Marine chronometer
A ship on the open sea knew how far it was from the equator, but not how far it was from home. The longitude problem was solved not by an astronomer but by a village carpenter with a clock.
- 15° = 1 hourhow much longitude the Sun covers in an hour
- £20,000the prize set by Parliament in 1714
- 5 secondsH4's error over 62 days to Jamaica
A night off the Scilly Isles
On 22 October 1707 Admiral Shovell's fleet was sailing home. The navigators believed the ships were safely west of Brittany; in fact they were heading straight for the rocks off the south-western tip of England.
Four ships were wrecked in a single night and around fourteen hundred sailors drowned. The fault lay neither in courage nor in charts. Nobody on board knew the longitude.
Latitude yes, longitude no
Latitude is easy: measure the height of the Sun at noon or of the Pole Star at night. Longitude doesn't work that way: the Earth turns, and the sky over every meridian looks the same, only at different times.
In 1714 Parliament offered £20,000 to whoever could find longitude on a voyage to the West Indies to within half a degree. For a clock that means drifting no more than three seconds a day.
A clock that remembers Greenwich
The Earth turns 15° an hour. Take a clock that keeps the time of your home port, look at it at local noon, and every hour of difference means 15° of longitude.
longitude = (TGreenwich − 12:00) × 15°/hThe hands in the drawing run on real Greenwich time, right now.
A pendulum is useless at sea
A ship's motion swings a pendulum harder than gravity does, and pendulum clocks at sea drift by minutes. A chronometer has a balance instead: a wheel with a spiral spring that swings to and fro twice a second.
So the seconds hand doesn't glide but jumps by half seconds. Look closely at the small dial at the bottom.
The box rolls, the clock doesn't
The brass bowl holding the movement hangs in a ring on two pins, and the ring hangs in the box on two more, at right angles. The bowl's heavy bottom always pulls it down.
The box heels over with the ship while the dial stays level. Move the pointer to the right for a rougher sea.
A strip of two metals
In the tropics the balance spring weakens and the clock loses. John Harrison invented the bimetallic strip for this: brass and steel riveted together bend as they warm and correct the rate by themselves.
His fourth timekeeper, H4, came out the size of a large pocket watch: 13 centimetres and about a kilogram and a half. The first three weighed tens of kilograms. Boxed chronometers like the one in the drawing were its descendants at the end of the 18th century.
Portsmouth, 18 November 1761
Harrison is 68, and his son William sails in his place. HMS Deptford sets course for Jamaica; H4 is wound every day and kept locked in a cabin.
Move the slider under the chart to sail the ship yourself. At every point of the voyage the longitude is worked out from the chronometer and the noon Sun, just as William worked it out.
An island right on time
By dead reckoning the ship's navigator was out by almost a degree and a half. From the watch, William predicted that the island would be sighted the next morning. In the morning Madeira rose over the horizon.
Three and a half hours apart
In mid-ocean the Sun reaches its highest point when the chronometer already reads half past three. Every 15° of westing takes exactly an hour off local time, and the meridians on the chart are labelled with those hours.
Five seconds in two months
At Jamaica H4 was 5.1 seconds slow. That is 1¼ minutes of longitude, about two kilometres. The law allowed an error more than twenty times larger.
The Board of Longitude called it luck and ordered a second trial. Harrison received the full prize only in 1773, at the age of 80, after King George III intervened.
What came next
James Cook took a copy of H4, made by Larcum Kendall, on his second voyage round the world and called it "our trusty friend the Watch". By the mid-19th century every naval ship and nearly every merchant ship carried a chronometer, and the Greenwich meridian became zero for the whole world.
Chronometers gave way to radio time signals only in the 20th century, and then to GPS satellites. Every one of those satellites carries clocks too, only atomic ones.