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Though Newton regarded action at a distance as
absurd, he was unable to find a satisfactory physical
model that would lead to his inverse square law of
gravitation. According to that law, every particle in
the Universe attracts every other particle with a gravi-
tational force that is utterly unaffected by intervening
matter. Or, to put it succinctly, gravitation does not
cast shadows.

Light does cast shadows, however, and this indicates
that it is something propagated. That it has finite speed
is by no means obvious. Important men like Kepler
and Descartes believed its speed infinite. Galileo's
pioneering experiment to measure its speed was incon-
clusive, and the first evidence that its speed was finite
came in 1676, when Roemer, to account for annual
variations observed in the rhythm of the eclipses of
the innermost moon of Jupiter, proposed that light is
propagated “gradually.” Since astronomical data
available at the time implied, if Roemer was correct,
a speed of some 130,000 miles per second, the word
“gradually” may sound like an understatement. Rela-
tivity will reveal in it an unexpected irony.

For the most part, Roemer's idea met with little
favor, though Huygens and Newton were among those
who took it seriously. Not till 1728 was independent
corroboration found of the finite yet stupendous speed
of light. In that year Bradley deduced from the aberra-
tion
of light (tiny annual elliptic apparent motions of
the stars) a speed close to the currently accepted value
of some 186,300 miles per second. Since aberration has
an important role to play, we briefly describe its es-
sence. If we stand still, vertically falling rain falls on
our hat. If, remaining upright, we run forward, it
strikes our face. If we ran in a circle, the rain would
seem to come from an ever-changing direction always
somewhat ahead of us. Analogously, because of the
orbital motion of the earth, light from a star seems
to come to us from a position always somewhat ahead
of where we would see it if we were not orbiting. The
stars thus seem to move in tiny ellipses once a year,
and from the size of the effect Bradley calculated the
ratio of the orbital speed of the earth to the speed
of light.

The discovery that light has a finite speed was to
prove of world-shaking importance. It lies at the heart
of the modern theory of relativity, with all its conse-
quences. That the discovery came from astronomers,

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as did the basis of Newton's law of gravitation, under-
lines the enormous practical consequences of the
astronomers' seemingly ivory-tower pursuits.

Ingenious laboratory methods have been devised for
measuring the speed of light with extraordinary preci-
sion, but the details need not concern us. It suffices
to know that the speed is finite and can be measured
in the laboratory.