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The challenge now was to find
observations to prove that one
theory was correct and the other
false. The discovery of moons
around Jupiter was great support for
a sun-centered system. It was now
clear that everything did not orbit
around a central Earth, but there
were still unanswered questions.
If the sun-centered system was
correct, Earth must be moving. If
Earth had to travel around the sun
every year, it had to have an orbital
speed of 20 miles/sec (30 km/sec).
In Galileo’s time, the exact distance
from Earth to the sun was not
known, but it was clearly far enough
that Earth would need to be moving
quickly, and humans cannot
apprehend this movement. Also,
this orbital motion should make the
stars appear to swing from side to
side every year in a phenomenon
called stellar parallax (p.102). This
again was not observed at the time.
Galileo and his contemporaries
did not suspect that the typical
distance between stars in the
Milky Way was about 500,000
times larger than the distance
between Earth and the sun, which
makes stellar parallax so small that
it is difficult to measure. It was not
until the mid-19th century that
vastly improved instruments made
it possible to detect this swing.
Despite these questions,
Galileo considered that his findings
had proved Copernicus correctTHE TELESCOPE REVOLUTION
Galileo Galilei Galileo Galilei was born in Pisa,
Italy on 15 February 1564. He
was appointed to the Chair of
Mathematics at the University
of Pisa in 1589, moving to the
University of Padua in 1590.
Galileo was an astronomer,
physicist, mathematician,
philosopher, and engineer,
who played a pivotal role in the
process of intellectual advances
in Europe now known as the
Scientific Revolution.
He was the first person to
effectively turn the refractor
telescope on the heavens.
During 1609–10, he discoveredthat the planet Jupiter had
four moons, Venus underwent
phase changes, the moon was
mountainous, and the sun was
spinning round once in about
a month. He was a prolific
writer and made his findings
accessible to a wide audience.Key works1610 The Starry Messenger
1632 Dialogue Concerning the
Two Chief World Systems
1638 The Discourses and
Mathematical Demonstrations
Relating to Two New SciencesStarting with the closest to Jupiter,
the Galilean moons are, from left
to right, Io, Europa, Ganymede, and
Callisto. Ganymede is larger than
the planet Mercury.beyond reasonable doubt. His
discoveries also included the
phases of Venus, which are best
explained if the planet is in orbit
around the sun, and the fact that
the sun is spinning, shown by the
movement of sunspots. By 1619,
Galileo’s pugnacious defense of
Copernicus had drawn him into
conflict with the Church, which had
declared in 1616 that heliocentricism
was heretical. In 1633, he appeared
before the Inquisition. His books
were banned, and he spent the last
10 years of his life under house arrest.New moons
Jupiter only had four known moons
for 283 years. A fifth satellite,
Amalthea, was discovered by the
American astronomer E. E. Barnard
in 1892, using the 36-in (91-cm)
refractor at the Lick Observatory
in California. It was the last solar
system satellite to be discovered
by direct observation. Subsequently,
satellites have been found by
the meticulous examination of
photographs. The number of known
Jupiter satellites had crept up to
12 by the mid-1950s, and has now
reached 67. Many smaller moons
may be found in the future. ■