a ron astronomer but by an ordinary carpenter from southern england john harrison at the age of twenty two john harrison built his first chronometer he spent sixty years of his life studying how time could be measured precisely his premise was that if a navigator at sea knew the exact time in his home port then from the difference between that time and the time on board ship he could calculate the position of the vessel in degrees latitude and long it cheap. for centuries no one had been able to make a clock that measured the time accurately at sea without being influenced by fluctuations in temperature and humidity well by the ship's motion and that was john harrison's great achievement. the h. four is the size of a pocket watch and it represents the sum of harrison skills on a twelve week trial voyage in seven hundred sixty one this little corner deviated