Answer 2:
Here are some good web
sites.
Original
by Philip Gibbs 1997.
How is the speed of light
measured? The speed of light in vacuum c is not
measured. It has an exact fixed value when given
in standard units. Since 1983 the meter has been
defined by international agreement as the length
of the path traveled by light in vacuum during a
time interval of 1/299792458 of a second. This
makes the speed of light exactly 299,792.458 km/s.
Since the inch is defined as 2.54 centimeters, the
speed of light also has an exact value in
nonmetric units. This definition only makes sense
because the speed of light in vacuum is constant;
a fact which is subject to experimental
verification (see relativity FAQ article Is the
speed of light constant?). Experiments are still
needed to measure the speed of light in media such
as air and water.
Before the seventeenth
century it was generally thought that light is
transmitted instantaneously. This was supported by
the observation that there is no noticeable lag in
the position of the Earth's shadow on the moon
during a lunar eclipse as would be expected if c
is finite. Now we know that light is just too fast
for the lag to be noticeable. Galileo doubted that
light speed is infinite and described an
experiment to measure its speed by covering and
uncovering lanterns observed at a distance of a
few miles. We don't know if he really attempted
the experiment, but again c is too high for such a
method to work.
The first successful
measurement of c was made by Olaus Roemer in 1676.
He noticed that the time between the eclipses of
the moons of Jupiter was less as the distance away
from Earth is decreasing than when it is
increasing. He correctly surmised that this is
due to the varying length of time it takes for
light to travel from Jupiter to Earth as the
distance changes. He obtained a value equivalent
to 214,000 km/s which was very approximate because
planetary distances were not accurately known at
that time.
In 1728 James Bradley made
another estimate by observing stellar aberration,
being the apparent displacement of stars due to
the motion of the Earth around the Sun. He
observed a star in Draco and found that its
apparent position changed during the year. All
stellar positions are affected equally in this
way. This distinguishes the effect from parallax
which affects nearby stars more noticeably. A
useful analogy to help understand aberration is to
imagine the effect of motion on the angle at which
rain falls. If you stand still in the rain when
there is no wind it comes down vertically on your
head. If you run through the rain it appears to
come at you from an angle and hit you on the
front. Bradley measured this angle for
starlight. Knowing the speed of the Earth around
the Sun he found a value for the speed of light of
301,000 km/s.
The first measurement of c on
Earth was by Armand Fizeau in 1849. He used a beam
of light reflected from a mirror 8 km away. The
beam passed through the gaps between teeth of a
rapidly rotating wheel. The speed of the wheel was
increased until the returning light passed through
the next gap and could be seen. Then c was
calculated to be 315,000 km/s. Leon Foucault
improved on this a year later by using rotating
mirrors and got the much more accurate answer of
298,000 km/s. His technique was good enough to
confirm that light travels slower in water than in
air.
After Maxwell published his theory of
electromagnetism it became possible to calculate
the speed of light indirectly from the magnetic
permeability and electric permittivity of free
space. This was first done by Weber and Kohlrausch
in 1857. In 1907 Rosa and Dorsey obtained 299,788
km/s in this way. It was the most accurate value
at that time.Many other methods were employed to
improve accuracy further. It soon became necessary
to correct for the refractive index of air. In
1958 Froome had the value of 299,792.5 km/s using
a microwave interferometer and a Kerr cell
shutter. After 1970 the development of lasers with
very high spectral stability and accurate cesium
clocks made even better measurements possible. Up
until then the changing definition of the meter
had always kept ahead of the accuracy in
measurements of the speed of light. Then the point
was reached where the speed of light was known to
within an error of plus or minus 1 m/s. It became
more practical to fix the value of c in the
definition of the meter and use atomic clocks and
lasers to measure accurate distances
instead.
Hope this helps.
