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The instrument that revolutionized astronomy before the telescope

Nature Astronomy volume 4, pages 111–112 (2020)Cite this article

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Over the past fifteen years, a group of recent graduates at St. John’s College in Santa Fe, New Mexico, have been pursuing the apparently quixotic goal of creating a functioning replica of one of the last pre-telescopic astronomical instruments — Tycho Brahe’s equatorial armillary sphere. Their aim was to incorporate the particular design features that allowed Brahe (1546–1601) to improve the precision of naked-eye observations by an order of magnitude, in a series of observations over more than thirty years. This increase in precision was crucial in permitting Kepler to demonstrate the elliptical shape of planetary orbits. The donors of the armillary wanted, above all, to give present-day students and researchers the means to experience and understand the functioning of the remarkable series of instruments that led to an entirely new view of how planets move.

How did Brahe himself come to take on such a project, and how did he achieve such a dramatic improvement? From an early age, he had an unusual interest in astronomy. As a teenaged student, he bought some of the astronomical textbooks current at the time, and began making his own observations. What was especially remarkable was that despite his rudimentary instruments — initially, nothing more than a tiny celestial globe and a piece of string — he set out to test the planetary positions given in the ephemerides he had acquired. Even on the globe — “no bigger than a fist”, as Brahe later wrote — he could tell that the positions in the books were wrong, compared to the fixed stars. From this time (he was then about 16 years old), he sought to acquire better instruments in order to provide observations that could serve as the basis for improved planetary theories.

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Fig. 1: Brahe’s armillary.

Astronomiae Instauratae Mechanica, courtesy of the Smithsonian Libraries (engraving)

Fig. 2: Tycho Brahe’s engravings of his two innovations.

Astronomiae Instauratae Mechanica, courtesy of the Smithsonian Libraries

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  1. Tutor Emeritus at St. John’s College, Santa Fe, NM, USA

    William Donahue

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  1. William Donahue
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Correspondence to William Donahue.

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Donahue, W. The instrument that revolutionized astronomy before the telescope. Nat Astron 4, 111–112 (2020). https://doi.org/10.1038/s41550-020-1020-8

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