Western Civilization

the numerical relationships existing between the plan-

ets, he focused much of his attention on discovering

the “music of the spheres.” Kepler was also a brilliant

mathematician and astronomer who took a post as im-

perial mathematician to Emperor Rudolf II. Using the

detailed astronomical data compiled by his predecessor,

Kepler derived laws of planetary motion that confirmed

the heliocentric theory. In his first law, he contradicted

Copernicus by showing that the orbits of the planets

around the sun were not circular but elliptical, with

the sun at one focus of the ellipse rather than at the

center.

Kepler’s work effectively eliminated the idea of

uniform circular motion as well as the idea of crystal-

line spheres revolving in circular orbits. The basic

structure of the traditional Ptolemaic system had

been disproved, and people had been freed to think

innewwaysofthepathsofplanetsrevolvingaround

the sun. By the end of Kepler’s life, the Ptolemaic sys-

tem was rapidly losing ground to the new ideas. Im-

portant questions remained unanswered, however.

What were the planets made of? And how could

motion in the universe be explained? It was an Italian

scientist who achieved the next important break-

throughtoanewcosmologybyansweringthefirst

question.

Galileo

Galileo Galilei (1564–1642) taught mathematics, first

at Pisa and later at Padua, one of the most prestigious

universities in Europe. Galileo was the first European

to make systematic observations of the heavens by

means of a telescope, thereby inaugurating a new age

in astronomy. He had heard of a Flemish lens grinder

who had created a “spyglass” that magnified objects

seen at a distance and soon constructed his own.

Instead of peering at terrestrial objects, Galileo turned

his telescope to the skies and made a remarkable series

of discoveries: mountains and craters on the moon,

four moons revolving around Jupiter, the phases of

Venus, and sunspots. Galileo’s observations demolished

yet another aspect of the traditional cosmology in that

the universe seemed to be composed of a material

The Copernican System.The

Copernican system was presented inOn

the Revolutions of the Heavenly Spheres,

published in 1543 shortly before

Copernicus’s death. As shown in this

illustration from the first edition of the

book, Copernicus maintained that the sun

was at the center of the universe and that

the planets, including the earth, revolved

around it. Moreover, the earth rotated

daily on its axis. (The circles read, from

the center outward: 1. Sun; 2. Mercury,

orbit of 80 days; 3. Venus; 4. Earth, with

the moon, orbit of 1 year; 5. Mars, orbit

of 2 years; 6. Jupiter, orbit of 12 years; 7.

Saturn, orbit of 30 years; 8. Immobile

Sphere of the Fixed Stars.)

Image Select/Art Resource, NY

Toward a New Heaven: A Revolution in Astronomy 389

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