The Observations of
As we have noted, modern astronomy is built on the interplay between
observations and testable theories that attempt to account for those
observations in a logical and mathematical way.
A crucial ingredient in the Copernican revolution was the acquisition of more
precise data on the motions of objects on the celestial sphere.
Precise Observations before the Invention of the Telescope
Tycho Brahe (1546-1601), made important contributions by devising the
most precise instruments available before the invention of the telescope for
observing the heavens. Brahe made his observations from
an island in the sound between
Denmark and Sweden called Hveen.
instruments of Brahe allowed him to determine more precisely than had been
possible the detailed motions of the planets. In particular, Brahe compiled
extensive data on the planet Mars, which would later prove crucial to
his formulation of the laws of planetary motion because it would be
sufficiently precise to demonstrate that the orbit of Mars was not a circle but
The Life and Times of Tycho Brahe
Brahe was by all accounts an extremely colorful character. He allegedly
challenged a fellow student to a duel with swords in a dispute over who was the
better mathematician. Brahe's nose was partially cut off, and he was said to
gold and silver replacement
he would continually rub oil. He fell out of favor when a new King came to
power in 1588, and moved to Prague shortly thereafter.
This is of great historical significance
because this move would eventually make Brahe's data available to
Kepler, who went to Prague also to become Brahe's assistant.
Brahe is thought to have died when he contracted a urinary infection
while attending a banquet hosted by a baron in Prague
in which he drank extensively but felt that etiquette
prevented him from leaving the table to relieve himself before the host left.
Summary of Brahe's Contributions
Among the important contributions of Brahe:
Thus, Brahe's ideas about his data were not always correct, but the quality
of the observations themselves was central to the development of modern
- He made the most precise observations that had yet been made by devising
the best instruments available before the invention of the telescope.
- His observations of planetary motion, particularly that of Mars,
provided the crucial data for later astronomers like Kepler to construct our
present model of the solar system.
- He made observations of a
(literally: nova= "new star")
(we now know that a supernova is an exploding star,
not a new star). This was a "star"
that appeared suddenly where none had been seen before, and was visible for
about 18 months before fading from view. Since this clearly represented a
change in the sky, prevailing opinion held that the supernova was not really a
star but some local phenomenon in the atmosphere (remember: the heavens were
supposed to be unchanging in the Aristotelian view). Brahe's meticulous
observations showed that the supernova did not change positions with respect to
the other stars (no parallax).
Therefore, it was a real star, not a local object. This was
early evidence against the immutable nature of the heavens, although Brahe did
not interpret the absence of parallax for stars
correctly, as we discuss below.
- Brahe made careful observations of a comet in 1577. By measuring the
parallax for the comet, he was able to show that the comet was further away
than the Moon. This contradicted the teachings of Aristotle, who had held that
comets were atmospheric phenomena ("gases burning in the atmosphere"
was a common
explanation among Aristotelians). As for the case of the supernova,
comets represented an obvious
change in a celestial sphere that was supposed to be
unchanging; furthermore, it was very difficult to ascribe uniform circular
motion to a comet.
- He made the best measurements that had yet been made in the search for
stellar parallax. Upon finding no parallax for the stars, he
(correctly) concluded that either
- the earth was motionless at the center
of the Universe, or
the stars were so far away that their parallax was too
small to measure.
Not for the only time in human thought, a great thinker
a pivotal question correctly,
but then made the wrong choice of possible answers:
did not believe that the stars could possibly be so far away and
so concluded that the Earth was the center
of the Universe and that Copernicus was wrong.
- Brahe proposed a model of the Solar System that was intermediate between
the Ptolemaic and Copernican models (it had the Earth at the center). It
proved to be incorrect, but was
the most widely accepted model of the Solar System for a time.