The speed of light
Light moves with a speed
c = 3 x 108 m/s.
This speed was first measured 1n 1675 by Danish astronomer Olaus Roemer,
using observations of the
moons of Jupiter.
Here is what Roemer saw:
He carefully timed the motions of the moons in their orbits. In order
to get a consistent picture from the measurements, he needed to account
for the time it takes the light to get from Jupiter to Earth, which
varies according to how far away Jupiter is away.
(But this gives a pretty crude measurement of the speed of light,
since the distance to Jupiter was not well known.)
ASTR 122 course home page
Updated 1 October 2007
Davison E. Soper, Institute of Theoretical Science,
University of Oregon, Eugene OR 97403 USA