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Further evidence of a changing
cosmos came from Tycho’s
observation of the Great Comet
in 1577. Aristotle had claimed
that comets were atmospheric
phenomena, and this was still
generally believed to be the case in
the 16th century. Tycho compared
measurements of the comet’s
position that he had taken on Hven
with those that had been taken
at the same time by Bohemian
astronomer Thaddaeus Hagecius
in Prague. In both instances, the
comet was observed in roughly
the same place, but the moon
was not, suggesting that the
comet was much farther away.THE TYCHONIC MODEL
Tycho Brahe’s observatory complex
on the island of Hven attracted scholars
and students from all over Europe
between its founding in 1576 and
its closure in 1597.Tycho’s observations of the way the
comet moved across the sky over
the months also convinced him
that it was traveling through the
solar system. This overturned
another theory that had been
believed for the previous 1,500
years. The great Graeco-Egyptian
astronomer Ptolemy had been
convinced that the planets were
embedded in real, solid, ethereal,
transparent crystalline spheres,
and that the spinning of these
spheres moved the planets across
the sky. However, Tycho observed
that the comet seemed to move
unhindered, and he concluded that
the spheres could not exist. He
therefore proposed that the planets
moved unsupported through space,
a daring concept at the time.No parallax
Tycho was also very interested
in Copernicus’s proposition
that the sun, rather than Earth,
was at the center of the cosmos.
If Copernicus was right, the nearby
stars should appear to swing from
side to side as Earth traveled
annually on its orbit around the
sun—a phenomenon known as
parallax. Tycho searched hard,
but could not find any stellar
parallax. There were two possible
conclusions. The first was that the
stars were too far away, meaning
that the change in their position
was too small for Tycho to measure
with the instruments of the day.
(It is now known that the parallax
of even the closest star is about
100 times smaller than the typical
accuracy of Tycho’s observations.)
The second possibility was that