1676

Speed of light

• Galileo had suggested
• using eclipses of the moons of Jupiter,
• as a celestial clock to determine longitude.
• Ole Rømer noted
• the times between eclipses of Io
• varied by the time of year,
• increasing as our distance to Jupiter increased
• and decreasing as our distance decreased.
• Realizing this meant that the speed of light is finite,
• Rømer calculated it using accumulated difference
• between actual and predicted eclipse times.

Historical speeds

• Rømer’s result in 1676
• was 26.6% slower than the true speed of light.
• James Bradley’s result in 1729
• was 0.4% faster.
• Hippolyte Fizeau’s result in 1849
• was 5.07% faster.
• Léon Foucault’s result in 1862
• was 0.6% slower.
• Rosa and Dorsey’s result in 1907
• was less than 0.03% slower.

In a vacuum

• It’s somewhat crude today to say
• outer space is a perfect vacuum.
• It’s a sea of all kinds of things.
• And the speed of light in an atmosphere
• depends on altitude, temperature,
• and whether the light’s continuous or pulsed.
• Do absolutes exist in real life?
• We can never judge nor be judged
• in a vacuum. Take it with a grain of salt.