# Light - We know the speed of light is constant because of orbits ## Romer's Observations - 18th century - Romer - Observing Galilean moons - 2,4,8,16 day period - Weren't quite constant - Ran early when Jupiter and Earth were on the same side of the sun and ran late when Jupiter and Earth were on opposite sides of the Sun (further apart) - Conclusion - light took longer to reach us, meaning the speed of light is finite and not instantaneous - From his observations, found an estimate within 25% of the accepted value today - Controversial at the time! - [Black Body Radiation](Black%20Body%20Radiation.md) - # References 1. # Footnotes