Controversy over the Comets

Elisabeth Hevelius: Observational Astronomer

Elisabeth Hevelius Learning Leaflet

How does the Sextant symbolize the person who worked on the famed Hevelius star catalog and star atlas throughout its production, from observation to publication? 

Elisabeth Hevelius, wife of Johann Hevelius, was an astronomer in her own right. They worked together in the observatory of their Gdansk home to measure angular widths and distances with a great sextant, which required two observers at a time. The Sextant was among the new constellations they proposed in Uranographia (1690), the most detailed and influential celestial atlas of the 17th century. The Uranographia contains 54 beautiful double page engraved plates of 73 constellations, and 2 oversized folding plates of planispheres.


Section 1: Systems of the World

Early modern astronomers debated diverse systems of the world.  Yet the competing models produced identical planetary predictions, so they were experimenting with geometrically equivalent cosmic systems.  Therefore astronomers searched for kinds of observations other than planetary predictions that might decide between them.  Perhaps clues might be found in the motion of comets, phases of Venus, or measurements of stellar parallax.

Section 3: Controversy

Mathematicians resisted the attempts of physicists and theologians to discount their conclusions.  Yet mathematical methods alone were not able to resolve the enigmas of comets, parallax, and diverse systems of the world. Galileo engaged in polemics against the system of Tycho Brahe that went beyond the evidence available at the time.

Six Cosmological Systems: Phases of Venus

Kircher-Cosmological Systems-Venus

It is often thought that Galileo's discovery of the phases of venus demonstrated the contested heliocentric model of the universe. However, such an understanding is overly simplistic of the early modern account of astronomy. Use this learning leaflet to learn more.


Phases of Venus: Riccioli, New Almagest

Phases of Venus-Riccioli

The phases of Venus were an item of discussion in early modern Europe as scientists sought to determine whether it was evidence of the heliocentric system. Yet among the scientists it was anything but conclusive that this evidence proved the sun-centered universe. Learn more in this learning leaflet.


Introduction to the Jesuits

Ignatius Loyola founded the Jesuits in the mid-16th century. The Jesuits were charged with teaching nothing in theology contrary to Thomas Aquinas, and nothing in natural science contrary to Aristotle. The cosmic section of Peter Apian, displayed in Music of the Spheres, represents the Aristotelian-Ptolemaic view of the cosmos the Jesuits were charged to uphold. Yet, in practice, things were more complicated.

Theory Choice and Underdetermination

For us today, with 20/20 hindsight, the Tychonic system may seem like an unstable compromise between Ptolemy and Copernicus. However,  Christopher Graney has shown that this impression ignores the force of Tycho’s argument from star sizes, given the means then available for measuring apparent star widths. In his private manuscripts, Galileo acknowledged that stars display detectable widths, even through the telescope, and he never publicly answered Tycho’s argument.

Sections 2: Comets

Since antiquity, comets posed an enigma. They appear without warning. They do not stay within the bounds of the Zodiac as do the planets. They arise from different directions. They change radically in speed and brightness. Their tails always point away from the Sun. 

Aristotle argued that comets are fiery vapors in the upper atmosphere.  Yet although parallax could be observed for the Moon, no parallax was apparent for comets. The absence of cometary parallax implied that comets are farther away than the Moon.