Re: [asa] geocentricity

  Randy,

  In Galileo's time, I believe the most important two pieces of evidence
were: the phases of Venus and parallax.

  Ptolemy's geocentric model and Copernicus' heliocentric model made
similar predictions about lots of observations, but they made very
different predictions about how Venus would appear from Earth. Like the
Moon, Venus is a sphere and it is visible because it reflects light from
the Sun. The Moon looks like a disk to us. Venus is farther away than
the Moon, so without a telescope Venus looks like a point of light. But
through a telescope Venus appears as a disk, just like the Moon. And like
the Moon, Venus can have phases, appearing full, new, or crescent,
depending on whether the sunlit side of Venus is mostly facing towards or
away from Earth.

  In Ptolemy's geocentric model, Venus orbits the Earth and is forced to
stay between the Earth and the Sun at all times. Thus, in the geocentric
model, the sunlit side of Venus would always face mostly away from Earth
and Venus should always appear as a crescent, never full. In the
heliocentric model Venus orbits the Sun, so that it is sometimes between
the Earth and the Sun and appears as a crescent. But at other times its
orbit will take it to the far side of the Sun, so Venus' sunlit side will
be facing the Earth and therefore appear nearly full. So in the
heliocentric model (but not in the geocentric model) we would see a nearly
full Venus once each orbit.

  What Galileo observed was that Venus moved between full phase and
crescent phase in a cycle that matched the prediction of the heliocentric
model. Other scientists of Galileo's time became similarly convinced by
this data.

  There is a third model of the solar system in which the earth is fixed,
the sun goes around the earth, and all the other planets go around the
sun. This model predicts correctly the phases of Venus like the
heliocentric model.

  The real clincher for heliocentrism is parallax due to the Earth's
motion. In Galileo's time, the stars were thought to be relatively close
to the Earth (just beyond Saturn), in which case, if the earth moved,
stars should show parallax. Galileo didn't see any. He knew this was a
problem for the heliocentric model. Galileo kept the heliocentric model,
despite his failure to observe parallax, and asserted that the stars must
be farther away. He was right, of course. It took significant advances
in telescope technology before this parallax was finally measured in 1838.
But even before then, many scientists became convinced of the heliocentric
model by other discoveries, such as the phases of Venus and Newton's law
of gravity.

Loren Haarsma

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Received on Thu Jul 5 10:54:40 2007