Heavenly spheres: a revolution in astronomical observation

Since humankind first looked up at the night sky, we’ve been fascinated by the stars. Endlessly distant, the constellations that make up the heavens are at once foreign and deeply familiar. 

We can date events from thousands of years ago based on descriptions of the sky (such as a solar eclipses) but understanding how celestial bodies move is a more recent innovation. The texts of Copernicus, Galileo and Johannes Kepler, offered in Printed and Manuscript Americana & Science at Christie’s New York from 13-27 January, laid the foundation for a new way of seeing the stars. 

Their work influenced not just future scientists but also generations of artists, whose work can be found in The Exceptional Sale at Christie’s New York on 27 January. They made inventive works based on our perspective on the great cosmic machine.

Our earliest astronomical documents come from the time of Aristotle (344-322 BC), who believed that between 43 and 55 crystalline spheres orbited on different axes around a stationary Earth — the geocentric model. 

Claudius Ptolemy (AD 120-180) in Alexandria, Egypt, expanded upon this model 400 years later, theorising that Earth travelled along a small circular path. As this model successfully predicted planetary movement within a few degrees’ difference, it remained the standard for the next 1500 years. 

In the 16th century, as Nicolaus Copernicus was studying astronomical mathematics at the University of Kraków, he noticed logical discrepancies within both the Ptolemaic and Aristotelian models of the universe. This research provided the basis of the most important scientific text of the 16th century: his landmark book De revolutionibus orbium coelestium (On the Revolutions of Celestial Spheres).

Published in 1543, the year of his death, the text uprooted the established systems of astronomy by stating that Earth was not the centre of the universe. Instead, it was the Sun, which was orbited by seven planets. This heliocentric model was one of the most important theories of the Renaissance, which would ultimately usher in the Scientific Revolution. 

The main opponent of Copernicus’ theory was the Church, with scripture often being cited to invalidate his theory, but Protestants were also vehement critics of the heliocentric model, with the prominent theologian Martin Luther calling Copernicus a fool. 

Despite this, his theories proved compelling to scientists. At the end of the 16th century, the German astronomer Johannes Kepler sought to prove and expand upon his model. Following the death of his mentor Tycho Brahe, Kepler was appointed the imperial mathematician of Holy Roman Emperor Rudolph II in 1601 and began studying planetary orbits around the Sun.

Among Kepler’s most important contributions to the field was his proof that planets did not travel along a circular path, but an elliptical one. This ‘first law of planetary motion’ was outlined in his 1609 book Astronomia nova. 

He showed that planets had different velocities, with those farther from the centre traveling more slowly than those closer. He built an early kind of telescope that allowed him to see that Venus, like the Moon, had phases of light and dark, showing that planets rotated as they orbited. 

At the same time that Kepler was researching heliocentrism through mathematics, the Italian polymath Galileo Galilei was refining the telescope in order to prove the theory through optics. The telescope allowed him to see Venus with greater detail than Kepler could. It also allowed him to see sunspots, which gave him cause to debate scientists of the time, who thought them to be satellites of the Sun, rather than parts of its surface.

Galileo’s observations led to his 1632 book Dialogo sopra i due massimi sistemi del mondo (Dialogue Concerning the Two Chief World Systems), which disproved the Aristotelian geocentric model and bolstered, with greater physical evidence than any of its predecessors, the heliocentric model. 

These observations were deeply troubling for the Church. Early versions of Galileo's theories had brought him to the capital to defend himself in front of Pope Paul V, but this inquisition was eventually cast aside with the election of his friend Pope Urban VIII. A decade later, however, the publication of his Dialogo infuriated Urban VII, who found the astronomer guilty on suspicion of heresy, condemning him to house arrest where he would remain until his death. To publish his final work, Galileo had to have the manuscript smuggled out of Italy to France and thence to Leiden for its first printing. This was the Discorsi e dimostrazioni matematiche of 1628, considered the first modern textbook of physics.

It is no exaggeration to say that the observations of Galileo, Kepler and Copernicus fundamentally altered humanity’s relationship to the cosmos. They showed courage in the face of fierce opposition from the Church and paved the way for scientists such as Isaac Newton and Albert Einstein in the modern scientific era. Further, their work gave fodder to artists and thinkers, enabling them to see what the heavens looked like and how small our world is in the great scale of celestial spheres.