How do you know where you are when land has been out of sight for weeks and the horizon is nothing but an endless expanse of ocean? In maritime navigation, orientation relies on two fundamental coordinates: latitude and longitude. While latitude is relatively easy to determine, the precise calculation of longitude represented one of the greatest scientific challenges of the 18th century.
Calculating position: methods of computation
Latitude is determined through astronomical observation. Observing the North Star, combined with a bit of trigonometry, makes it possible to determine how far one is from the equator. The method is proven, reproducible, and has been well mastered for more than 2,000 years.
Longitude calculations follow a very different logic. Longitude is determined by comparing the local time on board the ship with that of a reference meridian, already established at the time as Greenwich, a reflection of British maritime power.
The principle is simple in theory: the Earth completes one full rotation in 24 hours. Since a circle contains 360 degrees, the Sun therefore moves 15 degrees per hour. By determining local time through observation of the Sun, then comparing it with the time at the Greenwich meridian, one obtains a time difference which, multiplied by 15 degrees, gives the ship’s east–west position.
Whether on land or at sea, the reasoning remains the same. But at sea, everything depends on one thing: the clock. It must be set at departure and then maintain this reference with the greatest possible regularity. For when time becomes uncertain, so too does the ship’s position.
Time as the Only Compass
In the 18th century, ocean crossings were long. A voyage between a European port and the Caribbean lasted six to eight weeks, with no reliable stopovers and no possibility of precise recalibration. Throughout this entire period, the onboard clock remained the sole reference.
Measuring the passage of time with precision was therefore not an intellectual luxury. It determined the safety of ships, the success of commercial exchanges, and the survival of crews.
When longitude becomes a matter of State
Faced with this major challenge, the great maritime powers organized themselves. In England, the Board of Longitude was created to encourage any practical solution capable of accurately determining longitude at sea.
The rewards offered were commensurate with the challenge:
£10,000 for a method offering an accuracy of around 110 kilometers (one degree of longitude)
£15,000 for reducing the error below 75 kilometers
£20,000 for an error of less than 55 kilometers (half a degree of longitude)
Converted into today’s value, these sums represent several million Swiss francs. For better longitude calculation meant guaranteeing control of maritime routes, international trade, and a nation’s naval influence as a whole. At sea, these stakes played out far from the corridors of power, within the confined space of a ship’s cabin.
In the Captain's cabin
It is 1775. You are the captain of a merchant ship, tasked with leading your crew and a precious cargo from a European port to a distant destination.
In your cabin lies the most important object on board. A marine clock, suspended on gimbals and protected with the utmost care. It is placed under your direct responsibility. And it is never alone: a second clock accompanies it, as a safeguard. Redundancy of critical instruments is already an absolute rule.
Your shipowner has not hesitated. The cargo is worth a fortune, as is the ship, and he is not a man to leave such matters to chance. He has therefore acquired the very best available: a Ferdinand Berthoud marine chronometer, Watchmaker to the King and the Navy since 1770. A timekeeper capable of maintaining accuracy on the order of plus or minus one second per week. An exceptional result—one that could still rival many modern chronometers today.
Thanks to this chronometer, the time of the departure port remains your immutable reference. You calculate your position, adjust your course, and continue your journey with confidence. You will reach port safely—and you know it.
The reasons behind exceptional precision
Such a level of accuracy is based on decisive technical innovations. The first concerns the escapement. Berthoud adopted a pivoted detent escapement, considered the first true free escapement in history. Unlike traditional escapements, the regulator is not in permanent contact with the escapement. Friction is reduced, energy is used more efficiently, and rate stability is significantly improved.
Impulse is delivered by a deliberately oversized escape wheel, providing greater stability.
Next comes energy management, thanks to the fusee-and-chain mechanism. It delivers constant torque to the movement, an essential condition for maintaining a steady rate throughout the entire power reserve.
Finally, the physical stability of the calibre is ensured by gimbal suspensions. They keep the clock in a nearly constant position, regardless of the ship’s roll and pitch, allowing the movement to operate under conditions as close as possible to those of a workshop.
A living legacy
These marine chronometers form the foundation upon which contemporary watchmaking at Ferdinand Berthoud still rests today. Behind every precise timepiece lies the same obsession: controlling energy, reducing disturbances, and guaranteeing a stable rate in an environment that is inherently unstable.
Transposing these principles into a wristwatch remains a complex exercise, particularly when choosing to use a cylindrical hairspring. This topic is explored in greater detail in a dedicated article, which provides a concrete understanding of the challenges involved in moving from the marine chronometer to the wristwatch—a challenge far from being a simple matter of size or component layout.
From the high seas to the wrist, the objective remains unchanged: never lose track of time… nor of direction.
