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Limits RegistryDanish astronomer Ole Romer, in 1676, used systematic timing discrepancies in the observed eclipses of Jupiter's moon Io (which arrived later when Earth was farther from Jupiter in its orbit) to demonstrate that light has a finite speed and derive the first quantitative estimate, roughly 220,000 km/s — about 26% below the modern value, but the first empirical evidence that light's speed was finite and measurable at all.
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Danish astronomer Ole Romer, in 1676, used systematic timing discrepancies in the observed eclipses of Jupiter's moon Io (which arrived later when Earth was farther from Jupiter in its orbit) to demonstrate that light has a finite speed and derive the first quantitative estimate, roughly 220,000 km/s — about 26% below the modern value, but the first empirical evidence that light's speed was finite and measurable at all.
The first documented derivation of a finite, quantitative numerical value for the speed of light, as opposed to earlier debates over whether light propagated instantaneously.
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Limits Registry. LR-FIRST-MEASUREMENT-SPEED-OF-LIGHT. First quantitative measurement of the speed of light. 2026.