Dava Sobel · 1995

Longitude

The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time

Rough AI truth score

83/100

Harrison's timekeepers were extraordinary and central to practical longitude, but the subtitle's lone-genius story understates lunar methods, institutional support, replication, cost, and later makers.

Based on three central claims · medium-high confidence

The three claims

01Supported

Determining longitude at sea required either an accurate reference time or demanding astronomical observations that eighteenth-century navigation struggled to supply.

Longitude follows from the difference between local time and a reference meridian, but ship motion, temperature, friction, and imperfect clocks defeated ordinary timekeepers. Lunar distances became viable through instruments, tables, and almanacs, yet required more calculation and observation.

Royal Observatory: the quest for longitude Royal Museums Greenwich: Harrison's clocks

02Supported

John Harrison's marine timekeepers demonstrated that sufficiently accurate shipboard clocks could determine longitude.

Harrison's sequence from H1 through H4 culminated in a watch that met the Board's accuracy standard on sea trials and established the technical principle. Practical mass adoption depended on later, cheaper chronometers and continued trials rather than H4 alone.

Royal Museums Greenwich: Harrison's clocks Royal Museums Greenwich: Board of Longitude explainer

03Mixed

A hostile scientific establishment withheld recognition from a lone craftsman whose clock straightforwardly solved the longitude problem.

Harrison endured genuine conflict over tests, disclosure, replication, and payment, and ultimately appealed to Parliament. Historians show that the Board also gave him exceptional long-term financial support, that lunar methods worked, and that expensive prototypes were not yet a fleet-wide solution.

Challenging Tropes: Genius, Heroic Invention, and the Longitude Problem Prizes, patents and the search for longitude

Other claims worth checking

The 1707 Scilly disaster helped create political urgency for a longitude reward.
Nevil Maskelyne's role was more complex than the book's antagonist framing.

What this number means. It is an AI-generated first-pass judgment of three central factual or causal claims—not a rating, exhaustive fact-check, or human peer review. Claim credits are 100% for supported, 75% for mostly supported, 50% for mixed, and 25% for weak, then averaged and rounded. Lower confidence means the score should move more as better evidence arrives.

Method three-central-claims/0.1.0 · checked 2026-09-01 · 3/3 selected claims assessed · method and source audit