It had been discovered before the time of Bradley that the passage of
light through space is not an instantaneous phenomenon. Light
requires time for its journey. Galileo surmised that the sun may
have reached the horizon before we see it there, and it was indeed
sufficiently obvious that a physical action, like the transmission of
light, could hardly take place without requiring some lapse of time.
The speed with which light actually travelled was, however, so rapid
that its determination eluded all the means of experimenting which
were available in those days. The penetration of Roemer had
previously detected irregularities in the observed times of the
eclipses of Jupiter's satellites, which were undoubtedly due to the
interval which light required for stretching across the
interplanetary spaces. Bradley argued that as light can only travel
with a certain speed, it may in a measure be regarded like the wind,
which he noticed in the boat. If the observer were at rest, that is
to say, if the earth were a stationary object, the direction in which
the light actually does come would be different from that in which it
appears to come when the earth is in motion. It is true that the
earth travels but eighteen miles a second, while the velocity with
which light is borne along attains to as much as 180,000 miles a
second.
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