9.2: Astronomy

Ptolemy and Aristotle were the most influential ancient sources of scientific knowledge. Both argued that the Earth was at the center of the universe, which consisted of a giant rotating crystal sphere studded with the stars. The sun, moon, and planets were suspended and rotated around the Earth. Ptolemy, who lived centuries after Aristotle, elaborated on the Aristotelian system. He claimed that there were close to eighty spheres, one within the other, which was why the different heavenly bodies did not all move in the same direction or at the same speed. The idea that the earth is at the center of the universe is known as geocentrism.

Figure 9.2.1: The geocentric universe illustrated, with the sun and planets revolving around the Earth. Interestingly, the illustration was created in 1660, a few decades after Galileo popularized the fact that heliocentrism was completely inaccurate.

In this model of the universe, the earth was imperfect, chaotic, and changing, while the heavens were perfect and uniform. Thus, Christian thinkers embraced the Aristotelian model because it fit Christian theology so well. Also, from this Christianized version came the concept that God and heaven are "up in the sky" and hell is "below the ground". When the astronomers of the Scientific Revolution started detecting irregularities in the heavens, this totally contradicted how most learned people thought about the essential characteristics of the universe.

Nicolaus Copernicus

The ancient model did not match the observed paths taken by the stars. Nor did it align with the different planets' regular, circular orbits. Thus, medieval astronomers created even-more-elaborate caveats and modifications to the idea of simple perfect orbits, positing the existence of hugely complex paths supposedly taken by various heavenly objects.

A Polish priest, Nicolaus Copernicus (1473 – 1543), was the first to argue that the whole system would match reality if the sun was at the center of the orbits instead of the earth. This concept is called heliocentrism. Interestingly, he retained the idea of the crystal spheres, and used Ptolemy’s calculations in his own work. Copernicus was a quintessential Renaissance man; a medical doctor, an accomplished painter, fluent in Greek, and an astronomer.

Tycho Brahe

Copernicus’s theory was little known outside of astronomical circles, with most astronomers expressing dismay and skepticism at the idea. A Danish astronomer named Tycho Brahe (1546 – 1601) tried to refute the Heliocentric theory by demonstrating that the Earth was indeed at the center of the universe but that the heavenly bodies followed monstrously complex orbits. He spent twenty years carefully observing the heavens from his castle on an island near Copenhagen. Brahe’s work provided a wealth of data for later astronomers to work from, even though his central argument turned out to be inaccurate.

Johannes Kepler

German astronomer Johannes Kepler (1571 – 1630) ended up using Brahe’s data to argue against Brahe’s conclusion. Kepler noticed that there was some kind of force emanating from the sun that seemed to hold the planets in orbit. He concluded that some form of magnetism was likely the cause. (In fact, Kepler had noticed the role of gravity in space). Interestingly, Kepler was the official imperial mathematician of the Holy Roman Emperor Rudolph II, who overlooked the fact that Kepler was a Protestant because he (Rudolph) was so interested in science - all against the backdrop of the Thirty Years’ War.

Galileo Galilei

In the end, the most significant publicist of heliocentrism was an Italian, Galileo Galilei (1564 - 1642). Galileo built a telescope and discovered previously unknown aspects of the heavenly bodies, such as the fact that the moon and sun did not have smooth, perfect surfaces. His first major publication, The Starry Messenger (1610), conclusively demonstrated that the heavens were full of previously unknown objects (e.g. the moons of Jupiter) and that planets and moons appeared to be “imperfect” in the same manner as the earth.

In 1632, Galileo published the Dialogue, a work that quickly became much better known than Copernicus’s or Kepler’s. The Dialogue consisted of two imaginary interlocutors, one of whom presented the case for heliocentrism, the other for geocentrism. The supporter of heliocentrism wins every argument, and his debate partner, “Stupid” (Simplicio) is confounded. In publicizing his work, Galileo undermined the idea that the heavens were perfect, that the earth was central, and by extension, that ancient knowledge was reliable. All of these things were disruptive.

Galileo was accused of supporting a condemned doctrine, heliocentrism, not of heresy per se, and tried by the Inquisition in 1633. (In part because his former patron, the pope Urban VIII, thought that Galileo had been mocking him personally by naming the imaginary defender of the Ptolemaic view Stupid.) Galileo was forced to recant and his book was placed on the Catholic Index of banned books, where it would remain until 1822.

Galileo is less well remembered for his work in physics. Six years after the Dialogue was put on the Index, he published another work, Two New Sciences of Motion and Mechanics, that provided a theory and mathematical formulas of inertia and aspects of gravity. These theories refuted Aristotelian physics, which claimed that objects only stay in motion when there is a direct impetus.