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Observing the occasion.

Once you get past a sesquicentennial - the felicitously fabricated designation for a 150th anniversary - half-century acknowledgments are awkward and contrived commemorations. Masquerading as milestones, they are missing that zero in the tens place that tells you we really have something to celebrate. Ordinarily, then, I would have let this month's 450th anniversary of the birth of that great Dane Tycho Brahe - on December 14th - pass without cake and candle. But I was too young in 1946 to appreciate the quadricentennial, and I suspect that the quincentennial in 2046 may be observed without me. If I'm still around, I'll be 102 and grateful to carpe any diem at all. So, seizing the half century, I have decided to salute Tycho now.

I am not the only one mindful of Tycho's midcentenary. A little over a year ago, two scholars in the Czech Republic - Alena Hadravova and Petr Hadrava - announced their intention to honor him with a translation into Czech of Tycho's treatise on his innovative instruments, Astronomiae instauratae mechanica. Tycho spent the last two years of his life in Prague, now the capital of the Czech Republic, and is entombed there in Tyn Church.

Tycho has been called the greatest observational astronomer who lived before the invention of the telescope, and his instruments are what qualify him for the title. He established Uraniborg, Europe's first national observatory, on the Baltic island of Hven in 1576 with support from Frederick II, the Danish king. He left his astronomical headquarters 21 years later, however, when Frederick's successor put Tycho on waivers. This kind of thing still seems to go on at national observatories.

Despite the reversal of royal fortunes, December's sky is committed to high-level acclaim for Tycho by lifting aloft Cassiopeia, the Queen. Tycho's careful surveillance of the 1572 supernova in that constellation helped solidify the value of accurate observation and precise measurement.

Cassiopeia is near the top of the sky during the early evening hours this month, and you can use the all-sky map on pages 70 and 71 to find it when you're toasting Tycho on the 14th. It's where the meridian - the blue vertical line that splits the map in half - crosses the Milky Way, a little north of the zenith at the chart's center. Cassiopeia presides overhead more or less as she did on November 11, 1572, when Tycho spotted her stars hosting the supernova that became known as Tycho's Star. He later wrote he was "contemplating, as usual, the celestial vault," territory he knew as well as his own neighborhood, when he saw "with inexpressible astonishment, near the zenith, in Cassiopeia, a radiant star of extraordinary magnitude."

Tycho didn't realize he was witnessing the explosive, catastrophic finale of a star. He actually thought the stella nova, or new star, had condensed out of the cloudy vapors of the Milky Way. For two weeks the supernova was brighter than any other star in the sky. Changing color as it gradually faded, it completely disappeared by spring 1574. Although the true protocols of stellar evolution eluded him, Tycho understood the enormous significance of the new star. Never mind its initial brightness - the star's mere presence was enough to astonish Tycho. It contradicted Aristotle's authoritative judgment that stars populate a pure, unchanging kingdom.

Tycho was not the first person to notice the supernova, but he observed it more carefully than anyone else. He not only chronicled its unprecedented performance but also confirmed its exemption from the daily parallax that displaces the Moon with respect to the more distant stars. It could not be a planet vagrantly adrift from the ecliptic, nor could it be a comet dashing through the sky. Aristotle's conviction that comets are atmospheric phenomena also prevailed in the 16th century, but when Tycho failed to detect any daily parallax in the position of the comet of 1577, he argued that comets also introduce change into the celestial zone. So careful measurement, rather than ancient authority, revealed nature. No wonder Tycho was astonished.

Dedicated to the "renovation" of technical astronomy - according to Victor E. Thoren, his most recent biographer - Tycho "created almost single handedly the empirical ethic on which modern astronomy is founded." He believed that the price of accurate knowledge is greater precision, so he engineered instruments accurate to one arcminute. These were four times better than those he had used to observe the comet and supernova and 10 to 15 times better than those of most of his predecessors.

Tycho's observing regimen reflected an appreciation of observational error. Rather than rely upon a single measurement, he performed up to seven determinations of fundamental data, reported them all, and let the uncertainty scatter in an honest display of the limits of his procedure and apparatus. The quality and integrity of Tycho's observations eventually permitted Johannes Kepler to formulate accurately his three laws of planetary motion as well as a representation of the solar system's dynamics. These, in turn, let Isaac Newton explain everything with gravity.

In a duel with a rival over competing assertions of personal mathematical preeminence, Tycho lost the bridge of his nose. The unhappy slice forced him to don a gold-and-silver alloy prosthetic, making him a genuinely hard-nosed scientist and not a melancholy Dane. Romantics often criticize hard-nosed analysis as an attempt to overpower nature with intellect. They also may regard scientific observation as too disengaged to appreciate intuitive understanding. For Tycho, however, indulging in untestable romantic allegations and idiosyncratic thought was like cutting off your nose to spite your face.

Tycho's disciplined approach to the acquisition and understanding of knowledge is encapsulated in the famous illustration of the mural quadrant he installed in Uraniborg. Although a portrait of Tycho dominates the painting nested in the curve of the quadrant, the background tells the real story of science. A mountain profile with the Sun lodged in a horizon notch fills the top register and illustrates nature, which not only runs the show but is the show. The sky is doing its usual work, unencumbered by human activity. Below that we see the upper level of Uraniborg. Its balustraded terraces support astronomical instruments representing observation and accurate measurement of the natural world. On the next floor down, staff members gather at tables. Accompanied by Tycho's five-foot celestial globe, they analyze data, the next step after acquisition. Finally, in the basement we see Tycho's alchemical laboratory. Although today we regard alchemy as an occult enterprise, in Tycho's time it was regarded as respectable inquiry. In the mural, alchemy provides a reference to experimentation.

Tycho's belief in the coherence of nature and the unity of the cosmos persuaded him that accurate, detailed knowledge of nature could harmonize human affairs with natural law through the rhythms of the heavens. His vision of cosmic order was reflected in the layout of Uraniborg. With corners aligned to the cardinal directions, it belonged to an archaic tradition that brought the cosmos down to Earth in symbolic architecture. The astronomy, geometry, and symmetry of its garden, towers, turrets, and spires proclaim the principles of celestial harmony and congruence Tycho judged could be teased from nature through observation.

If Tycho were still with us, he would see in this anniversary of his birth 450 circuits of the Sun around the Earth, not 450 orbits of the Earth around the Sun. The world system he devised put the planets in solar orbits but centered the path of the Sun on a stationary Earth. He thought this picture was the best interpretation of the data, but the system capitulated to Keplerian motion and universal gravitation.

It is now hard to find any celestial acknowledgments of four and a half centuries of Tychonic legacy. Tycho's supernova is long gone, and no one knows when, if ever, the comet of 1577 will come back. But the statue of Tycho installed near the ruins of his observatory lifts his face to the zenith. He observes Cassiopeia every time she sails by, and he probably caught Comet Hyakutake in the corner of his eye.

E. C. Krupp takes careful measure of Griffith Observatory in Los Angeles.
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Title Annotation:450th birth anniversary of astronomer Tycho Brahe
Author:Krupp, E.C.
Publication:Sky & Telescope
Date:Dec 1, 1996
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