Galileo Galilei, famous as an astronomer and as an experimental
physicist, was born at Pisa, in Italy, Feb. 18, 1564. His talents
were most multifarious and remarkable; but his mathematical and
mechanical genius was dominant from the first. As a child he
constructed mechanical toys, and as a young man he made one of his
most important discoveries, which was that of the pendulum as an
agent in the measurement of time, and invented the hydrostatic
balance, by which the specific gravity of solid bodies might be
ascertained. At the age of 24 a learned treatise on the centre of
gravity of solids led to a lectureship at Pisa University. Driven
from Pisa by the enmity of Aristotelians, he went to Padua
University, where he invented a kind of thermometer, a proportional
compass, a microscope, and a telescope. The last invention bore
fruit in astronomical discoveries, and in 1610 he discovered four
of the moons of Jupiter. His promulgation of the Copernican
doctrine led to renewed attacks by the Aristotelians, and to
censure by the Inquisition. (See Religion, vol. xiii.)
Notwithstanding this censure, he published in 1632 his "Dialogues
on the System of the World." The interlocutors in the "Dialogues,"
with the exception of Salviatus, who expounds the views of the
author himself, represent two of Galileo's early friends. For the
"Dialogues" he was sentenced by the Inquisition to incarceration at
its pleasure, and enjoined to recite penitential psalms once a week
for three years. His life thereafter was full of sorrow, and in
1637 blindness added to his woes; but the fire of his genius still
burnt on till his death on January 8, 1642.
_Does the Earth Move_
SALVIATUS: Now, let Simplicius propound those doubts which dissuade him
from believing that the earth may move, as the other planets, round a
fixed centre.
SIMPLICIUS: The first and greatest difficulty is that it is impossible
both to be in a centre and to be far from it. If the earth move in a
circle it cannot remain in the centre of the zodiac; but Aristotle,
Ptolemy and others have proved that it is in the centre of the zodiac.
SALVIATUS: There is no question that the earth cannot be in the centre
of a circle round whose circumference it moves. But tell me what centre
do you mean?
SIMPLICIUS: I mean the centre of the universe, of the whole world, of
the starry sphere.
SALVIATUS: No one has ever proved that the universe is finite and
figurative; but granting that it is finite and spherical, and has
therefore a centre, we have still to give reasons why we should believe
that the earth is at its centre.
SIMPLICIUS: Aristotle has proved in a hundred ways that the universe is
finite and spherical.
SALVIATUS: Aristotle's proof that the universe was finite and spherical
was derived essentially from the consideration that it moved; and seeing
that centre and figure were inferred by Aristotle from its mobility, it
will be reasonable if we endeavour to find from the circular motions of
mundane bodies the centre's proper place. Aristotle himself came to the
conclusion that all the celestial spheres revolve round the earth, which
is placed at the centre of the universe. But tell me, Simplicius,
supposing Aristotle found that one of the two propositions must be
false, and that either the celestial spheres do not revolve or that the
earth is not the centre round which they revolve, which proposition
would he prefer to give up?