read here about his struggle with the Board of Longitude to actually be awarded the full prize money, it took the intervention of the King and another act of parliament).

John Harrison's Marine timekeeper, H4 completed in 1759 with its very stable, high frequency balance,
it proved the successful design. It was based on the design of a watch rather than a clock.

His chronometers worked at sea! Captain James Cook took K1 a replica of H4 with him on his sea faring expeditions.

Click here to read  in more detail about the development of Navigation.

Visit the museum in Greenwich to see John Harrison's chronometers, if you want to see one of his early Clocks or his paper drawing of the movement of H.4, go to the Clock Makers Museum at the Guildhall, City of London, where there is a wonderful collection of early clock, watches and sundials. Entrance is free!