Finite speed of light
Velocidad finita de la luz
- Date
- 1676 · confidence high
- Place
- Paris Observatory, France
- Credited to
- Ole Rømer
- People
- 👤 Ole Rømer1644–1710
- Field
- Communication
What was discovered
Delays in the eclipses of Io, depending on the Earth-Jupiter distance, show that light takes about 22 minutes to cross the Earth's orbit: light is not instantaneous. The first measurement of a fundamental constant of nature.
Cassini, head of the observatory, rejected the interpretation; Huygens accepted it and calculated the value of c. Merged with 'Finite speed of light from the eclipses of Io' (duplicate, 23-09-2026).
What it led to
- The constant c as a universal constant
- Stellar aberration (Bradley, 1729)
- Terrestrial measurements of c (Fizeau, Foucault)
- Special relativity
- Concept of distance in light-time
- Aberration of light
- Optics and relativity
Built on
- Galilean moons1610
- Cassini's tables of Jupiter's satellites
- Galileo's telescope
Made possible
- Wave theory of light and Huygens' principle1678-1690
- Aberration of starlight1729
- Time-of-flight distance measurement methods
- Clock synchronisation by light signals
- Laboratory measurement of c (Fizeau, Foucault)