John Harrison's marine chronometers

Have you ever wondered how longitude was calculated before the era of GPS navigation? The concept, known as the timekeeper method, is simple. The Earth, which revolves around the Sun once every 24 hours, is divided into 360° of longitude. Longitude can therefore be calculated by comparing the local time, as defined by the Sun's position at local noon, with the time at a known place, such as Greenwich, UK. Every hour of time difference corresponds to 15° of longitude, so a time difference of 6 hours corresponds to 90° of longitude and 12 hours corresponds to 180°.

John Harrison first proposed the timekeeper method in 1730. He was a “horologist”, a master craftsperson skilled in both the theory and practice of time, and was already building accurate clocks for use on land. Unfortunately, such clocks didn’t work at sea because their pendulum mechanisms couldn’t cope with ships’ motions.

Sixteen years earlier, in 1714, the British Government had created the “longitude awards'' with a grand prize of £20,000 for a method that could determine longitude within half a degree of accuracy. In 1730, Harrison was approached by the British government to develop a sufficiently accurate timepiece at sea, a “marine chronometer”. His first clock, completed five years later, was deemed good enough to warrant sea trials. Unfortunately, the clock lost time on the outward voyage but performed better on the return voyage.


Nevertheless, he received £250 to develop an improved version, known as H2. That, too, was inaccurate, but he persisted for another 19 years to construct his third clock, H3.


H3 had over 700 precisely engineered parts and he was awarded the Royal Society's Copley Medal for this work. Even before it was completed though, Harrison realized the heavy balances were unstable and he began working on a radical new prototype.


His fourth clock, the “sea watch”, was a masterpiece of timekeeping that worked at sea (top photo). It finally proved that the timekeeper method of finding longitude was practical. In 1772, in recognition of this great achievement, Parliament awarded Harrison most of the remainder of the £20,000 reward and his clocks quickly became popular with mariners during the 19th century.

John Harrison.

The timekeeper method is still used today, almost three centuries after it was first proposed by Harrison. GPS positioning also relies on accurate timekeeping, albeit using a different method. GPS satellites carry extremely accurate atomic clocks, which are used to insert times in each broadcast message. The receiver uses the time difference between the time the signal was received and the broadcast time to compute the distance from the receiver to the satellite. By using four satellites, receivers can compute latitude, longitude, altitude and local time.


PS All photos were taken by me during a visit to the Royal Observatory, Greenwich, UK in October 2022.