Astronomy

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Astronomy

Astronomy, the scientific study of bodies in space, underwent a revolution during the Renaissance. Three great astronomers—Nicolaus Copernicus, Johannes Kepler, and Galileo Galilei—challenged the old notion that Earth was the center of the universe by showing that Earth and the other planets revolve around the Sun. Other Renaissance thinkers developed the concepts of infinity and of a universe containing many worlds.

Since ancient times, people had learned astronomy alongside astrology and cosmology. Astrology is the study of the influences of the heavens on earthly events. Renaissance astrologers believed that they could predict the future by observing the stars. Cosmology is the study of the nature and structure of the universe. It is linked to theology* and philosophy. The ideas of philosophers, such as the ancient Greek thinker Aristotle, affected Renaissance views of the universe as much as the observations of scientists did. Aristotle believed that the world was changing and imperfect, but that the heavens were unchanging and pure. Ptolemy, the leading astronomer of the ancient world, thought that the study of heavenly motion should encourage people to think about its cause, which was God. Because they saw astronomy as being linked to faith, some Renaissance thinkers were very slow to change the ideas about the universe that they had inherited from Ptolemy.

From Ptolemy to Copernicus. Ptolemy had described his view of the universe in a book called Almagest. Renaissance scholars translated and commented on this text in the 1400s. Ptolemy's model of the universe was geocentric, or earth-centered. In it, the Sun and planets revolved around Earth. Most Renaissance scholars accepted this theory.

Copernicus, the greatest astronomer of the first half of the 1500s, had a new vision of the universe. In On the Revolutions of the Heavenly Spheres (1543), Copernicus described a heliocentric, or sun-centered, universe. In this model, the planets, including Earth, circled the Sun. Copernicus believed that this entire system existed within a sphere of unmoving stars. He used ancient, medieval*, and Renaissance astronomical works to defend his ideas. He combined these sources with his own observations of the sky to create a complex mathematical theory of the movements of the heavens.

Reactions to Copernicus. Most professional astronomers during the second half of the 1500s had high regard for Copernicus. However, they refused to believe in his heliocentric model.

The greatest astronomer of this period was Tycho Brahe. He possessed great skills as an observer and a maker of scientific instruments. Brahe presented a system called geoheliocentrism, in which the planets circled the Sun, which in turn circled an unmoving Earth. This theory enabled him to take advantage of some of Copernicus's discoveries. At the same time, Brahe rejected the disturbing notion that Earth was not the central and most important body of the universe.

Johannes Kepler (1571–1630) and Galileo Galilei (1564–1642) supported the heliocentric view. Kepler used mathematical principles to explain how planets move in a sun-centered system. He was also the first to describe gravity as a force that attracts all bodies in the universe to each other. Galileo used the telescope—a new invention—to study the heavens in greater detail than had ever been possible before. His observations led him to question established ideas about the universe. He saw that heavenly bodies were not perfect and unchanging when he found mountains and valleys on the Moon's surface. He realized that not everything in the heavens revolved around Earth when he discovered moons orbiting Jupiter. He carefully watched the phases of Venus (the cycle of changes in its appearance) and argued that they would not be visible in an earth-centered universe.

Galileo published his discoveries and theories in Dialogue Concerning the Two Chief World Systems in 1632. The book described Copernicus's view of a universe centered on the Sun as a physical reality, rather than simply a theory. By publishing this claim, Galileo disobeyed a papal* order. In response, the Roman Catholic Church banned his book and placed Galileo under house arrest for life.

Contributions from Theology. Modern views of the universe grew out of the findings of Copernicus, Kepler, and Galileo. However, they also reflect the ideas of Renaissance theologians.

Medieval models of the universe had claimed that the universe had boundaries. Cardinal Nicolas Cusanus (1401–1464) argued that the universe was boundless. He also believed that it had no single center and that it contained many inhabited worlds. In the late 1500s, Italian philosopher Giordano Bruno took up Cusanus's ideas. Bruno was the first to claim that the Sun was a star and that the stars were suns. He was burned at the stake in 1600. Some Renaissance astronomers, including Copernicus, continued to see the universe as limited in size. However, by the end of the 1600s, the idea of an infinite universe with many solar systems had become common.

(See alsoMagic and Astrology. )