The Impact of New and Better Data

Key points: Impact of new technology on astronomy; value of accurate, homogeneous observations

As Europeans emerged from the "Dark Ages," their technical skills improved. This painting of an astronomer (by Vermeer) shows the progress. The astronomer is looking at a book and an accurately rendered celestial globe, made by Jodocus Hondius in 1600. On the table, on a rich oriental tapestry, is an astrolabe, precursor of the large instruments developed by Tycho Brahe to measure planetary positions. ("The Astronomer," by Vermeer, photo by G. Rieke) astronomer.jpg (33550 bytes)
Tycho Brahe, a Dane, worked from ~ 1570 - 1601 and built the most accurate naked eye observatory ever. To the right, he is in discussion with his patron Rudolf II (seated).

(to left, From A. Van Helden

to right from Eduard Ender

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Tycho's great mural (wall-mounted) quadrant Tycho had huge instruments made to high accuracy to allow observers to sight positions accurately (from Landskrone official Tycho website and Astronomy, by Fred Hoyle). He was able to measure angles and hence positions of stars to an accuracy approaching 1 arcmin, equivalent to 1/30 the diameter of the full moon. Engraving of a large armillary (for sighting)

In addition, the first reasonably accurate clocks had been invented a few decades earlier and helped solve the age-old problem of relating observations to correct time.

Spring-Powered Clock by Peter Henlein, ~ 1530, from


In addition to making observations of unprecedented accuracy, Tycho also developed the concept of attaching an uncertainty to each of his measurements -- he would give a value and then state by what amount it could be wrong. This innovation made it possible to judge whether a model's disagreement with an observation is significant or not. See and for more about this strong personality.

Tycho observed the planets over a long number of years, and hoped to solve the problem of exactly how they move. He vehemently rejected the Copernican model because he had no sense that the earth moved and, more importantly, he could not detect parallax in his observations. He proposed a variant in which the earth is fixed, the moon and sun orbit it, and all the other planets orbit the sun. He made no effort to calculate planetary positions from his model to compare with his data.

tychomod.jpg (700348 bytes) Tycho's drawing to the left.. Mercury and Venus (and a comet) orbit the sun, marked with C and whose orbit is  labeled "SOL". The sun and the moon orbit the earth (from Landskrone official Tycho website and
Comparison of Tycho's and Copernicus' models

Tycho's model is geometrically identical to Copernicus's!! It is just a matter of changing the definition of what is fixed -- the sun or the earth. (Scientific American)

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Tycho's careful observations of the great comet of 1577 established that it (and presumably all comets) was above the atmosphere and part of the solar system. Here is a Turkish painting of the comet to the left and a woodcut made in Prague to the right. (from and Erol Pakin, Director, Istanbul Universitesi Rektorlugu and from the Intitut fur Planetforschung,
Another important event during Tycho's career was the explosion of a nearby star into a supernova. This event shattered the dogma that the heavens were unchanging. (From Flammarion's Astronomie Populaire (1880), reproduced in Sky and Telescope, November 1998

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copernicuscase.jpg (15819 bytes)Case for Copernicus' working copy of his book on the solar system

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Kepler's system has the sun at the center and all the planets orbiting it, from

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hypertext copyright.jpg (1684 bytes) G. H. Rieke

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