Longitude at Sea
The History of Timekeeping
Course 2 · Chapter 6 · Longitude at Sea
On the night of 22 October 1707, a British fleet returning from the Mediterranean ran onto the rocks off the Isles of Scilly, and well over a thousand sailors are reported to have died (check the date and the toll). The admiral and his officers had misjudged their position. They knew how far north they were, but not how far east or west. The disaster is often cited as one reason Parliament took the problem of longitude seriously. This chapter explains why finding longitude is a problem about time, follows the competing solutions, and tells the story of the clockmaker who built the machines that solved it.
Latitude Is Easy, Longitude Is Hard
A navigator can find latitude, the distance north or south of the equator, by measuring the height of the Sun at noon or of the Pole Star at night. Longitude, the distance east or west, has no fixed reference in the sky, because the Earth turns beneath it.
| Time error | Longitude error | Distance at the equator |
|---|---|---|
| 4 seconds | 1 minute of arc | About 1.9 km |
| 1 minute | 15 minutes of arc | About 28 km |
| 4 minutes | 1 degree | About 111 km |
These are approximate figures (check).
The Prize and the Board
In 1714 the British Parliament passed the Longitude Act, offering rewards, the largest of £20,000 (a vast sum then; check), for a method of finding longitude at sea to a stated accuracy, and set up the Board of Longitude to judge the claims (check the details). Many proposals came in, some from cranks, but the serious contenders fell into two groups.
The astronomical method
Use the Moon's position against the stars as a kind of clock face in the sky. Accurate tables and instruments were needed, and the sums were long and tiring at sea. The method of "lunar distances" was developed in the eighteenth century and used for decades (check).
The clock method
Carry a clock set to home time and compare it with local noon. Simple in principle, but a pendulum clock cannot work on a pitching ship, and ordinary watches were not good enough.
Other schemes
Proposals included signal ships, firing guns and even a wounded-dog scheme based on a "sympathetic powder", which is best treated as a curiosity (check).
John Harrison
John Harrison, a self-taught carpenter and clockmaker from Yorkshire (check), devoted decades to the clock method. His story is often told as a battle of a lone craftsman against the scientific establishment, which is partly true but oversimplified (check).
| Instrument | Date (approx.) | Notes |
|---|---|---|
| H1 | 1730s | A large brass clock with linked balances; tested on a voyage to Lisbon and back (check) |
| H2 and H3 | 1740s to 1750s | Refinements; H3 took many years and introduced ideas that later proved useful (check) |
| H4 | 1759 | A large pocket watch, far smaller than the earlier models, tested on a voyage to the West Indies in the early 1760s (check) |
H4 lost only a few seconds over a long voyage, well within the terms of the Act, but the Board was cautious and demanded further trials and copies, and Harrison received his payments in stages, with the final award coming only after the intervention of the king (check). The instruments can be seen at the Royal Observatory in Greenwich (check current displays).
From Prototype to Standard Equipment
Harrison's work showed that the clock method was possible but expensive. Other makers, among them Thomas Earnshaw and John Arnold in England, simplified the design in the later eighteenth century, making chronometers that could be produced in larger numbers at lower cost (check). By the early nineteenth century, a chronometer was standard on Royal Navy ships and on many merchant vessels. Captain James Cook took such timekeepers on his later voyages, and reported well of them (check).
The lunar distance method faded as chronometers became cheaper. A ship might carry several, and compare them to catch one that had gone astray.
Exercises
A navigator finds local noon at the moment the chronometer, set to Greenwich time, reads 15:00. How far east or west of Greenwich is the ship, and in which direction? Explain your reasoning.
A chronometer loses 3 seconds a day. After a voyage of 60 days with no correction, how large is the time error, and roughly how far off in longitude would it put the ship at the equator?
Make a list of four problems a clock faces at sea and, for each, one idea (ancient or modern) that might overcome it. Then write two sentences on which problem you think was the hardest.
Quick Reference
- Latitude: found from the Sun or Pole Star
- Longitude: a time difference; 15 degrees per hour
- 1707: the Scilly disaster, a spur to action
- 1714: the Longitude Act and the Board of Longitude
- Two methods: lunar distances and the chronometer
- Harrison: H1 to H4, a lifetime's work
- Later makers: Arnold and Earnshaw made chronometers affordable
- Check: the disaster details, prize amounts, dates of the instruments and the trials, the lunar method's timeline
Next chapter: Railways, Time Zones and the Modern Clock, which covers railway time, Greenwich, standard time zones and the quartz and atomic clocks.