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How Longitude Was Solved — and Why It Took So Long

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@garagelab | 2026-05-10 15:25:16 |

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## The Problem

For most of maritime history, sailors could calculate latitude fairly easily — just measure the sun's angle at noon. Longitude was different. Without knowing *what time it was* at a reference point while simultaneously knowing local time, you couldn't know how far east or west you were.

Ships were getting lost. Entire fleets were being wrecked on rocks that weren't supposed to be there. In 1707, four British warships under Admiral Shovell struck the Scilly Isles — the crew had miscalculated their position. Two thousand men drowned.

## The Prize

In 1714, the British Parliament established the Longitude Prize — up to £20,000 (roughly £4 million today) for anyone who could solve the problem to within 30 nautical miles.

## The Solutions Proposed

**The astronomical approach** — use the position of the Moon against background stars as a cosmic clock. Mathematically rigorous but required complex calculations that took hours per fix. Championed by Astronomer Royal Nevil Maskelyne, who genuinely believed it was the only practical solution.

**The mechanical approach** — build a clock accurate enough to keep time at sea despite temperature changes, humidity, and the constant motion of a ship. This is what John Harrison spent his life on.

## Harrison's Answer

Harrison built four chronometers over 40 years. His fourth — the H4, completed in 1759 — was the size of a large pocket watch and kept time to within 5 seconds over 81 days at sea. The error translated to about 1.25 nautical miles. The required standard was 30.

He had solved the problem. It took him another decade and an intervention by King George III to actually receive his prize.

## Why It Matters

The accurate marine chronometer made precise global navigation possible. It's one of the enabling technologies of the colonial era, global trade, and ultimately the modern world map.

The GPS satellite in your phone is solving the same fundamental problem — synchronizing clocks across distance — using atomic oscillators in orbit instead of a spring mechanism.

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