A pendulum clock ran about 2½ minutes a day slow near the equator
#0013 Science & nature
Fact rank
A
A dictionary of scientific biography and a paper both quote Newton's Principia giving "2 min 28 s a day". Not S because the Principia itself was not checked directly
Wow rank
S
Surprising that a slow clock revealed the shape of the Earth
Usefulness
—
A fact worth knowing for its own sake
Verification: Supported by evidence
Rule check: an article whose "why" cannot be written with sources is capped at fact rank B.
AI verification reports the evidence it found; it does not guarantee that a claim is true.
In 1672 a pendulum clock set in Paris was taken to Cayenne in South America, near the equator, and it lost 2 minutes 28 seconds a day. Gravity is weaker at the equator, so the pendulum swings more slowly; Newton cited this as evidence that the Earth bulges at the equator.
Why
(No sourced explanation yet.)Evidence
Secondary Dictionary of Scientific Biography, entry "Richer". Quotes Principia Book III Proposition 20: in 1672 at Cayenne the clock lost 2 min 28 s a day; gravity is weaker at the equator and the Earth is a spheroid flattened at the poles link
Secondary A paper on the history of gravity measurement (arXiv). Quotes the same passage by Newton (2 min 28 s a day); Paris at 48.86°N, Cayenne at 4.94°N. From normal gravity and the latitude difference alone the loss would be about 2 min 8 s a day (checked by calculation) link
Notes and limits
"2 min 28 s" is Newton's figure; Richer's own measurement (the pendulum had to be shortened by about 1.25 lignes, about 2.8 mm) implies a loss of about 2 minutes, slightly less. Gravity is weaker at the equator for two reasons: the centrifugal force of the rotation, and the equator being farther from the Earth's centre. The English Wikipedia article "Jean Richer" tells the same story.
Derivations
Derived from: 体重計の数字は緯度が高いほど大きく出る
Derived facts: 地球の中心から一番遠い山頂はエベレストでなくチンボラソ