19.3.3: Astronomy

The Emergence of Modern Astronomy

While astronomy is the oldest of the natural sciences, dating back to antiquity, its development during the period of the scientific revolution entirely transformed the views of society about nature. The publication of the seminal work in the field of astronomy, Nicolaus Copernicus’ De revolutionibus orbium coelestium (On the Revolutions of the Heavenly Spheres) published in 1543, is, in fact, often seen as marking the beginning of the time when scientific disciplines, including astronomy, began to apply modern empirical research methods, and gradually transformed into the modern sciences as we know them today.

The Copernican Heliocentrism

Copernican heliocentrism is the name given to the astronomical model developed by Nicolaus Copernicus and published in 1543. It positioned the sun near the center of the universe, motionless, with Earth and the other planets rotating around it in circular paths, modified by epicycles and at uniform speeds. The Copernican model departed from the Ptolemaic system that prevailed in western culture for centuries, placing Earth at the center of the universe. Copernicus’ De revolutionibus marks the beginning of the shift away from a geocentric (and anthropocentric) universe with Earth at its center. Copernicus held that Earth is another planet revolving around the fixed sun once a year, and turning on its axis once a day. But while he put the sun at the center of the celestial spheres, he did not put it at the exact center of the universe, but near it. His system used only uniform circular motions, correcting what was seen by many as the chief inelegance in Ptolemy’s system.

The Copernican Revolution

From 1543 until about 1700, few astronomers were convinced by the Copernican system. Forty-five years after the publication of De Revolutionibus, the astronomer Tycho Brahe went so far as to construct a cosmology precisely equivalent to that of Copernicus, but with Earth held fixed in the center of the celestial sphere instead of the sun. However, Tycho challenged the Aristotelian model when he observed a comet that went through the region of the planets. This region was said to only have uniform circular motion on solid spheres, which meant that it would be impossible for a comet to enter into the area. Following Copernicus and Tycho, Johannes Kepler and Galileo Galilei, both working in the first decades of the 17th century, influentially defended, expanded and modified the heliocentric theory.

Johannes Kepler

Johannes Kepler was a German scientist who initially worked as Tycho’s assistant. In 1596, he published his first book, the Mysterium cosmographicum, which was the first to openly endorse Copernican cosmology by an astronomer since the 1540s. The book described his model that used Pythagorean mathematics and the five Platonic solids to explain the number of planets, their proportions, and their order. In 1600, Kepler set to work on the orbit of Mars, the second most eccentric of the six planets known at that time. This work was the basis of his next book, the Astronomia nova (1609). The book argued heliocentrism and ellipses for planetary orbits, instead of circles modified by epicycles. It contains the first two of his eponymous three laws of planetary motion (in 1619, the third law was published). The laws state the following:

• All planets move in elliptical orbits, with the sun at one focus.
• A line that connects a planet to the sun sweeps out equal areas in equal times.
• The time required for a planet to orbit the sun, called its period, is proportional to long axis of the ellipse raised to the 3/2 power. The constant of proportionality is the same for all the planets.

Galileo Galilei

Galileo Galilei was an Italian scientist who is sometimes referred to as the “father of modern observational astronomy.” Based on the designs of Hans Lippershey, he designed his own telescope, which he had improved to 30x magnification. Using this new instrument, Galileo made a number of astronomical observations, which he published in the Sidereus Nuncius in 1610. In this book, he described the surface of the moon as rough, uneven, and imperfect. His observations challenged Aristotle’s claim that the moon was a perfect sphere, and the larger idea that the heavens were perfect and unchanging. While observing Jupiter over the course of several days, Galileo noticed four stars close to Jupiter whose positions were changing in a way that would be impossible if they were fixed stars. After much observation, he concluded these four stars were orbiting the planet Jupiter and were in fact moons, not stars. This was a radical discovery because, according to Aristotelian cosmology, all heavenly bodies revolve around Earth, and a planet with moons obviously contradicted that popular belief. While contradicting Aristotelian belief, it supported Copernican cosmology, which stated that Earth is a planet like all others.

In 1610, Galileo also observed that Venus had a full set of phases, similar to the phases of the moon, that we can observe from Earth. This was explainable by the Copernican system, which said that all phases of Venus would be visible due to the nature of its orbit around the sun, unlike the Ptolemaic system, which stated only some of Venus’s phases would be visible. Due to Galileo’s observations of Venus, Ptolemy’s system became highly suspect and the majority of leading astronomers subsequently converted to various heliocentric models, making his discovery one of the most influential in the transition from geocentrism to heliocentrism.

Copernicus was a polyglot and polymath who obtained a doctorate in canon law and also practiced as a physician, classics scholar, translator, governor, diplomat, and economist. In 1517 he derived a quantity theory of money–a key concept in economics–and in 1519, he formulated a version of what later became known as Gresham’s law (also in economics).

Uniting Astronomy and Physics: Isaac Newton

Although the motions of celestial bodies had been qualitatively explained in physical terms since Aristotle introduced celestial movers in his Metaphysics and a fifth element in his On the Heavens, Johannes Kepler was the first to attempt to derive mathematical predictions of celestial motions from assumed physical causes. This led to the discovery of the three laws of planetary motion that carry his name.

Isaac Newton developed further ties between physics and astronomy through his law of universal gravitation. Realizing that the same force that attracted objects to the surface of Earth held the moon in orbit around the Earth, Newton was able to explain, in one theoretical framework, all known gravitational phenomena. Newton’s Principia (1687) formulated the laws of motion and universal gravitation, which dominated scientists’ view of the physical universe for the next three centuries. By deriving Kepler’s laws of planetary motion from his mathematical description of gravity, and then using the same principles to account for the trajectories of comets, the tides, the precession of the equinoxes, and other phenomena, Newton removed the last doubts about the validity of the heliocentric model of the cosmos. This work also demonstrated that the motion of objects on Earth and of celestial bodies could be described by the same principles. His laws of motion were to be the solid foundation of mechanics; his law of universal gravitation combined terrestrial and celestial mechanics into one great system that seemed to be able to describe the whole world in mathematical formulae

Johannes Kepler Biography (1571-1630): Johannes Kepler was a German astronomer and mathematician, who played an important role in the 17th century scientific revolution.