Message #74557

[Bianca]

From September 1610, Galileo observed that Venus exhibited a full set of phases similar to that
of the Moon.

The heliocentric model of the solar system developed by Nicolaus Copernicus predicted that all
phases would be visible since the orbit of Venus around the Sun would cause its illuminated hemi-
sphere to face the Earth when it was on the opposite side of the Sun and to face away from the
Earth when it was on the Earth-side of the Sun.

In contrast, the geocentric model of Ptolemy predicted that only crescent and new phases would
be seen, since Venus was thought to remain between the Sun and Earth during its orbit around the Earth.

Galileo's observations of the phases of Venus proved that it orbited the Sun and lent support to
(but did not prove) the heliocentric model.

Galileo also observed the planet Saturn, and at first mistook its rings for planets, thinking it was a three-bodied system. When he observed the planet later, Saturn's rings were directly oriented at Earth, causing him to think that two of the bodies had disappeared. The rings reappeared when he observed the planet in 1616, further confusing him.

Galileo was one of the first Europeans to observe sunspots. He also reinterpreted a sunspot observation from the time of Charlemagne, which formerly had been attributed (impossibly) to a
transit of Mercury. The very existence of sunspots showed another difficulty with the unchanging perfection of the heavens as assumed in the older philosophy. And the annual variations in their motions, first noticed by Francesco Sizzi, presented great difficulties for both the geocentric system and that of Tycho Brahe.

A dispute over priority in the discovery of sunspots, and in their interpretation, led Galileo to a long
and bitter feud with the Jesuit Christoph Scheiner; in fact, there is little doubt that both of them
were beaten by David Fabricius and his son Johannes. Scheiner quickly adopted Kepler's 1615 pro-
posal of the modern telescope design, which gave larger magnification at the cost of inverted images; Galileo apparently never changed to Kepler's design.

Galileo was the first to report lunar mountains and craters, whose existence he deduced from the patterns of light and shadow on the Moon's surface. He even estimated the mountains' heights from these observations. This led him to the conclusion that the Moon was



                          "rough and uneven, and just like the surface of the Earth itself,"


rather than a perfect sphere as Aristotle had claimed.

Galileo observed the Milky Way, previously believed to be nebulous, and found it to be a multitude
of stars packed so densely that they appeared to be clouds from Earth.

He located many other stars too distant to be visible with the naked eye.

Galileo also observed the planet Neptune in 1612, but did not realize that it was a planet and took
no particular notice of it. It appears in his notebooks as one of many unremarkable dim stars.