By Sara Mineo
Marine chronometers— devices that can be used as a portable time standard and for calculation of longitude – have dramatically transformed nautical navigation. When the first chronometer was developed in the 18th century, it was heralded as a major technical achievement, and in an era that lacked electronic or communications aids, this changed the way mariners kept track of time. With the development of the chronometer, sailors could now set an accurate time frame that was vital for safe navigation for a long sea voyage. The first true chronometer was the life’s work of clockmaker John Harrison, which spanned 31 years of dogged experimentation and failed attempts that revolutionized naval (and later aerial) navigation, and enabled the expedition of the Age of Discovery.
Prior to Harrison’s revolutionary creation, mariners measured longitude by dead reckoning. Dead reckoning uses an estimated ship’s speed and elapsed time of a trip’s course to determine one’s position in relation to a fixed point. It’s a complicated and frequently erroneous method, prone to producing massive miscalculations and, consequently, fatal shipwrecks. In Harrison’s era, maritime shipping was the primary method of trade and commerce and, with ships making transglobal voyages, there was a desperate need for a device that accurately measured longitudinal lines.
When a ship travels 15° eastward, the local time advances one hour. Jointly, as a ship travels 15° westward, the local time retracts an hour. If a ship’s captain would know the local times for two places, he could theoretically calculate the longitudinal distance between two locations. With today’s abundance of high-tech, multifunctional gadgets, just a few swipes on our smartphones produce figures for travel distance, weather conditions, estimated trip time, and wind speeds. In the mid-1700s, however, constructing a clock so durable and reliable to produce accurate calculations under brutal – and unpredictable – sailing conditions was no small feat.
Harrison created three different versions of marine timekeepers before developing a smaller watch design. It took Harrison six years to create his first watch design, known as H4, which proved astoundingly accurate. It measured only 5.1 seconds slow during its test trip to Jamaica in 1762. The watch was so accurate, in fact, that Captain James Cook used a copy of this design for both his second and third voyages. With Harrison’s invention, Cook was able to accurately chart the South Pacific and paved the way for an era of future exploration, trade and colonization.