Tycho and Kepler Read Online Free

STRONOMERSIN THE ERA when Tycho lived thought of their subject as being separated into two parts, described as the
primum
and
secundum mobile
. The
primum
dealt with the way the celestial sphere as a whole “rose” and “set” every night, and the fact that the particular portion of that celestial sphere visible at night changes throughout the year in a regular annual cycle. One needed trigonometry, the mostadvanced form of mathematics then known, to understand these phenomena in detail, so classroom discussions usually took place on a more general, qualitative level. The
secundum mobile
, involving planetary positions and motions, did require trigonometry.
    Typical study of the
secundum mobile
began with Euclid’s
Geometry
, a work that had endured since around 300 B.C . (Euclidean geometry is stilltaught in basic geometry classes.) From there the course went on to trigonometry and planetary theory. In Tycho’s university years, planetary theory still meant theory according to Ptolemaic astronomy.
    When Greek and Alexandrian scholars such as Aristotle, Hipparchus, and Claudius Ptolemaeus (known as Ptolemy) peered at the night sky, they saw virtually the same panorama that is visible withthe naked eye on a clear night now, far enough away from city light. Thirteen centuries after Ptolemy, Copernicus and Tycho Brahe also had no other view than that, for they too lived before the advent of the telescope. Ancient sky-watchers, by scrutinizing the sky with care over long periods of time, had discovered that the motions of the heavenly bodies are not random. Stellar and planetary movementis intricate, but it was possible to calculate well in advance what paths these objects would take and where they would be at a future time. Close observers knew early on that though change, chance, and whim seem to be the rule on Earth, the heavens perform a complex but predictable dance. That dichotomy became a key part of the ancient and medieval worldview.
    The best way to describe andexplain what one observed in the skies with the naked eye was to think of Earth as the center, with everything else moving around it. That concept still works admirably for purposes of navigation. In fact, to think that things might operate differently demands a leap of fancy that would seem ludicrous to anyone not steeped since childhood in Sun-centered astronomy.
    Early astronomers knew,however, that there are phenomena that one observes looking at the sky with greater care over a period of time that seem at odds with a system in which Earth is the center and everything else is in motion around it. Rather than decide that these glitches were significant and stubborn enough to require one to discard the Earth-centered view of the universe entirely and look for another, they choseto attempt to explain the glitches, if they could,
within
an Earth-centered system. Ptolemy’s success in doing so was one of the most impressive intellectual achievements in history.
    Ptolemy did not begin with a tabula rasa in the second century A.D . by gazing up at the night sky as it appeared to him from near the mouth of the Nile at Alexandria. Instead, he drew together the results of centuriesof previous speculation and observation and pondered all of this afresh, applying his own superb mathematical talents. The result, set down in his
Almagest
and other works, was a cohesive explanation of the cosmos that endured and dominated Islamic and, later, Western thinking for fourteen centuries. Finally, even as it was rejected, it provided the springboard for Copernican astronomy and allthat has followed from that.
    Part of the intellectual worldview of the era in which Ptolemy lived was that the actual appearance of things had to be taken into account in trying to figure out what constitutes “reality.” To be plausible, an explanation had to “save the appearances,” not contradict them. Though in the early seventeenth century, after Tycho’s death, some Ptolemaic