Where the facts come from
Every factual claim on this site links to its source. This is the full list.
How we checked
Each fact was written in our own words alongside its source. Our build script stores the text of every source and, for each fact, checks that the key words and figures really appear in it. 146 of 146 checks passed when this site was last built (13 November 2026). Where we couldn’t find a named source saying something, we left it out. Where good sources disagree, we say so and give both.
Where sources differ
- When Galileo noticed the pendulum. About 1581 (Britannica, clock), c. 1583 with the lamp in Pisa cathedral (Britannica, pendulum), as early as 1582 (NIST).
- Who invented the pendulum clock. Some authorities say Galileo, others Huygens (Britannica); Huygens made the first in 1656, Galileo sketched one but never built it (NIST).
- The anchor escapement. Introduced by English clockmakers with the seconds pendulum in 1670 (Britannica), or built by William Clement in London from 1671 (NIST).
- The first spring-driven clocks. About 1450, probably in southern Germany or northern Italy (Britannica, clock); Peter Henlein of Nuremberg, 1500 to 1510 (NIST); the first watches shortly after 1500 (Britannica, watch).
- The balance spring. Hooke in the late 1650s, not shown to be a spiral, and Huygens probably first with a spiral in 1674–75 (Britannica); Huygens around 1675 (NIST).
- The first mechanical clock. Dunstable Priory, 1283, the oldest known (Britannica); Salisbury Cathedral, 1386, the oldest surviving in England (Britannica); Italian city towers in the first half of the 14th century (NIST).
- Su Song’s tower. 35 ft, 11 m (Britannica) or over 30 feet (NIST).
- Harrison and the prize. Won the 1714 prize (NIST); half the reward recommended in 1765, more tests, and a reward from Parliament in the end (Royal Museums Greenwich).
All sources (14)
- Encyclopaedia Britannica: Clock (37 facts)
- Encyclopaedia Britannica: Watch (17 facts)
- NIST, A Walk Through Time: A Revolution in Timekeeping (17 facts)
- Encyclopaedia Britannica: Escapement and Pendulum (16 facts)
- Encyclopaedia Britannica: Gear (16 facts)
- Royal Museums Greenwich: Longitude found, the story of Harrison’s timekeepers (13 facts)
- Encyclopaedia Britannica: Antikythera mechanism (9 facts)
- NIST, A Walk Through Time: Early Clocks (9 facts)
- Encyclopaedia Britannica: Su Song (3 facts)
- Royal Museums Greenwich: What made the search for longitude so important? (3 facts)
- Encyclopaedia Britannica: Christiaan Huygens (2 facts)
- Royal Museums Greenwich collections: H4 (2 facts)
- Encyclopaedia Britannica: Mainspring and 12-hour clock (1 fact)
- Royal Museums Greenwich: John Harrison’s marine timekeepers (1 fact)