December 7, 1676

Rømer's determination of the speed of light

The Journal des sçavans publishes Rømer's finite-speed-of-light argument

Rømer told the Academy that Io would come out ten minutes late. On 9 November the Paris observation recorded exactly that delay.

7 December 1676: four pages in the Journal des sçavans

On 7 December 1676 the Paris Journal des sçavans published "Démonstration touchant le mouvement de la lumière trouvé par M. Roemer de l'Académie Royale des Sciences" across four pages, at pp. 233–236.

Rømer did not write it. The report was in the third person, assembled by an anonymous reporter from his oral presentation to the Academy.

The argument from a moon's timetable

The report set out why Io's apparent orbital intervals — the moon's period runs about 42.5 hours — should lengthen as Earth moved away from Jupiter and shorten as Earth moved toward it, if light needed time to cross the changing distance.

It cited the decisive test directly: Io's emergence observed in Paris on 9 November came at 5:35:45, ten minutes later than a prediction based on August observations, as Rømer had forecast to the Academy.

What the number actually was

The published result was a travel time, not a speed: about 22 minutes for light to cross the diameter of Earth's orbit. The modern figure for that same crossing is 16 minutes 40 seconds.

Nothing in the report gave a value in distance per second, though it did bound light at about 3,000 Paris leagues in under a second. Publication produced no consensus — Cassini and others remained skeptical — but it handed other astronomers a testable correction for their Io eclipse tables.

Sources

Researched 23 Aug 2026 7 sources 2 audit passes

How this was checked

Researched from the web into a fact sheet, rewritten from that sheet, then audited against it by a different model. How the pipeline works →

What the sources leave uncertain

The exact 7 December event was publication, not the original observation or discovery. The crucial confirming observation was 9 November, and surviving Academy minutes place Rømer’s reading on 21 November; some later accounts give 22 November.
Rømer’s report supplied a travel-time ratio, not a numerical value of c in modern distance-per-second units. Christiaan Huygens subsequently performed that arithmetic.
The historical division of credit between Rømer and Cassini has been debated. A 2019 reanalysis of primary sources concluded that the discoveries were Rømer’s, while older accounts give Cassini a larger preliminary role.