On 7 January 1610, Galileo Galilei (1564-1642), professor of mathematics
at the University of Padua, aimed a new device of some glass lenses
mounted in a piece of organ pipe-he called it a “perspicillum”-toward
the southeastern sky. There, to the upper right of the gibbous Moon, was
Jupiter, a bright dot of light.
With his telescope yielding
about 20x magnification, Galileo noted three little stars near Jupiter,
one to the west and two to the east of it. The next night all three lay
to the west of Jupiter. Two nights later, he saw only two stars, both to
the east. This went on for two weeks before Galileo realized he was
seeing a total me four worlds orbiting Jupiter. No one had ever seen
these bodies before. Today, we know them as the Galileo moons (ganymede,
io, callisto, europa)
by the 1670s, astronomers had determined
the periods of revolution for the Galilean moons to within seconds of
their modern values. But the predictions for when a moon would enter or
leave Jupiter’s shadow were often up to several minutes in error. These
events seemed to occur earlier when Earth was nearer the Jovian system
and later when it was farther away from it. In 1675, Danish astronomer
Ole Römer realized these errors arose from the fact that the speed of
light was not infinitely quick. As a result of this study involving the
Galilean moons, the first scientific determination for the speed of
light was made.
Rabu, 05 September 2012
